The Flagships of the Space Fleet
By exploring planets, moons, asteroids and comets, these spacecraft are extending the frontiers of human knowledge
Few sights are as awe-inspiring as the liftoff of a space shuttle. Propped on its pair of solid-rocket boosters, the shuttle towers over the launchpad at the Kennedy Space Center in Cape Canaveral, Fla. Hundreds of engineers and technicians man the consoles in the Launch Control Center, monitoring the shuttle’s systems as the countdown proceeds. Half a minute before liftoff, the shuttle’s onboard computers take over the launch sequence, and at T minus six seconds they send the command to start the main engines. Fiery exhaust billows downward from the shuttle’s three rocket nozzles. At T minus zero, the solid-rocket boosters ignite, the umbilical lines retract and the shuttle climbs into the sky with 3.6 million kilograms (eight million pounds) of thrust. The space shuttle grabs the public’s attention and a big share of the budget of the National Aeronautics and Space Administration— because it carries astronauts into orbit. But it is by no means the only vessel in the space fleet. In recent years, NASA has sent unmanned spacecraft to explore Jupiter, Saturn, the asteroid belt and the moon. What these missions lack in personality they make up for with remarkable discoveries. The Galileo spacecraft, for example, has returned spectacular images of Jupiter’s moons and that planet’s Great Red Spot. Closer to home, the Lunar Prospector probe has found evidence of ice on the poles of Earth’s moon. Half a dozen of the most extraordinary unmanned spacecraft are profiled on the following pages.
Three of these probes Galileo, Cassini and the Chandra X-ray Observatory are large, expensive machines packed with scientific instrumentation. But the three others—Near Earth Asteroid Rendezvous, Lunar Prospector and Stardust are part of NASA’s new Discovery series of “faster, better, cheaper” spacecraft. Lunar Prospector is perhaps the best example of a cost-effective craft: the mission is being done for only $63 million. In contrast, a typical space shuttle mission costs about $420 million.
Over the next 10 years, about 50 more unmanned science probes are expected to blast off into space (for a comprehensive list, see pages 18 and 19). Many of these craft will venture across the solar system, and others will scan the heavens from Earth’s orbit. NASA will not be the only player the European Space Agency, Russia, Japan and others plan to launch their own vessels. This international armada will revolutionize our understanding of the universe and perhaps pave the way for manned missions to other worlds.
Three of these probes Galileo, Cassini and the Chandra X-ray Observatory are large, expensive machines packed with scientific instrumentation. But the three others—Near Earth Asteroid Rendezvous, Lunar Prospector and Stardust are part of NASA’s new Discovery series of “faster, better, cheaper” spacecraft. Lunar Prospector is perhaps the best example of a cost-effective craft: the mission is being done for only $63 million. In contrast, a typical space shuttle mission costs about $420 million.
Over the next 10 years, about 50 more unmanned science probes are expected to blast off into space (for a comprehensive list, see pages 18 and 19). Many of these craft will venture across the solar system, and others will scan the heavens from Earth’s orbit. NASA will not be the only player the European Space Agency, Russia, Japan and others plan to launch their own vessels. This international armada will revolutionize our understanding of the universe and perhaps pave the way for manned missions to other worlds.
The International Space Station:A WORK IN PROGRESS
The construction site in space that is for International Space Station is nothing if not ambitious. Writers have an array of superlatives they can choose from to describe the program: it is by far the most complex in-orbit project ever attempted and arguably one of the biggest engineering endeavors of any kind. More than 100 separate elements weighing 455,000 kilograms (over a million pounds) on Earth will be linked together during the assembly operation, making it the most massive thing in orbit: it will have the equivalent of two 747 jetliners’ worth of laboratory and living space. The job will need 45 flights by U.S. shuttles and Russian rockets, and over 50 more launches will take up supplies, crew and fuel to maintain the station in its orbit. Contributions come from 16 countries, making it the most cosmopolitan space program. Hooking the pieces together will take at least 1,700 hours of space walks, many more than have been made during the entire history of space exploration to date. Robotic arms and hands will be required, and free-flying robotic “eyes” might be employed for inspection flights.
Robots Vs Humans
The National Aeronautics and Space Administration has a difficult task. It must convince U.S. taxpayers that space science is worth $13.6 billion a year. To achieve this goal, the agency conducts an extensive public-relations effort that is similar to the marketing campaigns of America’s biggest corporations. NASA has learned a valuable lesson about marketing in the 1990s: to promote its programs, it must provide entertaining visuals and stories with compelling human characters. For this reason, NASA issues a steady stream of press releases and images from its human spaceflight program.
Every launch of the space shuttle is a media event. NASA presents its astronauts as ready-made heroes, even when their accomplishments in space are no longer groundbreaking. Perhaps the best example of NASA’s public-relations prowess was the participation of John Glenn, the first American to orbit Earth, in shuttle mission STS-95 last year. Glenn’s return to space at the age of 77 made STS-95 the most avidly followed mission since the Apollo moon landings. NASA claimed that Glenn went up for science he served as a guinea pig in various medical experiments— but it was clear that the main benefit of Glenn’s space shuttle ride was publicity, not scientific discovery.
Criticism of human spaceflight comes from many quarters. Some critics point to the high cost of manned missions. They contend that the National Aeronautics and Space Administration has a full slate of tasks to accomplish and that human spaceflight is draining funds from more important missions. Other critics question the scientific value of sending people into space. Their argument is that human spaceflight is an expensive “stunt” and that scientific goals can be more easily and satisfactorily accomplished by robotic spacecraft.But the actual experience of astronauts and cosmonauts over the past 38 years has decisively shown the merits of people as explorers of space. Human capability is required in space to install and maintain complex scientific instruments and to conduct field exploration. These tasks take advantage of human flexibility, experience and judgment. They demand skills that are unlikely to be automated within the foreseeable future. A program of purely robotic exploration is inadequate in addressing the important scientific issues that make the planets worthy of detailed study.
Every launch of the space shuttle is a media event. NASA presents its astronauts as ready-made heroes, even when their accomplishments in space are no longer groundbreaking. Perhaps the best example of NASA’s public-relations prowess was the participation of John Glenn, the first American to orbit Earth, in shuttle mission STS-95 last year. Glenn’s return to space at the age of 77 made STS-95 the most avidly followed mission since the Apollo moon landings. NASA claimed that Glenn went up for science he served as a guinea pig in various medical experiments— but it was clear that the main benefit of Glenn’s space shuttle ride was publicity, not scientific discovery.
Criticism of human spaceflight comes from many quarters. Some critics point to the high cost of manned missions. They contend that the National Aeronautics and Space Administration has a full slate of tasks to accomplish and that human spaceflight is draining funds from more important missions. Other critics question the scientific value of sending people into space. Their argument is that human spaceflight is an expensive “stunt” and that scientific goals can be more easily and satisfactorily accomplished by robotic spacecraft.But the actual experience of astronauts and cosmonauts over the past 38 years has decisively shown the merits of people as explorers of space. Human capability is required in space to install and maintain complex scientific instruments and to conduct field exploration. These tasks take advantage of human flexibility, experience and judgment. They demand skills that are unlikely to be automated within the foreseeable future. A program of purely robotic exploration is inadequate in addressing the important scientific issues that make the planets worthy of detailed study.
The Mars Pathfinder
Rocks, rocks, look at those rocks,” They exclaimed to everyone in the Mars Pathfinder control room at about 4:30 P.M. on July 4, 1997. The Pathfinder lander was sending back its first images of the surface of Mars, and everyone was focused on the television screens. We had gone to Mars to look at rocks, but no one knew for sure whether we would find any, because the landing site had been selected using orbital images with a resolution of roughly a kilometer. Pathfinder could have landed on a flat, rock-free plain. The first radio down-link indicated that the lander was nearly horizontal, which was worrisome for those of us interested in rocks, as most expected that a rocky surface would result in a tilted lander. The very first images were of the lander so that we could ascertain its condition, and it was not until a few tense minutes later that the first pictures of the surface showed a rocky plain exactly as we had hoped and planned for.
Why did we want rocks? Every rock carries the history of its formation locked in its minerals, so we hoped the rocks would tell us about the early Martian environment. The two-part Pathfinder payload, consisting of a main lander with a multi-spectral camera and a mobile rover with a chemical analyzer, was suited to looking at rocks. Although it could not identify the minerals directly its analyzer could measure only their constituent chemical elements our plan was to identify them indirectly based on the elemental composition and the shapes, textures and colors of the rocks. By landing Pathfinder at the mouth of a giant channel where a huge volume of water once flowed briefly, we sought rocks that had washed down from the ancient, heavily cratered highlands.Such rocks could offer clues to the early climate of Mars and to whether conditions were once conducive to the development of life .The most important requirement for life on Earth (the only kind we know) is liquid water. Under present conditions on Mars, liquid water is unstable: because the temperature and pressure are so low, water is stable only as ice or vapor; liquid would survive for just a brief time before freezing or evaporating. Yet Viking images taken two decades ago show drainage channels and evidence for lakes in the highlands.
Why did we want rocks? Every rock carries the history of its formation locked in its minerals, so we hoped the rocks would tell us about the early Martian environment. The two-part Pathfinder payload, consisting of a main lander with a multi-spectral camera and a mobile rover with a chemical analyzer, was suited to looking at rocks. Although it could not identify the minerals directly its analyzer could measure only their constituent chemical elements our plan was to identify them indirectly based on the elemental composition and the shapes, textures and colors of the rocks. By landing Pathfinder at the mouth of a giant channel where a huge volume of water once flowed briefly, we sought rocks that had washed down from the ancient, heavily cratered highlands.Such rocks could offer clues to the early climate of Mars and to whether conditions were once conducive to the development of life .The most important requirement for life on Earth (the only kind we know) is liquid water. Under present conditions on Mars, liquid water is unstable: because the temperature and pressure are so low, water is stable only as ice or vapor; liquid would survive for just a brief time before freezing or evaporating. Yet Viking images taken two decades ago show drainage channels and evidence for lakes in the highlands.
The way that the ROVER'S land on the mars is
Mars Exploration Rover
Spirit, MER-A (Mars Exploration Rover – A), is a robotic rover on Mars, active from 2004 to 2010. It was one of two rovers of NASA's ongoing Mars Exploration Rover Mission. It landed successfully on Mars at 04:35 Ground UTC on January 4, 2004, three weeks before its twin, Opportunity (MER-B), landed on the other side of the planet. Its name was chosen through a NASA-sponsored student essay competition. The rover became stuck in late 2009, and its last communication with Earth was sent on March 22, 2010.
The rover completed its planned 90-sol mission. Aided by cleaning events that resulted in higher power from its solar panels, Spirit went on to function effectively over twenty times longer than NASA planners expected following mission completion. Spirit also logged 7.73 km (4.8 mi) of driving instead of the planned 1 km (0.6 mi), allowing more extensive geological analysis of Martian rocks and planetary surface features. Initial scientific results from the first phase of the mission (the 90-sol prime mission) were published in a special issue of the journal Science. On May 1, 2009 (5 years, 3 months, 27 Earth days after landing; 21.6 times the planned mission duration), Spirit became stuck in soft soil. This was not the first of the mission's "embedding events" and for the following eight months NASA carefully analyzed the situation, running Earth-based theoretical and practical simulations, and finally programming the rover to make extrication drives in an attempt to free itself. These efforts continued until January 26, 2010 when NASA officials announced that the rover was likely irrecoverably obstructed by its location in soft soil, though it continued to perform scientific research from its current location.
The rover continued in a stationary science platform role until communication with Spirit stopped on sol 2210 (March 22, 2010).JPL continued to attempt to regain contact until May 24, 2011, when NASA announced that efforts to communicate with the unresponsive rover had ended. A formal farewell was planned at NASA headquarters after the Memorial Day holiday and was televised on NASA TV.
The Jet Propulsion Laboratory (JPL), a division of the California Institute of Technology in Pasadena, manages the Mars Exploration Rover project for NASA's Office of Space Science, Washington
The rover completed its planned 90-sol mission. Aided by cleaning events that resulted in higher power from its solar panels, Spirit went on to function effectively over twenty times longer than NASA planners expected following mission completion. Spirit also logged 7.73 km (4.8 mi) of driving instead of the planned 1 km (0.6 mi), allowing more extensive geological analysis of Martian rocks and planetary surface features. Initial scientific results from the first phase of the mission (the 90-sol prime mission) were published in a special issue of the journal Science. On May 1, 2009 (5 years, 3 months, 27 Earth days after landing; 21.6 times the planned mission duration), Spirit became stuck in soft soil. This was not the first of the mission's "embedding events" and for the following eight months NASA carefully analyzed the situation, running Earth-based theoretical and practical simulations, and finally programming the rover to make extrication drives in an attempt to free itself. These efforts continued until January 26, 2010 when NASA officials announced that the rover was likely irrecoverably obstructed by its location in soft soil, though it continued to perform scientific research from its current location.
The rover continued in a stationary science platform role until communication with Spirit stopped on sol 2210 (March 22, 2010).JPL continued to attempt to regain contact until May 24, 2011, when NASA announced that efforts to communicate with the unresponsive rover had ended. A formal farewell was planned at NASA headquarters after the Memorial Day holiday and was televised on NASA TV.
The Jet Propulsion Laboratory (JPL), a division of the California Institute of Technology in Pasadena, manages the Mars Exploration Rover project for NASA's Office of Space Science, Washington
Air-Breathing Engines
For years, engineers have dreamed of building an aircraft that could reach hypersonic speeds, greater than Mach 5, or five times the speed of sound. Propelled by a special type of air breathing jet engine, a high-performance hypersonic craft might even be able to “fly” into orbit a possibility first considered more than four decades ago. Recently, as the technology has matured and as the demand for more efficient Earth-to-orbit propulsion grows, scientists have begun seriously considering such systems for access to space.Air-breathing engines have several advantages over rockets.
Because the former use oxygen from the atmosphere, they require less propellant fuel, but no oxidizer resulting in lighter, smaller and cheaper launch vehicles. To produce the same thrust, air breathing engines require less than one seventh the propellant that rockets do. Furthermore, because air-breathing vehicles rely on aerodynamic forces rather than on rocket thrust, they have greater maneuverability, leading to higher safety: flights can be aborted, with the vehicle gliding back to Earth. Missions can also be more flexible. But air breathing engines for launch vehicles are relatively immature compared with rocket technology, which has continually evolved, with refinements and re refinements, over the past 40 years. Hypersonic air-breathing propulsion is just now finally coming of age. Of course, jet engines which work by compressing atmospheric air, combining it with fuel, burning the mixture and expanding the combustion products to provide thrust are nothing new. But turbojet engines, such as those found on commercial and fighter aircraft, are limited to Mach 3 or 4, above which the turbine and blades that compress the air suffer damage from overheating.
Fortunately, at such high supersonic speeds a turbine is not required if the engine is designed so that the air is “ram”-compressed. Such an engine has an air inlet that has been specially shaped to slow and compress the air when the vehicle is moving rapidly through the atmosphere. Because ramjets cannot work unless the vehicle is traveling at high speeds, they have been integrated in the same engine housing with turbojets, as in the French Griffon II experimental aircraft, which set a speed record of 1,640 kilometers per hour (1,020 miles per hour) around a course in 1959. Ramjets have also been combined with rockets in surface-to-air and air-to-surface missiles. But ramjets are limited to about Mach 6, above which the combustion chamber becomes so hot that the combustion products (water) decompose. To obtain higher speeds, supersonic-combustion ramjets, or scram jets, reduce the compression of the airflow at the inlet so that it is not slowed nearly as much. Because the flow remains supersonic, its temperature does not increase as dramatically as it does in ramjets. Fuel is injected into the supersonic airflow, where it mixes and must burn within a millisecond. The upper speed limit of scram jets has yet to be determined, but theoretically it is above the range required for orbital velocity (Mach 20 to 25). But at such extreme speeds, the benefits of scram jets over rockets become small and possibly moot because of the resulting severe structural stresses.
Hypersonic air-breathing engines can operate with a variety of fuel sources, including both hydrogen and hydrocarbons. Liquid hydrogen, which powers the U.S. space shuttle, is the choice for space launch because it can be used to cool the engine and vehicle before being burned. Hydrocarbons cannot be utilized so efficiently and are limited to speeds less than about Mach 8. For a scram jet-powered craft, which must be designed to capture large quantities of air, the is tinction between engine and vehicle blurs. The oncoming flow is deflected mainly by the underside of the craft, which increases the pressure of the diverted air. Generally, the change is great enough to cause a pressure discontinuity,called a shock wave, which originates at the ship’s nose and then propagates through the atmosphere. Most of the compressed air between the bottom of the vehicle and the shock wave is directed into the engine. The air gets hotter as its flow is slowed and as fuel is burned in the combustion region. The end product of the reaction expands through both an internal and an external nozzle, generating thrust. The high pressures on the underside of the
vehicle also provide lift.
To broaden the scram jet’s operating range, engineers have designed vehicles that can fly in either scram or ram mode. The dual-mode operation can be achieved either by constructing a combustor of variable geometry or by shifting the fuel flow between injectors at different locations. Because neither scram jets nor ramjets can operate efficiently when they are traveling below Mach 2 or 3, a third type of propulsion (perhaps turbojet or rocket) is required for takeoff. So-called rocket-based combined-cycle engines, which could be used in a space vehicle, rely on a rocket that is integrated within the scram jet combustor to provide thrust from takeoff through subsonic, low-supersonic and then ramjet speeds. Ramjet operation is then followed by scram jet propulsion to at least Mach 10 or 12, after which the rocket is utilized again to supplement the scram jet thrust. Above Mach 18, the rocket by itself propels the vehicle into orbit and enables it to maneuver in space.
The National Aeronautics and Space Administration is currently testing several variations of such a system. First, though, much work remains to validate scram jets. Sophisticated computational fluid-dynamic and engineering design methods have made it possible to develop a launch vehicle that has a scram jet built into its structure. Challenges remaining include developing lightweight, high-temperature materials, ensuring rapid and efficient fuel mixing and combustion, and minimizing the buildup of undesirable heat.
In the 1970s the NASA Langley Research Center demonstrated basic scram jet technology with models of hypersonic vehicles and a wind tunnel. Additional ground tests of prototype engines have been performed elsewhere in the U.S. as well as in England, France, Germany, Russia, Japan and Australia, with other related research under way in countries such as China, Italy and India. Today scientists routinely conduct ground tests of scram jet engines at simulated speeds up to Mach 15. In flight tests the Russians have demonstrated ramjet operation of a dual-mode scram jet up to Mach 6.4. To date, though, no vehicle has flown under scramjet power.
But this ultimate test is nearing reality. Through its Hyper-X research program at Langley and Dryden Flight Research Center, NASA is currently building the X-43A, a 3.6-meter-long aircraft that will demonstrate scram jet flight at Mach 7 and Mach 10 within the next three years. If all goes well, the tests will pave the way for future uses of scram jet propulsion, possibly in a vehicle designed for hypersonic flight into space.
Because the former use oxygen from the atmosphere, they require less propellant fuel, but no oxidizer resulting in lighter, smaller and cheaper launch vehicles. To produce the same thrust, air breathing engines require less than one seventh the propellant that rockets do. Furthermore, because air-breathing vehicles rely on aerodynamic forces rather than on rocket thrust, they have greater maneuverability, leading to higher safety: flights can be aborted, with the vehicle gliding back to Earth. Missions can also be more flexible. But air breathing engines for launch vehicles are relatively immature compared with rocket technology, which has continually evolved, with refinements and re refinements, over the past 40 years. Hypersonic air-breathing propulsion is just now finally coming of age. Of course, jet engines which work by compressing atmospheric air, combining it with fuel, burning the mixture and expanding the combustion products to provide thrust are nothing new. But turbojet engines, such as those found on commercial and fighter aircraft, are limited to Mach 3 or 4, above which the turbine and blades that compress the air suffer damage from overheating.
Fortunately, at such high supersonic speeds a turbine is not required if the engine is designed so that the air is “ram”-compressed. Such an engine has an air inlet that has been specially shaped to slow and compress the air when the vehicle is moving rapidly through the atmosphere. Because ramjets cannot work unless the vehicle is traveling at high speeds, they have been integrated in the same engine housing with turbojets, as in the French Griffon II experimental aircraft, which set a speed record of 1,640 kilometers per hour (1,020 miles per hour) around a course in 1959. Ramjets have also been combined with rockets in surface-to-air and air-to-surface missiles. But ramjets are limited to about Mach 6, above which the combustion chamber becomes so hot that the combustion products (water) decompose. To obtain higher speeds, supersonic-combustion ramjets, or scram jets, reduce the compression of the airflow at the inlet so that it is not slowed nearly as much. Because the flow remains supersonic, its temperature does not increase as dramatically as it does in ramjets. Fuel is injected into the supersonic airflow, where it mixes and must burn within a millisecond. The upper speed limit of scram jets has yet to be determined, but theoretically it is above the range required for orbital velocity (Mach 20 to 25). But at such extreme speeds, the benefits of scram jets over rockets become small and possibly moot because of the resulting severe structural stresses.
Hypersonic air-breathing engines can operate with a variety of fuel sources, including both hydrogen and hydrocarbons. Liquid hydrogen, which powers the U.S. space shuttle, is the choice for space launch because it can be used to cool the engine and vehicle before being burned. Hydrocarbons cannot be utilized so efficiently and are limited to speeds less than about Mach 8. For a scram jet-powered craft, which must be designed to capture large quantities of air, the is tinction between engine and vehicle blurs. The oncoming flow is deflected mainly by the underside of the craft, which increases the pressure of the diverted air. Generally, the change is great enough to cause a pressure discontinuity,called a shock wave, which originates at the ship’s nose and then propagates through the atmosphere. Most of the compressed air between the bottom of the vehicle and the shock wave is directed into the engine. The air gets hotter as its flow is slowed and as fuel is burned in the combustion region. The end product of the reaction expands through both an internal and an external nozzle, generating thrust. The high pressures on the underside of the
vehicle also provide lift.
To broaden the scram jet’s operating range, engineers have designed vehicles that can fly in either scram or ram mode. The dual-mode operation can be achieved either by constructing a combustor of variable geometry or by shifting the fuel flow between injectors at different locations. Because neither scram jets nor ramjets can operate efficiently when they are traveling below Mach 2 or 3, a third type of propulsion (perhaps turbojet or rocket) is required for takeoff. So-called rocket-based combined-cycle engines, which could be used in a space vehicle, rely on a rocket that is integrated within the scram jet combustor to provide thrust from takeoff through subsonic, low-supersonic and then ramjet speeds. Ramjet operation is then followed by scram jet propulsion to at least Mach 10 or 12, after which the rocket is utilized again to supplement the scram jet thrust. Above Mach 18, the rocket by itself propels the vehicle into orbit and enables it to maneuver in space.
The National Aeronautics and Space Administration is currently testing several variations of such a system. First, though, much work remains to validate scram jets. Sophisticated computational fluid-dynamic and engineering design methods have made it possible to develop a launch vehicle that has a scram jet built into its structure. Challenges remaining include developing lightweight, high-temperature materials, ensuring rapid and efficient fuel mixing and combustion, and minimizing the buildup of undesirable heat.
In the 1970s the NASA Langley Research Center demonstrated basic scram jet technology with models of hypersonic vehicles and a wind tunnel. Additional ground tests of prototype engines have been performed elsewhere in the U.S. as well as in England, France, Germany, Russia, Japan and Australia, with other related research under way in countries such as China, Italy and India. Today scientists routinely conduct ground tests of scram jet engines at simulated speeds up to Mach 15. In flight tests the Russians have demonstrated ramjet operation of a dual-mode scram jet up to Mach 6.4. To date, though, no vehicle has flown under scramjet power.
But this ultimate test is nearing reality. Through its Hyper-X research program at Langley and Dryden Flight Research Center, NASA is currently building the X-43A, a 3.6-meter-long aircraft that will demonstrate scram jet flight at Mach 7 and Mach 10 within the next three years. If all goes well, the tests will pave the way for future uses of scram jet propulsion, possibly in a vehicle designed for hypersonic flight into space.
Space Tethers
When humans begin to inhabit the moon and planets other than Earth, they may not use the modern technology of rockets. Instead space travel and settlement may depend on an ancient technology invented long before recorded history—string. How can mere string propel objects through space? Consider two scenarios. First, a thick strand connecting two satellites can enable one to “throw” the other into a different orbit, much like a hunter casting a stone with a sling. Such a concept could be adapted for transporting payloads to the moon and beyond. Second, if the string is a conductive wire, electricity flowing through it will interact with Earth’s magnetic field to generate propulsive forces. The great advantage of both types of tethers momentum transfer and electrodynamic is their economical operation. Instead of consuming huge quantities of propellant, they work by simply draining a little momentum from a body already in orbit or by using electrical energy supplied from solar panels. To date, 17 space missions have involved tethers. Most of these missions have been successful, but the general public has heard mainly about two failures. In 1992 a satellite built by the Italian Space Agency was to be released upward, away from Earth, from the space shuttle Atlantis at the end of a long tether made of insulated copper wire. But the spool mechanism jammed, halting the experiment. Four years later the National Aeronautics and Space Administration tried again. In that mission, as the tether approached its full 20-kilometer (12-mile) length, the motion of the shuttle through Earth’s magnetic field generated 3,500 volts in the tether Electronic devices on the shuttle and the Italian satellite provided an electrical conduit to the ionosphere, allowing ampere-level currents to flow through the tether.
The experiment demonstrated that such electrodynamic tethers can convert shuttle momentum into kilowatts of electrical power, and vice versa. Unfortunately, a flaw in the insulation allowed a high-power electric arc to jump from the tether to the deployment boom, and the arc burned through the tether. But although the break aborted the electrodynamic part of the project, it inadvertently triggered a spectacular display of momentum transfer. At the time, the Italian satellite was 20 kilometers above the shuttle and was being pulled along faster than the orbital speed for that higher altitude. Consequently, when the tether broke, the excess momentum made the satellite soar to seven times the tether length, or 140 kilometers, above the shuttle.
Other work has had greater success. In 1993, to test an idea proposed by Joseph A. Carroll of Tether Applications in San Diego, a payload attached to a 20-kilometer tether was deployed downward from a large satellite. Because the speed of the payload was then slower than that required for an object at that reduced orbital altitude, cutting the tether at the right moment caused the package to descend toward a predetermined point on Earth’s surface. Tether Applications is now developing a reentry capsule and tether that the International Space Station could use to send urgent deliveries to Earth, including scientific payloads that cannot wait for the next shuttle pickup. In a related mission in 1994, a payload was left hanging at the end of a 20-kilometer tether to see how long the connection as thick as a kite string would survive collisions with micro meteors and space debris. The expected lifetime of the tether, which could readily be cut by a particle the size of a sand grain traveling at high speed, was a meager 12 days. As things turned out, it was severed after only four.
The experiment demonstrated the need to make tethers out of many lines, separated so that they cannot all be cut by the same particle yet joined periodically so that when one line fails, the others take up the load. With that in mind, the Naval Research Laboratory (NRL) and the National Reconnaissance Office (NRO) fabricated a 2.5-millimeter-diameter hollow braid of Spectra fiber (a high-strength polymer used in fishing lines) loosely packed with yarn. A four-kilometer length linking two satellites that was launched in June 1996 has remained orbiting in space uncut for almost three years. In a follow-up experiment last October, NRL and NRO tested a tether with a different design: a thin plastic tape three centimeters wide with strong fiber strands running along its length. The six-kilometer tether should survive for many years in space, but the tape makes it heavy. Our company, Tethers Unlimited in Clinton, Wash., is working with Culzean Fabrics and Flemings Textiles, both in Kilmarnock, Scotland, to fabricate multi line tethers with an open, fish net like pattern that will weigh less and should last in space for many decades. Other tether demonstrations are scheduled.
The Michigan Technic Corporation in Holland, Mich., has plans in 2000 for a shuttle to release two science packages joined by a two-kilometer tether. In addition, the NASA Marshall Space Flight Center is investigating the use of electrodynamic tethers for propellant less space propulsion. In mid-2000 a mission will demonstrate that a conducting tether can lower the orbit of a Delta 2 upper stage. At Tethers Unlimited, we are developing a commercial version of the NASA concept: a small package that would be attached to a satellite or upper stage before launch. When the spacecraft completed its mission or malfunctioned, the conducting tether would unfurl and drag against Earth’s magnetic field, causing the craft to lose altitude rapidly until it burned up in the upper atmosphere. We will test such a tether de-orbit device in late 2000 on an upper stage built by the Lavochkin Association of Russia. NASA is also considering such electrodynamic tethers for upward propulsion. In the system, solar panels would supply a flow of electricity through the tether to push against Earth’s magnetic field. The resulting force could haul payloads around Earth indefinitely. This approach might be used to keep the International Space Station in orbit without refueling. How far can tethers take humankind in the future? We and others have analyzed a system of rapidly cartwheeling, orbiting tethers up to hundreds of kilometers long for delivering payloads to the moon and ever farther.
The idea is simple—think of Tarzan swinging from one vine to the next. First, a low Earthorbit tether picks up a payload from a reusable launch vehicle and hands the delivery to another tether in a more distant elliptical- Earth orbit. The second tether then tosses the object to the moon, where it is caught by a Lunavator tether in orbit there. The Lunavator would be cartwheeling around the moon at just the right velocity so that, after catching the payload, it could gently deposit the object onto the lunar surface a half rotation later. Simultaneously, the tether could pick up a return load. No propellant would be required if the amount of mass being delivered and picked up were balanced. Such a transportation mechanism could become a highway to the moon that might make frequent lunar travel commonplace. Obviously, there are many technological challenges that must be overcome before such a system becomes a reality, but its potential for opening up an economical expressway in space is tremendous. Perhaps someday there will be numerous cartwheeling tethers around many of the planets and their moons, carrying the hustle and bustle of interplanetary commerce. And it all will have begun with a piece of string.
The experiment demonstrated that such electrodynamic tethers can convert shuttle momentum into kilowatts of electrical power, and vice versa. Unfortunately, a flaw in the insulation allowed a high-power electric arc to jump from the tether to the deployment boom, and the arc burned through the tether. But although the break aborted the electrodynamic part of the project, it inadvertently triggered a spectacular display of momentum transfer. At the time, the Italian satellite was 20 kilometers above the shuttle and was being pulled along faster than the orbital speed for that higher altitude. Consequently, when the tether broke, the excess momentum made the satellite soar to seven times the tether length, or 140 kilometers, above the shuttle.
Other work has had greater success. In 1993, to test an idea proposed by Joseph A. Carroll of Tether Applications in San Diego, a payload attached to a 20-kilometer tether was deployed downward from a large satellite. Because the speed of the payload was then slower than that required for an object at that reduced orbital altitude, cutting the tether at the right moment caused the package to descend toward a predetermined point on Earth’s surface. Tether Applications is now developing a reentry capsule and tether that the International Space Station could use to send urgent deliveries to Earth, including scientific payloads that cannot wait for the next shuttle pickup. In a related mission in 1994, a payload was left hanging at the end of a 20-kilometer tether to see how long the connection as thick as a kite string would survive collisions with micro meteors and space debris. The expected lifetime of the tether, which could readily be cut by a particle the size of a sand grain traveling at high speed, was a meager 12 days. As things turned out, it was severed after only four.
The experiment demonstrated the need to make tethers out of many lines, separated so that they cannot all be cut by the same particle yet joined periodically so that when one line fails, the others take up the load. With that in mind, the Naval Research Laboratory (NRL) and the National Reconnaissance Office (NRO) fabricated a 2.5-millimeter-diameter hollow braid of Spectra fiber (a high-strength polymer used in fishing lines) loosely packed with yarn. A four-kilometer length linking two satellites that was launched in June 1996 has remained orbiting in space uncut for almost three years. In a follow-up experiment last October, NRL and NRO tested a tether with a different design: a thin plastic tape three centimeters wide with strong fiber strands running along its length. The six-kilometer tether should survive for many years in space, but the tape makes it heavy. Our company, Tethers Unlimited in Clinton, Wash., is working with Culzean Fabrics and Flemings Textiles, both in Kilmarnock, Scotland, to fabricate multi line tethers with an open, fish net like pattern that will weigh less and should last in space for many decades. Other tether demonstrations are scheduled.
The Michigan Technic Corporation in Holland, Mich., has plans in 2000 for a shuttle to release two science packages joined by a two-kilometer tether. In addition, the NASA Marshall Space Flight Center is investigating the use of electrodynamic tethers for propellant less space propulsion. In mid-2000 a mission will demonstrate that a conducting tether can lower the orbit of a Delta 2 upper stage. At Tethers Unlimited, we are developing a commercial version of the NASA concept: a small package that would be attached to a satellite or upper stage before launch. When the spacecraft completed its mission or malfunctioned, the conducting tether would unfurl and drag against Earth’s magnetic field, causing the craft to lose altitude rapidly until it burned up in the upper atmosphere. We will test such a tether de-orbit device in late 2000 on an upper stage built by the Lavochkin Association of Russia. NASA is also considering such electrodynamic tethers for upward propulsion. In the system, solar panels would supply a flow of electricity through the tether to push against Earth’s magnetic field. The resulting force could haul payloads around Earth indefinitely. This approach might be used to keep the International Space Station in orbit without refueling. How far can tethers take humankind in the future? We and others have analyzed a system of rapidly cartwheeling, orbiting tethers up to hundreds of kilometers long for delivering payloads to the moon and ever farther.
The idea is simple—think of Tarzan swinging from one vine to the next. First, a low Earthorbit tether picks up a payload from a reusable launch vehicle and hands the delivery to another tether in a more distant elliptical- Earth orbit. The second tether then tosses the object to the moon, where it is caught by a Lunavator tether in orbit there. The Lunavator would be cartwheeling around the moon at just the right velocity so that, after catching the payload, it could gently deposit the object onto the lunar surface a half rotation later. Simultaneously, the tether could pick up a return load. No propellant would be required if the amount of mass being delivered and picked up were balanced. Such a transportation mechanism could become a highway to the moon that might make frequent lunar travel commonplace. Obviously, there are many technological challenges that must be overcome before such a system becomes a reality, but its potential for opening up an economical expressway in space is tremendous. Perhaps someday there will be numerous cartwheeling tethers around many of the planets and their moons, carrying the hustle and bustle of interplanetary commerce. And it all will have begun with a piece of string.
Highways of Light
Today’s spacecraft carry their source of power. The cost of space travel could be drastically reduced by leaving the fuel and massive components behind and beaming high-intensity laser light or microwave energy to the vehicles. Experiments sponsored over the past year by the National Aeronautics and Space Administration and the U.S. Air Force have demonstrated what I call a light craft, which rides along a pulsed infrared laser beam from the ground. Reflective surfaces in the craft focus the beam into a ring, where it heats air to a temperature nearly five times hotter than the surface of the sun, causing the air to expand explosively for thrust.
Using an army 10-kilowatt carbon dioxide laser pulsing 2 times per second, Franklin B. Mead of the U.S. Air Force Research Laboratory and I have successfully propelled spin-stabilized miniature light craft measuring 10 to 15 centimeters(four to six inches) in diameter to altitudes of up to 30 meters (99 feet) in roughly three seconds. We have funding to increase the laser power to 100 kilowatts, which will enable flights up to a 30-kilometer altitude. Although today’s models weigh less than 50 grams (two ounces), our five-year goal is to accelerate a one-kilogram micro satellite into low Earth orbit using a custom-built, one mega watt ground based laser expending just a few hundred dollars’ worth of electricity. Current light craft demonstration vehicles are made of ordinary air craft grade aluminum and consist of a forward aero shell, or covering, an annular (ring-shaped) cowl and an aft part consist ng of an optic and expansion nozzle. During atmospheric flight, the forward section compresses the air and directs it to the engine inlet. The annular cowl takes the brunt of the thrust. The aft section serves as a parabolic collection mirror that concentrates the infrared laser light into an annular focus, while providing another surface against which the hot-air exhaust can press. The design offers automatic steering: if the craft starts to move outside the beam, the thrust inclines and pushes the vehicle back. A one-kilogram light craft will accelerate this way to about Mach 5 and reach 30 kilometers’ altitude, then switch to on board liquid hydrogen for propellant as air becomes scarce. One kilogram of hydrogen should suffice to take the craft to orbit. Aversion 1.4 meters in diameter should be able to orbit micro satellites of up to 100 kilograms by riding a 100-megawatt laser beam.
Because the beams we use are pulsed, this power might be achieved fairly easily by combining the output from a group of lasers. Such lasers could launch communications satellites and deorbit them when their electronics become obsolete. Light craft with different geometries can move toward their energy source rather than away from it—or even sideways. These variant vehicles have potential for moving cargo economically around the planet. Light craft could also be powered by microwaves. Microwaves cannot achieve such high power densities as lasers, so the vehicles would have to be larger. But microwave sources are considerably less expensive and easier to scale to very high powers. I have also designed more sophisticated beamed energy craft, operating on a different principle, that could transport passengers. These craft would be better for carrying larger cargoes because they can produce thrust more efficiently.A mirror in the craft focuses some of the incoming beamed energy at a point one vehicle-diameter ahead of the vehicle.
The intense heat creates an “air spike” that diverts oncoming air past the vehicle, decreasing drag and reducing the heating of the craft. This craft taps some additional beamed energy to generate powerful electric fields around the rim, which ionizes air. It also uses superconducting magnets to create strong magnetic fields in that region. When ionized air moves through electric and magnetic fields in this configuration, magneto hydrodynamic forces come into play that accelerate the slipstream to create thrust. By varying the amount of energy it reflects forward, the light craft can control the airflow around the vehicle. I demonstrated reduction of drag by an air spike in April 1995 in a hypersonic shock tunnel at Rensselaer Polytechnic Institute, though with an electrically heated plasma torch rather than with laser power. Tests aimed at generating magneto hydrodynamic thrust, using a 15- centimeter-diameter device, have just begun. A person-size light craft of this type driven by microwaves or by a 1,000- megawatt pulsed laser should be able to operate at altitudes up to 50 kilometers and to accelerate easily to orbital velocities.
Light craft could revolutionize transportation if they are driven from orbiting solar-power stations. But the cost of assembling the orbital infrastructure eventually must be reduced below a few hundred dollars per kilogram. It now costs about$20,000 to put a kilogram of payload in orbit by means of the space shuttle, about 100 times too much.I think we can bridge the gap by making the first orbital power station one that is specialized for enabling cheap access to space. Imagine a one-kilometer-diameter structure built like a giant bicycle wheel and orbiting at
an altitude of 500 kilometers. Its mass would be about 1,010 metric tons, and it would slowly spin to gain gyroscopic stability. Besides the structural “spokes,” the wheel would have a disk made from 55 large, pie-slice segments of 0.32-millimeter-thick silicon carbide. Completely covering one side of the silicon carbide would be 30 percent efficient, thin-film solar photo voltaic cells capable of supplying 320 megawatts of electricity. (Such devices are expected within a decade.) On the other side would be 13.2 billion miniature solid-state transmitters, each just 8.5 millimeters across and delivering 1.5 watts of microwave power. Today’s heavy-lift chemical rockets could loft this entire structure over about 55 launches, at an affordable cost of perhaps $5.5 billion.
The station would be ringed by an energy storage device consisting of two superconducting cables, each with a mass of 100 metric tons, that could be charged up with counter flowing electric currents. (This arrangement would eliminate the titanic magnetic torque that would be produced by a single cable.) During two orbits of Earth, the station would completely charge this system with 1,800 gigajoules of energy. It would then beam down 4.3 gigawatts of microwave power onto a light craft at a range of about 1,170 kilometers.Torquing forces produced by shifting small amounts of current from one cable to the other would crudely point the power station, but fine control would come from a beacon mounted on the light craft. It would send a signal that would coordinate the individual transmitters on the power station to create a spot 10 meters in diameter at the launch site. The vehicle could reach orbit in less than five minutes, subjecting occupants to no more than three g’s of acceleration, about the same that shuttle astronauts experience. Or the solar-power station could unload all its energy in a 54-second burst that should offer a nearly vertical 20-g boost to geostationary orbit or even to escape velocity.The first orbital solar-power station will pave the way for a whole industry of orbital stations, launched and assembled from specialized lightcraft. Within decades, a fleet of these will make feasible rapid, low-cost travel around the globe, to the moon and beyond.
Using an army 10-kilowatt carbon dioxide laser pulsing 2 times per second, Franklin B. Mead of the U.S. Air Force Research Laboratory and I have successfully propelled spin-stabilized miniature light craft measuring 10 to 15 centimeters(four to six inches) in diameter to altitudes of up to 30 meters (99 feet) in roughly three seconds. We have funding to increase the laser power to 100 kilowatts, which will enable flights up to a 30-kilometer altitude. Although today’s models weigh less than 50 grams (two ounces), our five-year goal is to accelerate a one-kilogram micro satellite into low Earth orbit using a custom-built, one mega watt ground based laser expending just a few hundred dollars’ worth of electricity. Current light craft demonstration vehicles are made of ordinary air craft grade aluminum and consist of a forward aero shell, or covering, an annular (ring-shaped) cowl and an aft part consist ng of an optic and expansion nozzle. During atmospheric flight, the forward section compresses the air and directs it to the engine inlet. The annular cowl takes the brunt of the thrust. The aft section serves as a parabolic collection mirror that concentrates the infrared laser light into an annular focus, while providing another surface against which the hot-air exhaust can press. The design offers automatic steering: if the craft starts to move outside the beam, the thrust inclines and pushes the vehicle back. A one-kilogram light craft will accelerate this way to about Mach 5 and reach 30 kilometers’ altitude, then switch to on board liquid hydrogen for propellant as air becomes scarce. One kilogram of hydrogen should suffice to take the craft to orbit. Aversion 1.4 meters in diameter should be able to orbit micro satellites of up to 100 kilograms by riding a 100-megawatt laser beam.
Because the beams we use are pulsed, this power might be achieved fairly easily by combining the output from a group of lasers. Such lasers could launch communications satellites and deorbit them when their electronics become obsolete. Light craft with different geometries can move toward their energy source rather than away from it—or even sideways. These variant vehicles have potential for moving cargo economically around the planet. Light craft could also be powered by microwaves. Microwaves cannot achieve such high power densities as lasers, so the vehicles would have to be larger. But microwave sources are considerably less expensive and easier to scale to very high powers. I have also designed more sophisticated beamed energy craft, operating on a different principle, that could transport passengers. These craft would be better for carrying larger cargoes because they can produce thrust more efficiently.A mirror in the craft focuses some of the incoming beamed energy at a point one vehicle-diameter ahead of the vehicle.
The intense heat creates an “air spike” that diverts oncoming air past the vehicle, decreasing drag and reducing the heating of the craft. This craft taps some additional beamed energy to generate powerful electric fields around the rim, which ionizes air. It also uses superconducting magnets to create strong magnetic fields in that region. When ionized air moves through electric and magnetic fields in this configuration, magneto hydrodynamic forces come into play that accelerate the slipstream to create thrust. By varying the amount of energy it reflects forward, the light craft can control the airflow around the vehicle. I demonstrated reduction of drag by an air spike in April 1995 in a hypersonic shock tunnel at Rensselaer Polytechnic Institute, though with an electrically heated plasma torch rather than with laser power. Tests aimed at generating magneto hydrodynamic thrust, using a 15- centimeter-diameter device, have just begun. A person-size light craft of this type driven by microwaves or by a 1,000- megawatt pulsed laser should be able to operate at altitudes up to 50 kilometers and to accelerate easily to orbital velocities.
Light craft could revolutionize transportation if they are driven from orbiting solar-power stations. But the cost of assembling the orbital infrastructure eventually must be reduced below a few hundred dollars per kilogram. It now costs about$20,000 to put a kilogram of payload in orbit by means of the space shuttle, about 100 times too much.I think we can bridge the gap by making the first orbital power station one that is specialized for enabling cheap access to space. Imagine a one-kilometer-diameter structure built like a giant bicycle wheel and orbiting at
an altitude of 500 kilometers. Its mass would be about 1,010 metric tons, and it would slowly spin to gain gyroscopic stability. Besides the structural “spokes,” the wheel would have a disk made from 55 large, pie-slice segments of 0.32-millimeter-thick silicon carbide. Completely covering one side of the silicon carbide would be 30 percent efficient, thin-film solar photo voltaic cells capable of supplying 320 megawatts of electricity. (Such devices are expected within a decade.) On the other side would be 13.2 billion miniature solid-state transmitters, each just 8.5 millimeters across and delivering 1.5 watts of microwave power. Today’s heavy-lift chemical rockets could loft this entire structure over about 55 launches, at an affordable cost of perhaps $5.5 billion.
The station would be ringed by an energy storage device consisting of two superconducting cables, each with a mass of 100 metric tons, that could be charged up with counter flowing electric currents. (This arrangement would eliminate the titanic magnetic torque that would be produced by a single cable.) During two orbits of Earth, the station would completely charge this system with 1,800 gigajoules of energy. It would then beam down 4.3 gigawatts of microwave power onto a light craft at a range of about 1,170 kilometers.Torquing forces produced by shifting small amounts of current from one cable to the other would crudely point the power station, but fine control would come from a beacon mounted on the light craft. It would send a signal that would coordinate the individual transmitters on the power station to create a spot 10 meters in diameter at the launch site. The vehicle could reach orbit in less than five minutes, subjecting occupants to no more than three g’s of acceleration, about the same that shuttle astronauts experience. Or the solar-power station could unload all its energy in a 54-second burst that should offer a nearly vertical 20-g boost to geostationary orbit or even to escape velocity.The first orbital solar-power station will pave the way for a whole industry of orbital stations, launched and assembled from specialized lightcraft. Within decades, a fleet of these will make feasible rapid, low-cost travel around the globe, to the moon and beyond.
Compact Nuclear Rockets
Someday, in exploring the outer planets of our solar system, humankind will want to do more than send diminutive probes that merely fly rapidly by them. In time, we will want to send spacecraft that go into orbit around these gaseous giants, land robots on their moons and even return rock and soil samples back to Earth. Eventually, we will want to send astronauts to their intriguing moons, on at least a couple of which liquid water the fundamental requirement for life as we know it is believed to be abundant. For missions such as these, we will need rockets powered by nuclear fission rather than chemical combustion. Chemical rockets have served us well. But the relatively low amount of energy that they can deliver for a given mass of fuel imposes severe restrictions on spacecraft. To reach the outer planets, for example, a chemically powered space vehicle must have very limited mass and make extensive use of planetary gravitational “assists,” in which the craft maneuvers close enough to a planet for the planet’s gravitational field to act like a slingshot, boosting the speed of the craft. To take advantage of these assists, mission planners must wait for “windows” short periods within which a craft can be launched toward planets appropriately positioned to speed it on its way to ore distant bodies.
In technical terms, chemical rockets have a low maximum velocity increment, which means that their exhaust velocities are not high enough to impart very high speeds to the rocket. The best chemical rockets, which are based on the reaction between hydrogen and oxygen, impart a maximum velocity increment of about 10 kilometers (six miles) a second to spacecraft departing from Earth orbit. Nuclear rockets, in contrast, could impart a maximum velocity increment of upto about 22 kilometers a second. Such a high value would make possible a direct path to, say, Saturn, reducing travel time from about seven years to as little as three. A nuclear rocket such as this would be inherently safe and environmentally benign: contrary to popular belief, a nuclear rocket need not be strongly radioactive when launched.
The spacecraft, with its nuclear thrusters, would be launched as a payload atop a conventional chemical rocket. Then, once the payload was in high Earth orbit, above about 800 kilometers, the nuclear reactor would start up. The technology required to build a rocket motor powered by nuclear fission is not far beyond current capabilities. In fact, my colleagues and I have designed a compact nuclear rocket engine, which we call Mitee (deriving the letters loosely from the words “miniature reactor engine”), that could be built in about six or seven years at a cost of $600 million to $800 million actually quite modest in the context of space launches. In fact, the costs of developing the engine would be offset by savings in future launch costs. The reason is that nuclear spacecraft powered by the engine would not need to haul along a large mass of chemical propellant, meaning that launching it would not require a Titan IV vehicle costing $250 million to $325 million. Instead a lower-priced rocket, such as a Delta or an Atlas in the range of $50 million to $125 million, could be used. In our design, the reactor’s nuclear fuel would be in the form of perforated metal sheets in an annular roll, in a configuration similar to a jelly roll with a hollow center. A jacket of lithium 7 hydride around the outside of the fuel roll would act as a moderator, reducing the speed of the neutrons emitted by the nuclear fission occurring inside the fuel.
The coolant liquid hydrogen would flow from the outside of the roll inward, quickly turning into a gas as it heated up and flowed toward the center. The superheated gas, at about 2,700 degrees Celsius (4,900 degrees Fahrenheit), would flow at a high velocity along a channel at the center axis of the roll and then out through a small nozzle at the end. A key attraction of nuclear propulsion is that its propellant hydrogen is widely available in gaseous form in the giant planets of the outer solar system and in the water ice of distant moons and planets. Thus,because the nuclear fuel would be relatively long-lasting, a nuclear-powered craft could in theory tour the outer solar system for 10 or 15 years, replenishing its hydrogen propellant as necessary. A vehicle could fly for months in the atmospheres of Jupiter, Saturn, Uranus and Neptune, gathering detailed data on their composition, weather patterns and other characteristics. Alternatively, a craft could fly to Europa, Pluto or Titan to collect rock samples and also accumulate hydrogen, by electrolyzing water from melted ice, for the trip back to Earth. Because its reactor would start up well away from Earth, a nuclear-powered spacecraft could actually be made safer than some deep-space probes that are powered by
chemical thrusters.
In the outer reaches of the solar system, the sun’s rays are too feeble to provide energy for a spacecraft’s instruments. So they generally run on plutonium 238 power sources, which are highly radioac-tive even during launch. In a probe with nuclear thrusters, on the other hand, the instruments would be run off the same reactor that provides thrust. Moreover, the amount of radioactive waste produced would be negligible—amounting to about a gram of fission products for a deep-space mission and in any event the material would never come back to Earth. Nuclear rockets are not new. Among the U.S. Department of Defense’s projects in this area was the Space Nuclear Thermal Propulsion program in the late 1980s. Its goal was to develop a compact, lightweight nuclear engine for defense applications, such as launching heavy payloads into high-Earth orbit. The cornerstone of the design was a particle bed reactor (PBR), in which the fuel consisted of small, packed particles of uranium carbide coated with zirconium carbide. Although the PBR work ended before a full-scale nuclear engine was built, engineers did successfully build and operate low-power reactors based on the concept and demonstrated that high-power densities could be achieved. Indeed, our Mitee engine owes much to the PBR effort, on which my colleagues and I worked for nearly a decade at Brookhaven National Laboratory.
In addition to the same basic annular configuration of fuel elements, the Mitee also would use lightweight, thermally stable lithium 7 hydride as a moderator. To be conservative, however, we designed the Mitee’s fuel assembly to have a power density of about 10 megawatts per liter instead of the PBR’s 30. It is an easily provable fact that with only chemical rockets, our ability to explore the outer planets and their moons is meager. In the near term, only nuclear rockets could give us the kind of power, reliability and flexibility that we would need to improve dramatically our understanding of the still largely mysterious worlds at the far edges of our solar system.
In technical terms, chemical rockets have a low maximum velocity increment, which means that their exhaust velocities are not high enough to impart very high speeds to the rocket. The best chemical rockets, which are based on the reaction between hydrogen and oxygen, impart a maximum velocity increment of about 10 kilometers (six miles) a second to spacecraft departing from Earth orbit. Nuclear rockets, in contrast, could impart a maximum velocity increment of upto about 22 kilometers a second. Such a high value would make possible a direct path to, say, Saturn, reducing travel time from about seven years to as little as three. A nuclear rocket such as this would be inherently safe and environmentally benign: contrary to popular belief, a nuclear rocket need not be strongly radioactive when launched.
The spacecraft, with its nuclear thrusters, would be launched as a payload atop a conventional chemical rocket. Then, once the payload was in high Earth orbit, above about 800 kilometers, the nuclear reactor would start up. The technology required to build a rocket motor powered by nuclear fission is not far beyond current capabilities. In fact, my colleagues and I have designed a compact nuclear rocket engine, which we call Mitee (deriving the letters loosely from the words “miniature reactor engine”), that could be built in about six or seven years at a cost of $600 million to $800 million actually quite modest in the context of space launches. In fact, the costs of developing the engine would be offset by savings in future launch costs. The reason is that nuclear spacecraft powered by the engine would not need to haul along a large mass of chemical propellant, meaning that launching it would not require a Titan IV vehicle costing $250 million to $325 million. Instead a lower-priced rocket, such as a Delta or an Atlas in the range of $50 million to $125 million, could be used. In our design, the reactor’s nuclear fuel would be in the form of perforated metal sheets in an annular roll, in a configuration similar to a jelly roll with a hollow center. A jacket of lithium 7 hydride around the outside of the fuel roll would act as a moderator, reducing the speed of the neutrons emitted by the nuclear fission occurring inside the fuel.
The coolant liquid hydrogen would flow from the outside of the roll inward, quickly turning into a gas as it heated up and flowed toward the center. The superheated gas, at about 2,700 degrees Celsius (4,900 degrees Fahrenheit), would flow at a high velocity along a channel at the center axis of the roll and then out through a small nozzle at the end. A key attraction of nuclear propulsion is that its propellant hydrogen is widely available in gaseous form in the giant planets of the outer solar system and in the water ice of distant moons and planets. Thus,because the nuclear fuel would be relatively long-lasting, a nuclear-powered craft could in theory tour the outer solar system for 10 or 15 years, replenishing its hydrogen propellant as necessary. A vehicle could fly for months in the atmospheres of Jupiter, Saturn, Uranus and Neptune, gathering detailed data on their composition, weather patterns and other characteristics. Alternatively, a craft could fly to Europa, Pluto or Titan to collect rock samples and also accumulate hydrogen, by electrolyzing water from melted ice, for the trip back to Earth. Because its reactor would start up well away from Earth, a nuclear-powered spacecraft could actually be made safer than some deep-space probes that are powered by
chemical thrusters.
In the outer reaches of the solar system, the sun’s rays are too feeble to provide energy for a spacecraft’s instruments. So they generally run on plutonium 238 power sources, which are highly radioac-tive even during launch. In a probe with nuclear thrusters, on the other hand, the instruments would be run off the same reactor that provides thrust. Moreover, the amount of radioactive waste produced would be negligible—amounting to about a gram of fission products for a deep-space mission and in any event the material would never come back to Earth. Nuclear rockets are not new. Among the U.S. Department of Defense’s projects in this area was the Space Nuclear Thermal Propulsion program in the late 1980s. Its goal was to develop a compact, lightweight nuclear engine for defense applications, such as launching heavy payloads into high-Earth orbit. The cornerstone of the design was a particle bed reactor (PBR), in which the fuel consisted of small, packed particles of uranium carbide coated with zirconium carbide. Although the PBR work ended before a full-scale nuclear engine was built, engineers did successfully build and operate low-power reactors based on the concept and demonstrated that high-power densities could be achieved. Indeed, our Mitee engine owes much to the PBR effort, on which my colleagues and I worked for nearly a decade at Brookhaven National Laboratory.
In addition to the same basic annular configuration of fuel elements, the Mitee also would use lightweight, thermally stable lithium 7 hydride as a moderator. To be conservative, however, we designed the Mitee’s fuel assembly to have a power density of about 10 megawatts per liter instead of the PBR’s 30. It is an easily provable fact that with only chemical rockets, our ability to explore the outer planets and their moons is meager. In the near term, only nuclear rockets could give us the kind of power, reliability and flexibility that we would need to improve dramatically our understanding of the still largely mysterious worlds at the far edges of our solar system.
Reaching for the Stars
ANTIMATTER-POWERED interstellar craft would put some distance between the payload and
the power plant. Ring is part of the magnetic nozzle that would direct charged particles to create thrust.
The notion of traveling to the stars is a concept compelling enough to recur in countless cultural artifacts, from Roman poetry to 20th-century popular music. So ingrained has the concept become that when novelists, poets or lyricists write of reaching for the stars, it is instantly understood as a kind of cultural shorthand for striving for the unattainable. Although interstellar travel remains a glorious if futuristic dream, a small group of engineers and scientists is already exploring concepts and conducting experiments that may lead to technologies capable of propelling spacecraft to speeds high enough to travel far beyond the edge of our solar system. A propulsion system based on nuclear fusion could carry humans to the outer planets and could propel robotic spacecraft thousands of astronomical units into interstellar space (an astronomical unit, at 150 million kilometers, or 93 million miles, is the average distance from Earth to the sun). Such a system might be built in the next several decades. Eventually, even more powerful engines fueled by the mutual annihilation of matter and antimatter might carry spacecraft to nearby stars, the closest of which is Proxima Centauri, some 270,000 astronomical units distant.
The attraction of these exotic modes of propulsion lies in the fantastic amounts of energy they could release from a given mass of fuel. A fusion-based propulsion system, for example, could in theory produce about 100 trillion joules per kilogram of fuel—an energy density that is more than 10 million times greater than the corresponding figure for the chemical rockets that propel today’s spacecraft. Matter-antimatter reactions would be even more difficult to exploit but would be capable of generating an astounding 20 quadrillion joules from a single kilogram of fuel—enough to supply the entire energy needs of the world for about 26 minutes.
In nuclear fusion, very light atoms are brought together at temperatures and pressures high enough, and for long enough, to fuse them into more massive atoms. The difference in mass between the reactants and the products of the reaction corresponds to the amount of energy released, according to Albert Einstein’s famous formula E = mc2 . The obstacles to exploiting fusion, much less antimatter, are daunting. Controlled fusion concepts, whether for rocket propulsion or terrestrial power generation, can be divided into two general classes. These categories indicate the technique used to confine the extremely hot, electrically charged gas, called a plasma, within which fusion occurs. In magnetic confinement fusion,
strong magnetic fields contain the plasma. Inertial confinement fusion, on the other hand, relies on laser or ion beams to heat and compress a tiny pellet of fusion fuel. In November 1997 researchers exploiting the magnetic confinement approach created a fusion reaction that produced 65
percent as much energy as was fed into it to initiate the reaction.
This milestone was achieved in England at the Joint European Torus, a tokamak facility—a doughnut-shaped vessel in which the plasma is magnetically confined. A commercial fusion reactor would have to produce far more energy than went into it to start or maintain the reaction. But even if commercial fusion power becomes a reality here on Earth, there will be several problems unique to developing fusion rockets. A key one will be directing the energetic charged particles created by the reaction to produce usable thrust. Other important challenges include acquiring and storing enough fusion fuel and maximizing the amount of power produced in relation to the mass of the spacecraft. Since the late 1950s, scientists have proposed dozens of fusion rocket concepts.
Although fusion produces enormous amounts of very energetic particles, the reaction will accelerate a spacecraft only if these particles can be directed so as to produce thrust. In fusion systems based on magnetic confinement, the strategy would be to feed in fuel to sustain the reaction while allowing a portion of the plasma to escape to generate thrust. Because the plasma would destroy any material vessel it touched, strong magnetic fields, generated by an assembly that researchers call a magnetic nozzle, would direct the charged particles out of the rocket. In an engine based on the inertial confinement approach, high power lasers or ion beams would ignite tiny fusion fuel capsules at a rate of perhaps 30 per second. A magnetic nozzle might also suffice to direct the plasma out of the engine to create thrust. The particles created in a fusion reaction depend on the fuels used.
The easiest reaction to initiate is between deuterium and tritium,two heavy isotopes of hydrogen whose atomic nuclei include one and two neutrons, respectively, besides a proton. The reaction products are neutrons and helium nuclei (also known as alpha particles). For thrust, the positively charged alpha particles are desirable, whereas the neutrons are not. Neutrons cannot be directed; they carry no charge. Their kinetic energy can be harnessed for propulsion, but not directly to do so would involve stopping the min a material and making use of the heat generated by their capture. Neutron radiation also poses a danger to a human crew and would necessitate a large amount of shielding for piloted missions. These facts lead to a key difficulty in fusion fuel selection. Although it is easiest to initiate fusion between deuterium and tritium, for many propulsion concepts it would be more desirable to use deuterium and the isotope helium 3 (two protons, one neutron). Fusion of these nuclei produces an alpha particle and a proton, both of which can be manipulated by magnetic fields. The problem is that helium 3 is exceedingly rare on Earth. In addition, the deuterium–helium 3 reaction is more difficult to ignite than the deuterium-tritium reaction. But regardless of the fusion fuel selected, a spacecraft of thousands of tons much of it fuel would be necessary to carry humans to the outer reaches of the solar system or deep into interstellar space (for comparison, the International Space Station will have a mass of about 500 tons).
The attraction of these exotic modes of propulsion lies in the fantastic amounts of energy they could release from a given mass of fuel. A fusion-based propulsion system, for example, could in theory produce about 100 trillion joules per kilogram of fuel—an energy density that is more than 10 million times greater than the corresponding figure for the chemical rockets that propel today’s spacecraft. Matter-antimatter reactions would be even more difficult to exploit but would be capable of generating an astounding 20 quadrillion joules from a single kilogram of fuel—enough to supply the entire energy needs of the world for about 26 minutes.
In nuclear fusion, very light atoms are brought together at temperatures and pressures high enough, and for long enough, to fuse them into more massive atoms. The difference in mass between the reactants and the products of the reaction corresponds to the amount of energy released, according to Albert Einstein’s famous formula E = mc2 . The obstacles to exploiting fusion, much less antimatter, are daunting. Controlled fusion concepts, whether for rocket propulsion or terrestrial power generation, can be divided into two general classes. These categories indicate the technique used to confine the extremely hot, electrically charged gas, called a plasma, within which fusion occurs. In magnetic confinement fusion,
strong magnetic fields contain the plasma. Inertial confinement fusion, on the other hand, relies on laser or ion beams to heat and compress a tiny pellet of fusion fuel. In November 1997 researchers exploiting the magnetic confinement approach created a fusion reaction that produced 65
percent as much energy as was fed into it to initiate the reaction.
This milestone was achieved in England at the Joint European Torus, a tokamak facility—a doughnut-shaped vessel in which the plasma is magnetically confined. A commercial fusion reactor would have to produce far more energy than went into it to start or maintain the reaction. But even if commercial fusion power becomes a reality here on Earth, there will be several problems unique to developing fusion rockets. A key one will be directing the energetic charged particles created by the reaction to produce usable thrust. Other important challenges include acquiring and storing enough fusion fuel and maximizing the amount of power produced in relation to the mass of the spacecraft. Since the late 1950s, scientists have proposed dozens of fusion rocket concepts.
Although fusion produces enormous amounts of very energetic particles, the reaction will accelerate a spacecraft only if these particles can be directed so as to produce thrust. In fusion systems based on magnetic confinement, the strategy would be to feed in fuel to sustain the reaction while allowing a portion of the plasma to escape to generate thrust. Because the plasma would destroy any material vessel it touched, strong magnetic fields, generated by an assembly that researchers call a magnetic nozzle, would direct the charged particles out of the rocket. In an engine based on the inertial confinement approach, high power lasers or ion beams would ignite tiny fusion fuel capsules at a rate of perhaps 30 per second. A magnetic nozzle might also suffice to direct the plasma out of the engine to create thrust. The particles created in a fusion reaction depend on the fuels used.
The easiest reaction to initiate is between deuterium and tritium,two heavy isotopes of hydrogen whose atomic nuclei include one and two neutrons, respectively, besides a proton. The reaction products are neutrons and helium nuclei (also known as alpha particles). For thrust, the positively charged alpha particles are desirable, whereas the neutrons are not. Neutrons cannot be directed; they carry no charge. Their kinetic energy can be harnessed for propulsion, but not directly to do so would involve stopping the min a material and making use of the heat generated by their capture. Neutron radiation also poses a danger to a human crew and would necessitate a large amount of shielding for piloted missions. These facts lead to a key difficulty in fusion fuel selection. Although it is easiest to initiate fusion between deuterium and tritium, for many propulsion concepts it would be more desirable to use deuterium and the isotope helium 3 (two protons, one neutron). Fusion of these nuclei produces an alpha particle and a proton, both of which can be manipulated by magnetic fields. The problem is that helium 3 is exceedingly rare on Earth. In addition, the deuterium–helium 3 reaction is more difficult to ignite than the deuterium-tritium reaction. But regardless of the fusion fuel selected, a spacecraft of thousands of tons much of it fuel would be necessary to carry humans to the outer reaches of the solar system or deep into interstellar space (for comparison, the International Space Station will have a mass of about 500 tons).
The Best Targets for Future Exploration
The Sun
Upcoming missions will investigate the sun and the powerful solar wind that it hurls toward the planets Like an ill-tempered king, the sun is prone to violent outbursts. Shifts in the sun’s intense magnetic fields send monstrous streams of charged particles hurtling through space. This solar wind buffets the planets and sparks the aurora borealis in Earth’s Northern Hemisphere. Occasional surges in the solar wind can also silence communications satellites and cause power blackouts on Earth. In the next decade, space agencies in the U.S., Europe and Asia expect to launch a small fleet of spacecraft to study the sun and its fierce flare-ups. One of those probes will even venture into the corona, the sun’s fiery outer atmosphere. Recent solar missions have paved the way.
For the past three years, the Solar and Heliospheric Observatory (SOHO) has provided breathtaking images of the sun and its corona. And the Ulysses probe has measured the solar wind and the sun’s magnetic field while moving in a distant orbit that allows it to view the sun’s north and south poles. These missions suggest that the fastest solar winds, flowing at up to 800 kilometers (500 miles) per second, may arise all over the sun’s surface and not just from its poles, as astronomers had previously thought. But scientists still don’t understand the physical processes that produce the solar wind, and they cannot predict the occurrence of the solar storms that wreak such havoc on Earth. In 2001 NASA plans to launch Genesis, a spacecraft that will collect solar-wind particles from a near-Earth orbit. After a three-year mission, the probe will return the samples to Earth, where scientists can measure the abundance of various elements and isotopes. Russia, Japan and Germany are also developing spacecraft that will study thesun from a variety of vantage points. But the most ambitious mission is NASA’s Solar Probe, scheduled for launch in 2007. This spacecraft will go into an eccentric orbit that in 2010 will send it through the corona, less than three million kilometers from the sun’s surface about one-twentieth the distance between the sun and Mercury. During its first flyby of the sun, 14 hours from pole to pole, Solar Probe’s heat shields will have to withstand temperatures of up to 2,000 degrees Celsius (3,600 degrees Fahrenheit). The spacecraft will measure the sun’s magnetic fields and take high-resolution photographs of the sun’s surface. The probe will also carry several spectrometers and an instrument to measure the sun’s plasma waves. “It’s the first mission to a star our star,” says Bruce Tsurutani, Solar Probe project scientist at the Jet Propulsion Laboratory in Pasadena, Calif.
The spacecraft will return for a second flyby in 2015, when it will speed through the coronal holes where the fastest solar winds appear to originate. Scientists hope the spacecraft will help explain how the solar windis accelerated to such incredible speeds. The mission may also illuminate the most puzzling paradox of solar physics: why the sun’s outer atmosphere is hundreds of times hotter than the sun’s surface. And David Hathaway, head of solar physics at the NASA Marshall Space Flight Center, says the new data may help scientists forecast potentially damaging solar storms. “These scientific mysteries aren’t just intellectual curiosities,” Hathaway remarks.
For the past three years, the Solar and Heliospheric Observatory (SOHO) has provided breathtaking images of the sun and its corona. And the Ulysses probe has measured the solar wind and the sun’s magnetic field while moving in a distant orbit that allows it to view the sun’s north and south poles. These missions suggest that the fastest solar winds, flowing at up to 800 kilometers (500 miles) per second, may arise all over the sun’s surface and not just from its poles, as astronomers had previously thought. But scientists still don’t understand the physical processes that produce the solar wind, and they cannot predict the occurrence of the solar storms that wreak such havoc on Earth. In 2001 NASA plans to launch Genesis, a spacecraft that will collect solar-wind particles from a near-Earth orbit. After a three-year mission, the probe will return the samples to Earth, where scientists can measure the abundance of various elements and isotopes. Russia, Japan and Germany are also developing spacecraft that will study thesun from a variety of vantage points. But the most ambitious mission is NASA’s Solar Probe, scheduled for launch in 2007. This spacecraft will go into an eccentric orbit that in 2010 will send it through the corona, less than three million kilometers from the sun’s surface about one-twentieth the distance between the sun and Mercury. During its first flyby of the sun, 14 hours from pole to pole, Solar Probe’s heat shields will have to withstand temperatures of up to 2,000 degrees Celsius (3,600 degrees Fahrenheit). The spacecraft will measure the sun’s magnetic fields and take high-resolution photographs of the sun’s surface. The probe will also carry several spectrometers and an instrument to measure the sun’s plasma waves. “It’s the first mission to a star our star,” says Bruce Tsurutani, Solar Probe project scientist at the Jet Propulsion Laboratory in Pasadena, Calif.
The spacecraft will return for a second flyby in 2015, when it will speed through the coronal holes where the fastest solar winds appear to originate. Scientists hope the spacecraft will help explain how the solar windis accelerated to such incredible speeds. The mission may also illuminate the most puzzling paradox of solar physics: why the sun’s outer atmosphere is hundreds of times hotter than the sun’s surface. And David Hathaway, head of solar physics at the NASA Marshall Space Flight Center, says the new data may help scientists forecast potentially damaging solar storms. “These scientific mysteries aren’t just intellectual curiosities,” Hathaway remarks.
Venus and Mercury
Venus provides a good example of the horrific effects of runaway global warming. The planet is a hellish place, with a carbon dioxide choked atmosphere, clouds of sulfuric acid and a surface hot enough to melt lead. But planetary scientists believe that Venus started out much like Earth and simply evolved differently, like a twin gone bad. Venus offers researchers a unique opportunity to compare the planet with Earth and perhaps discover why the histories of the two bodies diverged. In 2002 a proposed mission called the Venus Sounder for Planetary Exploration (VESPER) may travel to Earth’s closest neighbor, following the trail blazed by the Mariner, Pioneer and Magellan spacecraft. VESPER is expected to orbit Venus for two and a half years, measuring atmospheric gases, wind speeds, air pressure and temperature in short, recording the planet’s weather. Mounted on a three-axis platform, VESPER’s spectrometers, cameras and other instruments will pivot their fields of view to study Venus’s environment from every angle. VESPER will focus its instruments on Venus’s middle atmosphere, 60 to 120 kilometers above the surface. It is here that yellow clouds of sulfuric acid form, causing the greenhouse effect that heats up the planet. Gordon Chin, VESPER’s principal investigator at the NASA Goddard Space Flight Center, says the spacecraft could help scientists understand how to prevent such disastrous global warming on Earth. “For that, Venus is a wonderful laboratory,” Chin observes. Mercury, the planet closest to the sun, also intrigues scientists. It is the second densest planet in the solar system, next to Earth, and contains a much higher proportion of iron than any other planet or satellite does. Astronomers have developed several hypotheses to explain Mercury’s unusual density. Some scientists speculate that early in the solar system’s history, the sun vaporized the outer part of the planet, leaving only the metallic core intact. Others believe that a comet or asteroid impact may have blasted away Mercury’s outer crust and mantle.
Only one spacecraft has ever visited Mercury: Mariner 10, which flew by the planet three times in 1974 and 1975. But NASA is now considering the Mercury Surface, Space Environment, Geochemistry and Ranging mission (MESSENGER), which is scheduled for launch in 2004. After flying by Venus and Mercury twice, the 300-kilogram spacecraft would go into orbit around Mercury in 2009. For the next year, MESSENGER would use its instruments including an imaging system, a magnetometer and four spectrometers to gather data on Mercury’s surface features, magneticfield and tenuous atmosphere. Because Mercury is so close to the sun about one third as far from it as Earth MESSENGER will carry a huge sunshade to protect the spacecraft’s instruments from the intense solar radiation. Scientists hope that the probe can solve the mystery of Mercury’s geologic past by determining the abundance of elements in the planet’s crust. “It’s just one example of the formation and evolution questions we can ask about terrestrial planets in the inner solar system,” explains Sean Solomon, the Carnegie Institution of Washington geophysicist who is the mission’s principal investigator. “And like so many questions, this one can only be answered in space.”
Only one spacecraft has ever visited Mercury: Mariner 10, which flew by the planet three times in 1974 and 1975. But NASA is now considering the Mercury Surface, Space Environment, Geochemistry and Ranging mission (MESSENGER), which is scheduled for launch in 2004. After flying by Venus and Mercury twice, the 300-kilogram spacecraft would go into orbit around Mercury in 2009. For the next year, MESSENGER would use its instruments including an imaging system, a magnetometer and four spectrometers to gather data on Mercury’s surface features, magneticfield and tenuous atmosphere. Because Mercury is so close to the sun about one third as far from it as Earth MESSENGER will carry a huge sunshade to protect the spacecraft’s instruments from the intense solar radiation. Scientists hope that the probe can solve the mystery of Mercury’s geologic past by determining the abundance of elements in the planet’s crust. “It’s just one example of the formation and evolution questions we can ask about terrestrial planets in the inner solar system,” explains Sean Solomon, the Carnegie Institution of Washington geophysicist who is the mission’s principal investigator. “And like so many questions, this one can only be answered in space.”
Europa
Europa is no ordinary moon. The surface of Jupiter’s fourth-largest satellite is sheathed with a layer of scarred and fractured ice. Many scientists believe that at one point in Europa’s past and possibly still today a briny ocean roiled under the ice pack. If still present, the ocean could be the first found on another world. It could even be home to extraterrestrial life, which might thrive near undersea volcanic vents. In 1979 NASA’s Voyager 1 probe first glimpsed Europa’s craggy surface. Over the past four years, the Galileo spacecraft has repeatedly flown by Europa during its orbits around Jupiter and transmitted clearer images of the moon’s icy shell. The ice is streaked with stress cracks, ridges and salt deposits all evidence, scientists say, of a turbulent ocean underneath the ice. Although the temperature at Europa’s surface is a chilly –160 degrees Celsius (–256 degrees Fahrenheit), friction generated by Jupiter’s enormous gravity which causes Europa’s surface to rise and fall in a kind of tide may be warming the moon’s interior. Unfortunately, scientists do not know for certain whether an ocean of liquid water or slush lies below Europa’s surface. Galileo’s cameras cannot peer through the ice to find out. So NASA is going diving. In 2003 the agency plans to launch a spacecraft called Europa Orbiter that will aim ice-penetrating radar at the moon. After the probe goes into orbit around Europa, a three-antenna radar array will beam signals of various frequencies toward the moon’s surface. By recording the reflections of the signals, the instrument will measure the thickness of the ice layer and determine whether an ocean lies below it. If an ocean exists, the radar will provide a three-dimensional map of its distribution. In addition, a laser altimeter on board the spacecraft will measure the tidal deformation of Europa’s surface caused by Jupiter’s gravity. The tidal bulge should be much larger if there is an ocean beneath the ice. Here on Earth, oceans mean life. Researchers have found hardy microbes, dubbed extremophiles, lurking in even the most punishing oceanic environments, from Antarctic sea ice to deep-sea hydrothermal vents. Could organisms do the same on a moon that is 780 million kilometers from the sun? Probably, says Torrence Johnson, project scientist for Europa Orbiter at the Jet Propulsion Laboratory. “Europa may be the only place where we can find extraterrestrial life in an ocean.” Europa Orbiter, slated to arrive at the moon in 2007, will stop short of looking for life. It will, however, identify prime landing spots for future missions. One idea for a follow-up mission is to use hydrobots, or remote controlled underwater probes, that would penetrate the ice, possibly by melting their way through, and look for signs of life in the water below. JPL scientists have already designed a prototype, a 20-centimeter-wide cylinder equipped with a camera. They recently tested the probe at an undersea volcano off the coast of Hawaii. A research submersible lowered the probe to a depth of nearly 1.3 kilometers, then inserted it into a hydrothermal vent so that it could search for microbes in the superheated water. The scientists hope to test a similar hydrobot in Antarctica and finally on Europa.
Earth-like Planets in Other Solar Systems
Perhaps the most exciting astronomical discovery would be the sighting of an Earth-like planet orbiting another star. If a futuristic telescope could find such a planet and analyze its atmosphere, it might be able to determine whether the planet is home to
extraterrestrial life. Ground-based telescopes have recently detected evidence of a handful of planets circling stars outside our solar system. But these observations
have been indirect the astronomers inferred a planet’s presence based on the gravity-induced wobble of the star being observed. And because a planet must be very massive to produce a discernible wobble, all the planets detected so far are closer in size to Jupiter than to Earth.
In 2005 NASA plans to improve its searching ability with the Space Interferometry
Mission (SIM), an observatory that would travel around the sun in a near-Earth orbit. SIM would capture images of unprecedented resolution by combining the light from two telescopes that are 10 meters apart. The observatory would be able to measure star positions so precisely that astronomers could detect the wobble caused by an Earth-like planet orbiting a nearby star. SIM would set the stage for the Terrestrial Planet Finder (TPF), an instrument that could directly observe the light reflected off Earth-like planets in other solar systems. The main challenge facing TPF is glare. A nearby star would shine one million times brighter than its surrounding planets, even in the infrared range of the spectrum, where planets are brightest. According to Charles Beichman, co-chair of the TPF science team, observing a planet in another solar system would be like trying to spot a firefly that is sitting on the rim of a searchlight. What is more, interstellar dust tends to scatter starlight, adding extra glare and making it harder to isolate a planet’s faint glow.
Fortunately, TPF has a way to block the stars’ glare. The observatory would consist of five spacecraft flying in formation in a near-Earth orbit around the sun. Four of the spacecraft would carry 3.5-meter-wide telescope mirrors that would be aimed at the target star. Each of the mirrors would reflect the star’s infrared light toward the fifth spacecraft, a vessel flying in the middle of the group, where the image would be focused. The four beams would be combined so that the light waves interfered with one another, canceling out the starlight in the center of the image but preserving the light from any planets on the periphery. NASA hopes to launch TPF in 2010, after SIM has identified the solar systems most likely to have Earth-like planets.
TPF would observe several hundred stars up to 50 light-years away, spending a few hours at each star. After completing the survey, the group of spacecraft wouldpay closer attention to any discovered planet that is about the size ofEarth. The observatory would then use spectrographic instruments to try to determine the chemical composition of the planet’s atmosphere. Carbon dioxide, water vapor and ozone are all promising signs of life that can be detected in the infrared spectrum. Ozone, for example,forms when light reacts with oxygen, which can be made by plants. “If you have ozone in the atmosphere, that’s circumstantial evidence for primitive life on the planet,” Beichman says. TPF will get about five years in space to conduct its search. Mission scientists believe that if they focus on the right stars and planets, they are bound to discover whether there is evidence of life in other solar systems.
Super Galaxies
Can astronomers observe the birth of the universe? In 2003 NASA plans to begin building the Next Generation Space Telescope (NGST), a deep-space observatory that will allow scientists to peer into the farthest reaches of the cosmos, nearly 12 billion light-years from Earth. The new telescope would use an eight-meter-wide mirror to capture images of the very first galaxies, which astronomers believe started generating their light just a few hundred million years after the big bang. The Hubble Space Telescope, which has been orbiting Earth since 1990, has revealed some tantalizing hints about the early history of the universe. Hubble has observed fully formed galaxies dating as far back as a billion years after the big bang. Astronomers want to know how those first galaxies coalesced from the dark primordial nebula. “Hubble whetted our appetite for the cosmic dark ages,” says John Mather, the NGST project scientist at the NASA
Goddard Space Flight Center. “NGST will help us see farther and sharper to learn about the history and shape of the universe.” Because the universe is expanding, the light from distant objects is redshifted that is, converted to longer wavelengths. The amount of redshift is measured as the ratio of the change in wavelength to the original wavelength. The farthest galaxies have the greatest redshifts. The best current telescopes have spotted galaxies with redshifts of about five, but NGST will be able to observe objects with redshifts of 10 to 20. To see such objects, the new telescope will be designed to scan from the far visible to the mid-infrared range of the spectrum. (Hubble detects light in the visible to near-infrared range.)
NGST’s lightweight, flexible mirror will be at least twice as wide as Hubble’s and will gather 10 times more light. Because the new telescope will operate in the infrared range, the optics and cameras must be kept as cold as possible to prevent background heat from obscuring the images. The spacecraft will carry a massive sunshade to prevent overheating and will be located far from Earth to avoid the sunlight reflected from the planet’s surface. Most likely, the telescope will orbit the sun near the L2 Lagrange point, one of five points where the sun’s and Earth’s gravity are in equilibrium. L2 is about 1.5 million kilometers farther from the sun than Earth is. The images from NGST may help unravel the mystery of how density fluctuations left over from the big bang evolved into the large-scale structure of the universe. Astronomers are not sure whether galaxies formed from the contraction of larger clouds of matter or from the aggregation of smaller star clusters. The telescope may also provide new observations of stellar and planetary formation, which take place inside massive clouds of dust. Because dust does not absorb infrared light as much as it absorbs light of other wavelengths, NGST will be able to see deeper inside the clouds. “With the infrared, we can peer into dust clouds, learn about dark matter and find faint planets,” Mather says. “There’s a lot out there to discover.” Several groups are vying to construct NGST, which is expected to be launched in 2008. Lifted off Earth by an expendable rocket, NGST would shoot skyward in a folded-up position. Once in space, it would unfold like a giant bird opening its wings, pop up its sunshade and settle into its frigid orbit. If all goes well, the telescope will begin collecting images within days and operate for about a decade.
Goddard Space Flight Center. “NGST will help us see farther and sharper to learn about the history and shape of the universe.” Because the universe is expanding, the light from distant objects is redshifted that is, converted to longer wavelengths. The amount of redshift is measured as the ratio of the change in wavelength to the original wavelength. The farthest galaxies have the greatest redshifts. The best current telescopes have spotted galaxies with redshifts of about five, but NGST will be able to observe objects with redshifts of 10 to 20. To see such objects, the new telescope will be designed to scan from the far visible to the mid-infrared range of the spectrum. (Hubble detects light in the visible to near-infrared range.)
NGST’s lightweight, flexible mirror will be at least twice as wide as Hubble’s and will gather 10 times more light. Because the new telescope will operate in the infrared range, the optics and cameras must be kept as cold as possible to prevent background heat from obscuring the images. The spacecraft will carry a massive sunshade to prevent overheating and will be located far from Earth to avoid the sunlight reflected from the planet’s surface. Most likely, the telescope will orbit the sun near the L2 Lagrange point, one of five points where the sun’s and Earth’s gravity are in equilibrium. L2 is about 1.5 million kilometers farther from the sun than Earth is. The images from NGST may help unravel the mystery of how density fluctuations left over from the big bang evolved into the large-scale structure of the universe. Astronomers are not sure whether galaxies formed from the contraction of larger clouds of matter or from the aggregation of smaller star clusters. The telescope may also provide new observations of stellar and planetary formation, which take place inside massive clouds of dust. Because dust does not absorb infrared light as much as it absorbs light of other wavelengths, NGST will be able to see deeper inside the clouds. “With the infrared, we can peer into dust clouds, learn about dark matter and find faint planets,” Mather says. “There’s a lot out there to discover.” Several groups are vying to construct NGST, which is expected to be launched in 2008. Lifted off Earth by an expendable rocket, NGST would shoot skyward in a folded-up position. Once in space, it would unfold like a giant bird opening its wings, pop up its sunshade and settle into its frigid orbit. If all goes well, the telescope will begin collecting images within days and operate for about a decade.
Here’s what you can look forward to in the next ten years of space exploration
We’re about to enter one of the most exciting eras in the history of space exploration. From private spaceflight to journeys into the outer Solar System, find out what missions will be of most interest in the next ten years in our extensive (but not exhaustive) timeline below. Bear in mind that space is an unpredictable business, so all of these dates are subject to change.
2013
- November/December – NASA’s next Mars orbiter, MAVEN, will launch towards the Red Planet.
MAVEN will study the climate history of Mars. Image credit: NASA - December – China’s Chang’e 3 probe is expected to land on the Moon, the first to do so since Russia’s Luna 24 in 1976.
The Lynx spaceplane will complete its first flight to sub-orbit.
2014
- April – In-flight abort test of SpaceX’s crewed spacecraft DragonRider, with a manned flight to follow at an unspecified date.
- July – JAXA’s Hayabusa 2 spacecraft, the successor to Hayabusa, will begin its journey to an asteroid on a mission to land and return samples.
- July – Curiosity’s primary mission on Mars is expected to come to an end, but it is almost certain that the mission will be extended.
- September – First test of NASA’s Orion spacecraft, which will launch unmanned on a Delta IV Heavy rocket.
- Planetary Resources will launch its Arkyd-3 mini satellites to test technologies for later telescopes to look for asteroids to mine.
- Virgin Galactic will begin scheduled flights of SpaceShipTwo to sub-orbit.
Virgin Galactic completed a 'cold flow' test of SpaceShipTwo in April 2013. Image credit: MarsScientific.com and Clay Center Observatory 2015
- February – NASA’s Dawn spacecraft will enter orbit around Ceres, the first spacecraft to visit a dwarf planet.
- May – The Indian Space Research Organisation (ISRO) will launch an orbiter towards Venus.
- July – NASA’s New Horizons spacecraft will arrive at Pluto, becoming the first spacecraft to flyby the dwarf planet and also returning the first close-up images of this distant world and its moons.
- October – The Google Lunar X Prize may be won by Astrobotic Technology, who have a contract with SpaceX to use one of their rockets to take a lunar rover to the Moon.
- The ESA’s Don Quijote spacecraft will launch, a mission to impact an asteroid and study the change in its trajectory to see if such a method could be used to deflect an asteroid away from Earth.
The ESA's Don Quijote mission will include an orbiter and an impactor. Image credit: ESA - AOES Medialab - China will land its unmanned Chang’e 4 spacecraft on the Moon.
- Russia is expect to land two unmanned probes on the Moon, Luna-Glob 1 and 2.
- The ISRO will land its Chandrayaan-2 rover on the Moon, in tandem with a lunar orbiter.
- April – In-flight abort test of SpaceX’s crewed spacecraft DragonRider, with a manned flight to follow at an unspecified date.
- July – JAXA’s Hayabusa 2 spacecraft, the successor to Hayabusa, will begin its journey to an asteroid on a mission to land and return samples.
- July – Curiosity’s primary mission on Mars is expected to come to an end, but it is almost certain that the mission will be extended.
- September – First test of NASA’s Orion spacecraft, which will launch unmanned on a Delta IV Heavy rocket.
- Planetary Resources will launch its Arkyd-3 mini satellites to test technologies for later telescopes to look for asteroids to mine.
- Virgin Galactic will begin scheduled flights of SpaceShipTwo to sub-orbit.
Virgin Galactic completed a 'cold flow' test of SpaceShipTwo in April 2013. Image credit: MarsScientific.com and Clay Center Observatory 2015
- February – NASA’s Dawn spacecraft will enter orbit around Ceres, the first spacecraft to visit a dwarf planet.
- May – The Indian Space Research Organisation (ISRO) will launch an orbiter towards Venus.
- July – NASA’s New Horizons spacecraft will arrive at Pluto, becoming the first spacecraft to flyby the dwarf planet and also returning the first close-up images of this distant world and its moons.
- October – The Google Lunar X Prize may be won by Astrobotic Technology, who have a contract with SpaceX to use one of their rockets to take a lunar rover to the Moon.
- The ESA’s Don Quijote spacecraft will launch, a mission to impact an asteroid and study the change in its trajectory to see if such a method could be used to deflect an asteroid away from Earth.
The ESA's Don Quijote mission will include an orbiter and an impactor. Image credit: ESA - AOES Medialab - China will land its unmanned Chang’e 4 spacecraft on the Moon.
- Russia is expect to land two unmanned probes on the Moon, Luna-Glob 1 and 2.
- The ISRO will land its Chandrayaan-2 rover on the Moon, in tandem with a lunar orbiter.
2016
- NASA’s next Mars lander, InSight, will launch to the Red Planet to study beneath the surface using a drill.
- The first part of the ESA’s ExoMars mission, the Trace Gas Orbiter (to study the atmosphere) and the EDM lander (to test landing technologies), will launch to Mars.
- NASA will launch its own asteroid sample return mission, called OSIRIS-REx.
- Bearing the same name as its successful probes in the 60s, 70s and 80s, Russia will launch a new orbiter called Venera-D to Venus.
- Sierra Nevada Corporation’s Dream Chaser spacecraft will complete its first flight, launching on an Atlas V rocket.
SNC's Dream Chaser will launch on a modified Atlas V. Image credit: SNC - NASA’s solar powered Juno spacecraft will arrive at Jupiter.
- NASA’s next Mars lander, InSight, will launch to the Red Planet to study beneath the surface using a drill.
- The first part of the ESA’s ExoMars mission, the Trace Gas Orbiter (to study the atmosphere) and the EDM lander (to test landing technologies), will launch to Mars.
- NASA will launch its own asteroid sample return mission, called OSIRIS-REx.
- Bearing the same name as its successful probes in the 60s, 70s and 80s, Russia will launch a new orbiter called Venera-D to Venus.
- Sierra Nevada Corporation’s Dream Chaser spacecraft will complete its first flight, launching on an Atlas V rocket.
SNC's Dream Chaser will launch on a modified Atlas V. Image credit: SNC - NASA’s solar powered Juno spacecraft will arrive at Jupiter.
2017
- January – The ESA’s new solar orbiter, SOLO, will launch to the Sun.
- December – First flight of NASA’s Space Launch System rocket, taking the Orion spacecraft on an unmanned flight around the Moon.
NASA's Space Launch System will be used to take astronauts to an asteroid, and possibly the Moon and Mars as well. Image credit: NASA - China will launch another mission to the Moon, Chang’e 5, this time with the goal of returning lunar samples to Earth.
2018
- January – Inspiration Mars will launch two people to Mars on a 501-day mission to flyby the Red Planet and return to Earth.
- NASA’s Solar Probe Plus will launch and will approach the Sun to within 8.5 solar radii (0.034 AU, 5.9 million kilometres), the closest any spacecraft has been to the Sun.
- The ESA’s ExoMars rover will launch to the Red Planet, using the previously launched Trace Gas Orbiter as its means to communicate with Earth.
The ESA's ExoMars rover will be the agency's first landing on the Red Planet. Image credit: ESA - NASA’s James Webb Space Telescope (JWST), the successor to the Hubble Space Telescope, will launch.
- January – The ESA’s new solar orbiter, SOLO, will launch to the Sun.
- December – First flight of NASA’s Space Launch System rocket, taking the Orion spacecraft on an unmanned flight around the Moon.
NASA's Space Launch System will be used to take astronauts to an asteroid, and possibly the Moon and Mars as well. Image credit: NASA - China will launch another mission to the Moon, Chang’e 5, this time with the goal of returning lunar samples to Earth.
2018
- January – Inspiration Mars will launch two people to Mars on a 501-day mission to flyby the Red Planet and return to Earth.
- NASA’s Solar Probe Plus will launch and will approach the Sun to within 8.5 solar radii (0.034 AU, 5.9 million kilometres), the closest any spacecraft has been to the Sun.
- The ESA’s ExoMars rover will launch to the Red Planet, using the previously launched Trace Gas Orbiter as its means to communicate with Earth.
The ESA's ExoMars rover will be the agency's first landing on the Red Planet. Image credit: ESA - NASA’s James Webb Space Telescope (JWST), the successor to the Hubble Space Telescope, will launch.
2019
- A proposed NASA telescope called EXCEDE (Exoplanetary Circumstellar Environments and Disk Explorer) will launch to observe planet formation around nearby stars.
2020
- The ISS will be decommissioned and de-orbited at some point after 2020.
The ISS will exceed its operational use in the next decade. Image credit: NASA - Russia will launch an orbiter, lander and rover to the Moon on the Luna-Grunt 1 mission.
- ESA and NASA may attempt a Mars sample return mission.
2021
- Russia’s next lunar lander, Luna-Grunt 2, will return samples to Earth.
- NASA’s Orion spacecraft will fly with a crew for the first time, possibly taking astronauts to visit an asteroid.
NASA's Orion spacecraft will be able to carry at least four astronauts. Image credit: NASA 2022
- ESA will launch its new deep-space probe JUICE (Jupiter Icy Moon Explorer) to study the Jovian system, specifically Ganymede, Callisto and Europa.
2023
- Mars One expects to land the first human settlers on the Red Planet.
- A proposed NASA telescope called EXCEDE (Exoplanetary Circumstellar Environments and Disk Explorer) will launch to observe planet formation around nearby stars.
2020
- The ISS will be decommissioned and de-orbited at some point after 2020.
The ISS will exceed its operational use in the next decade. Image credit: NASA - Russia will launch an orbiter, lander and rover to the Moon on the Luna-Grunt 1 mission.
- ESA and NASA may attempt a Mars sample return mission.
2021
- Russia’s next lunar lander, Luna-Grunt 2, will return samples to Earth.
- NASA’s Orion spacecraft will fly with a crew for the first time, possibly taking astronauts to visit an asteroid.
NASA's Orion spacecraft will be able to carry at least four astronauts. Image credit: NASA 2022
- ESA will launch its new deep-space probe JUICE (Jupiter Icy Moon Explorer) to study the Jovian system, specifically Ganymede, Callisto and Europa.
2023
- Mars One expects to land the first human settlers on the Red Planet.
More about Exploration...
Useful links for u.................
SPACE AGENCIES
www.nasa.gov/
Home page of the National Aeronautics and Space Administration(NASA)
www.rka.ru/english/eindex.htm
Home page of the Russian Space Agency (RSA), in English
www.esa.int/
Home page of the European Space Agency (ESA), in English
www.nasda.go.jp/index_e.html
Home page of the National Space Development Agency of Japan (NASDA), in English
SPACE SHUTTLE AND STATIONS
www.ksc.nasa.gov/shuttle/missions/missions.html
Summaries of all space shuttle flights to date and plans for future missions
www.ksc.nasa.gov/shuttle/technology/sts-newsref/stsref-toc.html
A technical reference manual for the space shuttle
spaceflight.nasa.gov/index.html
News about the International Space Station, and real-time tracking of its orbit
liftoff.msfc.nasa.gov/
Information on space stations and satellites as well as a section for children
UNMANNED MISSIONS AND OBSERVATORIES
www.hq.nasa.gov/office/oss/missions/index.htm
A thorough list of NASA’s space science missions—past, present and future
mpfwww.jpl.nasa.gov/
Mars missions managed by the Jet Propulsion Laboratory
www.jpl.nasa.gov/ice_fire//
Information on the “Ice and Fire Missions”: Europa Orbiter,Pluto-Kuiper Express and Solar Probe
www.stsci.edu/
The latest discoveries made with the Hubble Space Telescope
nssdc.gsfc.nasa.gov/
Space research data from the National Space Science Data Center
SPACE ORGANIZATIONS
www.marssociety.org/
Information on the Mars Society’s efforts to increase support for exploration of the Red Planet
www.nss.org/
Educational resources and more from the National Space Society, the oldest space advocacy group
www.space-frontier.org/
Home page of the Space Frontier Foundation, with links to companies in the space industry
www.prospace.org/
Home page of ProSpace, a lobbying group for space exploration
www-ssi.colorado.edu/
Resources for scientists and teachers from the Space Science Institute
www.asi.org/
Information on the Artemis Project, which is dedicated to establishing human communities on the moon
www.reston.com/nasa/watch.html
A non-NASA site about NASA activities, offering news about the agency’s programs, budget and administrators
HISTORY OF SPACE EXPLORATION
www.ksc.nasa.gov/history/mercury/mercury.html
Historical information on the Mercury missions, the first U.S. manned flights into space
www.ksc.nasa.gov/history/gemini/gemini.html
Site for facts about the Gemini missions
www.ksc.nasa.gov/history/apollo/apollo.html
Descriptions of the Apollo missions and the first moon landings
www.ksc.nasa.gov/history/skylab/skylab.html
A history of Skylab, the first U.S. space station
ASTRONAUTS AND COSMONAUTS
www.jsc.nasa.gov/Bios/astrobio.html
Biographies of all active-duty U.S. astronauts and mission specialists
from other countries who have flown on the space shuttle
www.jsc.nasa.gov/Bios/cosmo.html
Biographies of cosmonauts involved in U.S.-Russian joint projects
38.201.67.70/history/shuttle-mir/ops/crew/
Photographs and biographies of the crews who served on Mir,the Russian space station
GREAT PICTURES
photojournal.jpl.nasa.gov
At least 1,400 images of planets, moons, the sun and more
www.jpl.nasa.gov/pictures/archive.html
Illustrations of Mars Pathfinder, Galileo, Cassini and other spacecraft
nssdc.gsfc.nasa.gov/photo_ gallery/
The National Space Science Data Center’s archive of astronomy photographs
www.nasa.gov/
Home page of the National Aeronautics and Space Administration(NASA)
www.rka.ru/english/eindex.htm
Home page of the Russian Space Agency (RSA), in English
www.esa.int/
Home page of the European Space Agency (ESA), in English
www.nasda.go.jp/index_e.html
Home page of the National Space Development Agency of Japan (NASDA), in English
SPACE SHUTTLE AND STATIONS
www.ksc.nasa.gov/shuttle/missions/missions.html
Summaries of all space shuttle flights to date and plans for future missions
www.ksc.nasa.gov/shuttle/technology/sts-newsref/stsref-toc.html
A technical reference manual for the space shuttle
spaceflight.nasa.gov/index.html
News about the International Space Station, and real-time tracking of its orbit
liftoff.msfc.nasa.gov/
Information on space stations and satellites as well as a section for children
UNMANNED MISSIONS AND OBSERVATORIES
www.hq.nasa.gov/office/oss/missions/index.htm
A thorough list of NASA’s space science missions—past, present and future
mpfwww.jpl.nasa.gov/
Mars missions managed by the Jet Propulsion Laboratory
www.jpl.nasa.gov/ice_fire//
Information on the “Ice and Fire Missions”: Europa Orbiter,Pluto-Kuiper Express and Solar Probe
www.stsci.edu/
The latest discoveries made with the Hubble Space Telescope
nssdc.gsfc.nasa.gov/
Space research data from the National Space Science Data Center
SPACE ORGANIZATIONS
www.marssociety.org/
Information on the Mars Society’s efforts to increase support for exploration of the Red Planet
www.nss.org/
Educational resources and more from the National Space Society, the oldest space advocacy group
www.space-frontier.org/
Home page of the Space Frontier Foundation, with links to companies in the space industry
www.prospace.org/
Home page of ProSpace, a lobbying group for space exploration
www-ssi.colorado.edu/
Resources for scientists and teachers from the Space Science Institute
www.asi.org/
Information on the Artemis Project, which is dedicated to establishing human communities on the moon
www.reston.com/nasa/watch.html
A non-NASA site about NASA activities, offering news about the agency’s programs, budget and administrators
HISTORY OF SPACE EXPLORATION
www.ksc.nasa.gov/history/mercury/mercury.html
Historical information on the Mercury missions, the first U.S. manned flights into space
www.ksc.nasa.gov/history/gemini/gemini.html
Site for facts about the Gemini missions
www.ksc.nasa.gov/history/apollo/apollo.html
Descriptions of the Apollo missions and the first moon landings
www.ksc.nasa.gov/history/skylab/skylab.html
A history of Skylab, the first U.S. space station
ASTRONAUTS AND COSMONAUTS
www.jsc.nasa.gov/Bios/astrobio.html
Biographies of all active-duty U.S. astronauts and mission specialists
from other countries who have flown on the space shuttle
www.jsc.nasa.gov/Bios/cosmo.html
Biographies of cosmonauts involved in U.S.-Russian joint projects
38.201.67.70/history/shuttle-mir/ops/crew/
Photographs and biographies of the crews who served on Mir,the Russian space station
GREAT PICTURES
photojournal.jpl.nasa.gov
At least 1,400 images of planets, moons, the sun and more
www.jpl.nasa.gov/pictures/archive.html
Illustrations of Mars Pathfinder, Galileo, Cassini and other spacecraft
nssdc.gsfc.nasa.gov/photo_ gallery/
The National Space Science Data Center’s archive of astronomy photographs