The Galileo spacecraft used the gravity of Earth and Venus to accelerate and build up enough speed to reach its destination of Jupiter. Launched in 1989, Galileo finally reached orbit around Jupiter in 1995, where it released a probe to study Jupiter’s atmosphere. Despite the failure of its main antenna to open completely, limiting the speed at which information is transmitted back to Earth, Galileo has provided much new information about the Jovian system. Galileo's initial mission ended in 1997, but the spacecraft is continuing to study Jupiter and its moons on an extended mission.
Using its own thrusters, a spacecraft can raise or lower its orbit by adding or removing energy, respectively. To add energy, the spacecraft orients itself and fires its thrusters so that it accelerates in its direction of flight. To subtract energy, the craft fires its engines against the direction of flight. Any change in the height of a spacecraft’s orbit also produces a change in its speed and vice versa. The craft moves more slowly in a higher orbit than it does in a lower one. By firing its rockets perpendicular to the plane of its orbit, the craft can change the orientation of its orbit in space.
To travel from one planet to another, a spacecraft must follow a precise path, or trajectory, through space. The amount of energy that a spacecraft’s launch rocket and onboard thrusters must provide varies with the type of trajectory. The trajectory that requires the least amount of energy is called a Hohmann transfer. A Hohmann transfer follows the shape of an ellipse, or a flattened circle, whose sides just touch the orbits of the two planets.
The trajectory must also take into account the motion of the planets around the Sun. For example, a probe traveling from Earth to Mars must aim for where Mars will be at the time of the spacecraft’s arrival, not where Mars is at the time of launch.
In many interplanetary missions, a spacecraft flies past a third planet and uses the planet’s gravitational field to bend the craft’s trajectory and accelerate it toward its target planet. This is known as a gravitational slingshot maneuver. The first spacecraft to use this technique was the Mariner 10 probe (see Mariner), which flew past Venus on its way to Mercury in 1974.
Navigation and Guidance
Most spacecraft depend on a combination of internal automatic systems and commands from ground controllers to keep on the correct path. Normally, ground controllers can communicate with a spacecraft only when it is within sight of an Earth-based receiving station. This poses problems for spacecraft in low Earth orbit—that is, within 2,000 km (1,200 mi) of the planet’s surface—as such craft are only within sight of a relatively small portion of the globe at any given moment. One way around this restriction is to place special satellites in orbit to act as relays between the orbiting spacecraft and ground stations, allowing continuous communications. NASA has done this for the U.S. space shuttle with the Tracking and Data Relay Satellite System (TDRSS).
At an altitude of about 35,800 km (about 22,200 mi), a satellite’s motion exactly matches the speed of Earth’s rotation. As a result, the satellite appears to hover over a specific spot on Earth’s surface. This so-called stationary, or geosynchronous, orbit is ideal for communications satellites, whose job is to relay information between widely separated points on the globe.
Spacecraft on interplanetary trajectories may travel millions or even billions of kilometers from Earth. In these cases their radio signals are so weak that giant receiving stations are necessary to detect them. The largest stations have antenna dishes in excess of 70 m (230 ft) across. NASA and the Jet Propulsion Laboratory operate the Deep Space Network, a system of three tracking stations with several antennas each. The stations are in California, Spain, and Australia, providing continuous contact with distant spacecraft as Earth spins on its axis.
Much of the work of ground controllers involves monitoring a spacecraft’s health and flight path. Using a process called telemetry, a spacecraft can transmit data about the functioning of its internal components. In addition, engineers can use a spacecraft’s radio signals to assess its flight path. This is possible because of the Doppler effect. Because of the Doppler effect, a spacecraft’s motion causes tiny shifts in the frequency of its radio signals—just as the motion of a passing car causes the apparent pitch of its horn to go up as the car approaches an observer and down as the car moves away. By analyzing Doppler shifts in a spacecraft’s radio signals, controllers can determine the craft’s speed and direction. Over time, controllers can combine the Doppler shift data with data on the spacecraft’s position in the sky to produce an accurate picture of the craft’s path through space.
The guidance system helps control the craft’s orientation in space and its flight path. In the early days of spaceflight, guidance was accomplished by means of radio signals from Earth. The Mercury spacecraft and its Atlas booster utilized such radio guidance signals broadcast from ground stations. During launch, for example, the Atlas received steering commands that it used to adjust the direction of its engines. However, Mercury flight controllers found that radio guidance was limited in accuracy because interference with the atmosphere tends to make the signals weaker.
Beginning with Gemini, engineers used a system called inertial guidance to stabilize rockets and spacecraft. This system takes advantage of the tendency of a spinning gyroscope to remain in the same orientation. A gyroscope mounted on a set of gimbals, or a mechanism that allows it to move freely, can maintain its orientation even if the spacecraft’s orientation changes. An inertial guidance system contains several gyroscopes, each oriented along a different axis. When the spacecraft rotates along one or more of its axes, measuring devices tell how far it has turned from the gyroscopes’ own orientations. In this way, the gyroscopes provide a constant reference by which to judge the craft’s orientation in space. Signals from the guidance system are fed into the spacecraft’s onboard computer, which uses this information to control the craft’s maneuvers.
The Global Positioning System satellites, which enable ships, airplanes, and even hikers to know their positions with extreme accuracy, should play a similar role in spacecraft. The space shuttle Atlantis was equipped with GPS receivers during an upgrade in late 1998.
Once in orbit, a spacecraft relies on its own rocket engines to change its orientation (or attitude) in space, the shape or orientation of its orbit, and its altitude. Of these three tasks, changes in orientation require the least energy. Relatively small rockets called thrusters control a spacecraft’s attitude. In a massive spacecraft, the attitude control thrusters may be full-fledged liquid-fuel rockets. Smaller spacecraft often use jets of compressed gas. Depending on which combination of thrusters is fired, the spacecraft turns on one or more of its three principal axes: roll, pitch, and yaw. Roll is a spacecraft’s rotation around its longitudinal axis, the horizontal axis that runs from front to rear. (In the case of the space shuttle orbiter, a roll maneuver resembles the motion of an airplane dipping its wing.) Pitch is rotation around the craft’s lateral axis, the horizontal axis that runs from side to side. (On the shuttle, a pitch maneuver resembles an airplane raising or lowering its nose.) Yaw is a spacecraft’s rotation around a vertical axis. (A space shuttle executing a yaw maneuver would appear to be sitting on a plane that is turning to the left or right.) A change in attitude might be required to point a scientific instrument at a particular target, to prepare a spacecraft for an upcoming maneuver in space, or to line the craft up for docking with another spacecraft.
When an orbiting spacecraft needs to drop out of orbit and descend to the surface, it must slow down to a speed less than orbital velocity. The craft slows down by using retrorockets in a process called a deorbit maneuver. On early piloted spacecraft, retrorockets used solid fuel because solid-fuel rockets were generally more reliable than liquid-fuel rockets. Vehicles such as the Apollo spacecraft and the space shuttle have used liquid-fuel retrorockets. In the deorbit maneuver, the retrorocket acts as a brake by firing into the line of flight. The duration of the firing is carefully controlled, because it will affect the path that the spacecraft takes into the atmosphere. The same technique has been used by Apollo lunar modules and by unpiloted planetary landers to leave orbit and head for a planet’s surface.
Spacecraft have used a variety of technologies to provide electrical power for running onboard systems. Engineers have used batteries and solar panels since the early days of space exploration. Often, spacecraft use a combination of the two: Solar panels provide power while the spacecraft is in sunlight, and batteries take over during orbital night. The solar panels also recharge the batteries, so the craft has an ongoing source of power. However, solar panels are impractical for many interplanetary spacecraft, which may travel vast distances from the Sun. Many of these craft have relied on thermonuclear electric generators, which create power from the decay of radioactive isotopes and have lifetimes measured in years or even decades. The twin Voyager spacecraft, which explored the outer solar system, used generators such as these. Thermonuclear electric generators are controversial because they carry radioactive substances. The radioactivity poses no danger once the spacecraft reaches space, but some people worry that an accident during launch or during an unplanned reentry into Earth’s atmosphere could release harmful radiation into the atmosphere. Concerned groups protested the 1997 launch of the Cassini spacecraft, which carried its radioactive material in explosion-proof graphite containers.
Effects of Space Travel on Humans
Space is a hostile environment for humans. Piloted spacecraft must supply oxygen, food, and water for their occupants. For longer flights, a spacecraft must provide a way to dispose of or recycle wastes. For very long flights, spacecraft will eventually have to become almost totally self-sufficient. For healthy spaceflight, the spacecraft must provide far more than just the core physical needs of astronauts. Exercise equipment, comfortable sleeping and recreation areas, and well-designed work areas are some of the amenities that soften spaceflight’s effects on humans.
The effort to save weight is so inherent to spacecraft design that it even affects the food supply. Much of the food eaten by astronauts is dehydrated to save both weight and space. In space, astronauts use a device like a water gun to rehydrate these items. Many food items are also carried in conventional form, ranging from bread to candy to fruit.
On many spacecraft, including the U.S. space shuttle, drinkable water is produced by fuel cells that also provide electrical power. The reaction between hydrogen and oxygen that creates electricity produces water as a byproduct. A small supply of water for emergency use is also carried in onboard storage tanks.
For very long-duration missions aboard space stations, water is recycled. Drinkable water can be extracted from a combination of waste water, urine, and moisture from the cabin atmosphere. This kind of system was used on the Mir space station and is used on the International Space Station. See also Space Station.
Perhaps the question most frequently asked of astronauts is, “How do you go to the bathroom in space?” The answer has changed over the years. On early missions such as Mercury, Gemini, and Apollo, the bathroom facilities were relatively crude. For urine collection, the astronauts, all of whom were men, used a hose with a condom-like fitting at one end. Urine was then dumped overboard. Feces were collected in plastic bags and brought back to Earth for medical analyses. The Skylab space station featured a toilet that used forced air for suction. Mir used similar toilets, with special fittings for men and women, as does the space shuttle.
Skylab was also the first spacecraft to offer astronauts the chance to bathe in space, by means of a collapsible shower. To prevent globs of water from escaping and floating around inside the cabin, the astronaut sealed the shower once inside. The astronaut used a handheld nozzle to dispense water and a small vacuum to remove it. On the space shuttle astronauts and cosmonauts have had to make do with sponge baths. The International Space Station has a shower in its habitation module.
Most piloted spacecraft have carried oxygen in onboard tanks in liquid form at cryogenic (super-cold) temperatures to save space. Liquid oxygen is about 800 times smaller in volume than gaseous oxygen at everyday temperatures. The Russian Mir space station used an additional source of oxygen: Special generators aboard Mir separated water into oxygen and hydrogen, and the hydrogen was vented overboard.
On Mercury, Gemini, and Apollo, the cabin atmosphere was pure oxygen at about 0.3 kg/sq cm (about 5 lb/sq in). On the space shuttle a mixture of oxygen and nitrogen provides a pressure of 1.01 kg/sq cm (14.5 lb/sq in), slightly less than atmospheric pressure on Earth at sea level. Shuttle astronauts who go on spacewalks must pre-breathe pure oxygen to purge nitrogen from their bloodstream. This eliminates the risk of decompression sickness, called the bends, because the shuttle space suit operates at a lower pressure (0.30 kg/sq cm, or 4.3 lb/sq in) than inside the cabin. Sudden decompression can cause nitrogen bubbles to form in blood and tissues, a painful and potentially lethal condition. The International Space Station has an oxygen-nitrogen atmosphere at a pressure similar to that in the shuttle.
In the past, astronauts on missions of a few days or less have often worked long hours. Some found that their need for sleep was reduced because of the minimal exertion required to move around in microgravity. However, the intense concentration required to complete busy flight plans can be tiring. On longer missions, proper rest is essential to the crew’s performance. Even on the Moon, astronauts on extended exploration missions—with surface stay times of three days—knew that they could not afford to go without a good night’s sleep. Redesigned space suits, which were easier to take off and put on, and hammocks that were strung across the lunar module cabin helped the Moon explorers get their rest.
On the Skylab space station, each astronaut had a small sleeping compartment with a sleeping restraint attached to the wall. On Mir, cosmonauts and astronauts sometimes took their sleeping bags and moved them to favorite locations inside one module or another. The International Space Station, like Skylab, has private sleeping quarters, and these will be expanded in the future to accommodate a greater number of people.
Recreation is also essential on long missions, and it takes many forms. Weightlessness provides an ongoing source of fascination and enjoyment, offering the opportunity for acrobatics, experimentation, and games. Looking out the window is perhaps the most popular pastime for astronauts orbiting Earth, providing ever-changing vistas of their home planet. On some flights, astronauts and cosmonauts read books, play musical instruments, watch videos, and engage in two-way conversations with family members on the ground.
Work in Space
Humans face many challenges when working in space. These challenges include communicating with Earth and other spacecraft, creating suitable environments for scientific experiments and other tasks, moving around in the microgravity of space, and working within cumbersome spacesuits.
Spacecraft in orbit around Earth cannot communicate continuously with the ground unless special relay satellites provide a link between the spacecraft and ground receiving stations. This problem disappears when astronauts leave Earth orbit. As Apollo astronauts traveled to the Moon, they were in constant touch with mission control. However, when they entered lunar orbit, communications were interrupted whenever the spacecraft flew over the far side of the Moon, because the Moon stood between the spacecraft and Earth. Lunar landing sites were on the near side of the Moon, so Earth was always overhead and the astronauts could maintain continuous contact with mission control. For astronauts who venture to other planets, the primary difficulty in communications will be one of distance. For example, radio signals from Mars will take as long as 20 minutes to reach Earth, making ordinary conversations impossible. For this reason, planetary explorers will have to be able to solve many problems on their own, without help from mission control.
The design of spacecraft interiors has changed as more powerful booster rockets have become available. Powerful boosters allow bigger spacecraft with roomier cabins. In Mercury and Gemini, for example, astronauts could not even stretch their legs completely. Their cockpits resembled those of jet fighters. The Apollo command module offered a bit of room in which to move around, and included a lower equipment bay with navigation equipment, a food pantry, and storage areas. The Soviet Vostoks had enough room for their sole occupant to float around, and Soyuz includes both a fairly cramped reentry module and a roomier orbital module. The orbital module is jettisoned prior to the cosmonauts’ return to Earth. The space shuttle has two floors—a flight deck with seats, controls, and windows and a middeck with storage lockers and space to perform experiments.
For the Skylab space station, designers had the luxury of creating several different kinds of environments for different purposes. For example, Skylab had its own wardroom, bathroom, and sleeping quarters. Designers have tried several different approaches to work spaces on spacecraft. Most rooms on Skylab were designed like rooms on Earth with a definite floor and ceiling. However, Skylab’s multiple docking adaptor had instrument panels on each wall, and each had its own frame of reference. Thanks to weightlessness, this was not a problem: Astronauts reported that they were able to shift their own sense of up and down to match their surroundings. When necessary, ceiling became floor and vice versa. On Salyut and Mir, the ceilings and floors were painted different colors to aid cosmonauts in orienting themselves. Because simulators on Earth were given the same color scheme, the cosmonauts were accustomed to it when they lifted off.
To help astronauts anchor themselves while they work in weightlessness, designers have equipped spacecraft with a variety of devices, including handholds, harnesses, and foot restraints. Foot restraints have taken a number of forms. Skylab crews used special shoes that could lock into a grid-like floor. Apollo astronauts used shoes equipped with strips of Velcro that stuck to Velcro strips on the capsule floor. Space shuttle astronauts have even used strips of tape on the floor as temporary foot restraints.
Astronauts and cosmonauts who perform spacewalks use a variety of devices to aid in mobility and in anchoring the body in weightlessness. Any surface along which astronauts move is fitted with handholds, which the astronauts use to pull themselves along. Foot restraints allow astronauts to remain anchored in one spot, something that is often essential for tasks requiring the use of both hands. During many spacewalks, astronauts use tethers to keep themselves from drifting away from the spacecraft. Sometimes, however, astronauts fly freely as they work by wearing backpacks with thrusters to control their direction and movement.
Astronauts who have conducted spacewalks report that the most difficult tasks are those that involve using their gloved hands to grip or manipulate tools and other gear. Because the suit—including its gloves—is pressurized, closing the hand around an object requires constant effort, like squeezing a tennis ball. After a few hours of this work, forearms and hands become fatigued. The astronauts must also keep careful track of tools and parts to prevent them from floating away. In general, designers of space hardware strive to make any kind of assembly or repair work in space as simple as possible.
THE POLITICS OF SPACE EXPLORATION
Space exploration requires more than just science—it requires an enormous amount of money. The amount of money that a country is willing to invest in space exploration depends on the political climate of the time. During the Cold War, a period of tense relations between the United States and the USSR, both countries poured huge amounts of money into their space programs, because many of the political and public opinion battles were being fought over superiority in space. After the Cold War, space exploration budgets in both countries shrank dramatically.
The Space Race and the Cold War
Space exploration became possible at the height of the Cold War, and superpower competition between the United States and the USSR gave a boost to space programs in both nations. Indeed, the primary impact of Sputnik was political—in the United States Sputnik triggered nationwide concern about Soviet technological prowess. When the USSR succeeded in putting the first human into space, it only added to the disappointment and shame felt by many Americans, and especially by President Kennedy. Against this background, Alan Shepard’s Mercury flight on May 5, 1961, was a welcome cause for celebration. Twenty days later Kennedy told Congress, “I believe that this nation should commit itself to achieving the goal, before this decade is out, of landing a man on the Moon and returning him safely to the Earth.” This was the genesis of the Apollo program. Although there were other motivations for going to the Moon—scientific exploration among them—Cold War geopolitics was the main push behind the Moon race. Cold War competition also affected the unpiloted space programs of the United States and USSR.
The Moon Race
During the piloted programs of the Moon race, the pressure of competition caused Soviet leaders to order a number of “space spectaculars,” as much for their propaganda value as for their contributions. Each Voskhod flight entailed significant risks to the cosmonauts—the Voskhod 1 crew flew without space suits, while Voskhod 2’s Alexei Leonov was almost unable to reenter his craft following his historic spacewalk. But the space spectacular the Soviets wanted most of all—a piloted mission around the Moon in time for the 50th anniversary of the Russian revolution—never came to pass. By December 1968, when the Apollo 8 astronauts flew around the Moon, it was clear that victory in the Moon race had gone to the United States.
The achievement of Kennedy’s goal, with the Apollo 11 lunar landing mission, signaled a new era in space exploration in the United States—but not as NASA had hoped. Instead of accepting NASA’s proposals for a suite of ambitious post-Apollo space programs, Congress backed off on space funding, with the space shuttle as the only major space program to gain approval. In time it became clear that the lavish space budgets of the 1960s had been a product of a unique time in history, in which space was the most visible arena for superpower competition.
After the Moon
Tensions between the superpowers eased somewhat in the early 1970s, and the United States and USSR joined forces for the Apollo-Soyuz mission in 1975. Nevertheless, Cold War suspicions continued to influence space planners in both nations in the 1970s and 1980s. Both sides continued to spend enormous sums on missiles and nuclear warheads. Missiles of the Cold War arms race were designed to fly between continents on a path that took them briefly into space during their journeys. In the United States, a great deal of research went into a space-based antimissile system called the Strategic Defense Initiative (known to the public as Star Wars), which was never built. The stockpiling of missiles was eventually slowed by the Strategic Arms Limitation Talks (SALT) treaties.
In the USSR, concerns over possible offensive uses of the U.S. space shuttle helped prompt the development of the heavy-lift launcher Energia and the space shuttle Buran. Economic hardships, however, forced the suspension of both programs. The economy worsened after the collapse of the USSR in 1991, threatening the now-Russian space program with extinction.
After the Cold War
In 1993 the U.S. government redefined NASA’s plans for an international space station to include Russia as a partner, a development that would not have been possible before the end of the Cold War. An era of renewed cooperation in space between Russia and the United States followed, highlighted by flights of cosmonauts on the space shuttle and astronauts on the Mir space station.
Meanwhile, other nations have staged their own programs of unpiloted and piloted space missions. Many have been conducted by the European Space Agency (ESA), formed in 1975, whose 13 member nations include France, Italy, Germany, and the United Kingdom. European astronauts visited Mir and have flown on shuttle missions. Since the late 1970s, a series of European rockets called Ariane have launched a significant percentage of commercial satellites. ESA’s activities in planetary exploration have included probes such as Huygens, which is scheduled to land on Saturn’s moon Titan in 2004 as part of NASA’s Cassini mission.
China, Japan, and India have each developed satellite launchers. None have created rockets powerful enough to put piloted spacecraft into orbit. However, Japan has joined Canada, Russia, and the ESA in contributing hardware and experiments to the International Space Station.
The High Cost of Space Exploration
One aspect of space exploration that has changed little over time is its cost. To some extent the ability to carry out a vigorous space program is a measure of a nation’s economic vitality. For example, Russia has had difficulties staying on schedule with its contributions to the International Space Station—a reflection of the unstable Russian economy.
Cost has always been a central factor in the political standing of space programs. The enormous expense of the Apollo Moon program (roughly $100 billion in 1990s dollars) prompted critics to say that the program could have been carried out far more cheaply by robotic missions. While that claim is oversimplified—no robot has yet equaled the performance of a skilled observer—it reveals how vulnerable space programs are to budget cuts. The reusable space shuttle failed to significantly lower the cost of placing satellites in low Earth orbit, as compared with throwaway launchers like the Saturn V and the Titan III. Cost, not scientific potential, is usually the most significant factor for a nation in deciding whether to adopt a major space program. In the United States budgetary process, space funding must compete in a very visible way with expenditures for social programs and other concerns. Taking inflation into account, Congress has steadily trimmed NASA’s allotments, forcing the agency to reduce its number of employees to pre-Apollo levels by the year 2000.
In response to the high cost of space access, the late 1990s saw renewed efforts to develop a single-stage, reusable space vehicle. The situation also strengthened arguments that in the future, the most expensive space programs should be carried out by a consortium of nations. Most scientists envision a program for sending humans to Mars as an international one, primarily as a cost-sharing measure. Still, the mix of scientific, political, and other motivations has yet to bring about such a venture, and it may be years or even decades before international piloted interplanetary voyages become reality.
FUTURE OF SPACE EXPLORATION
The future of space exploration depends on many things. It depends on how technology evolves, how political forces shape rivalries and partnerships between nations, and how important the public feels space exploration is. The near future will see the continuation of human spaceflight in Earth orbit and unpiloted spaceflight within the solar system. Piloted spaceflight to other planets, or even back to the Moon, still seems far away. Any flight to other solar systems is even more distant, but a huge advance in space technology could propel space exploration into realms currently explored only by science fiction.
The 1968 film 2001: A Space Odyssey depicted commercial shuttles flying to and from a giant wheel-shaped space station in orbit around Earth, bases on the Moon, and a piloted mission to Jupiter. The real space activities of 2001 will not match this cinematic vision, but the 21st century will see a continuation of efforts to transform humanity into a spacefaring species.
The International Space Station was scheduled to become operational in the first years of the new century. NASA plans to operate the space shuttle fleet at least through the year 2012 before phasing in a replacement—possibly a single-stage-to-orbit (SSTO) vehicle. However, some experts predict that the SSTO is too difficult a goal to be achieved that soon, and that a different kind of second-generation shuttle would be necessary—perhaps a two-stage, reusable vehicle much like the current shuttle. In a two-stage launcher, neither stage is required to do all the work of getting into orbit. This results in less stringent specifications on weight and performance than are necessary for an SSTO.
Perhaps the most difficult problem space planners face is how to finance a vigorous program of piloted space exploration, in Earth orbit and beyond. In 2001 no single government or international consortium had plans to send people back to the Moon, much less to Mars. Such missions are unlikely to happen until the perceived value exceeds their cost.
Some observers, such as Apollo 11 astronaut Buzz Aldrin, believe the solution may lie in space tourism. By conducting a lottery for tickets on Earth-orbit “vacations,” a nonprofit corporation could generate revenue to finance space tourism activities. In addition, the vehicles developed to carry passengers might find later use as transports to the Moon and Mars. Several organizations are pushing for the development of commercial piloted spaceflight. In 1996 the U.S. X-Prize Foundation announced that it would award $10 million to the first private team to build and fly a reusable spacecraft capable of carrying three individuals to a height of at least 100 km (62 mi). By 2000, 16 teams had registered for the competition, with estimates of first flights in 2001.
One belief shared by Aldrin and a number of other space exploration experts is that future lunar and Martian expeditions should not be Apollo-style visits, but rather should be aimed at creating permanent settlements. The residents of such outposts would have to “live off the land,” obtaining necessities such as oxygen and water from the harsh environment. On the Moon, pioneers could obtain oxygen by heating lunar soil. In 1998 the Lunar Prospector discovered evidence of significant deposits of ice—a valuable resource for settlers—mixed with soil at the lunar poles. On Mars, oxygen could be extracted from the atmosphere and water could come from buried deposits of ice.
The future of piloted lunar and planetary exploration remains largely unknown. Most space exploration scientists believe that people will be on the Moon and Mars by the middle of the 21st century, but how they get there—and the nature of their visits—is a subject of continuing debate. Clearly, key advances will need to be made in lowering the cost of getting people off Earth, the first step in any human voyage to other worlds.
The space agencies of the world planned a wide array of robotic missions for the final years of the 20th century and the opening decade of the 21st century. NASA’s Mission to Planet Earth (MTPE) Enterprise is designed to study Earth as a global system, and to document the effects of natural changes and human activity on the environment. The Earth Observing System (EOS) spacecraft form the cornerstone of the MTPE effort. Terra, the first EOS spacecraft, was launched in December 1999. It began providing scientists with data and images in April 2000.
Mars will be visited by a succession of landers and orbiters as part of NASA’s Discovery Program, of which the Mars Pathfinder lander was a part. The program suffered setbacks in 1999 that jeopardized NASA’s goal of retrieving a sample of Martian rocks and soil in 2003 and bringing it to Earth. Although NASA planned future missions to Mars, the missions may face delays as engineers work to ensure they do not lose more spacecraft to human error or inadequate testing.
The Discovery program also includes the Near Earth Asteroid Rendezvous mission (NEAR). This spacecraft entered orbit around the asteroid Eros in 2000. In 2004 a spacecraft called Stardust, launched on February 7, 1999, is scheduled to fly past Comet Wild 2 (pronounced Vilt 2) and gather samples of the comet’s dust to bring back to Earth (see Comet).
Jupiter’s moon Europa is also likely to receive increased scrutiny, because of strong evidence for a liquid-water ocean beneath its icy crust. Among the missions being studied is a lander to drill through the ice and explore this suspected ocean. As with Mars, scientists are especially eager to find any evidence of past or present life on Europa. Such investigations will be difficult, but the discovery of any form of life beyond Earth would undoubtedly spur further explorations.
Saturn will be visited by the Cassini orbiter in the summer of 2004. The spacecraft is to deploy a probe called Huygens that will enter the atmosphere of Saturn’s largest moon, Titan, in December 2004. During its trip to the surface, Huygens will analyze the cloudy atmosphere, which is rich in organic molecules.
NASA is also considering orbiters to survey Mercury, Uranus, and Neptune. Pluto, the only planet that has never been visited by a spacecraft, is the target for a proposed Pluto Express mission. A pair of lightweight probes would be launched at high speed, reaching Pluto and its moon Charon as early as 2010.
NASA’s New Millennium program is aimed at creating new technologies for space exploration and swiftly incorporating them into spacecraft. In its first mission, the Deep Space 1 spacecraft used solar-electric propulsion to fly by an asteroid in July 1999 and was scheduled to visit comet Borelly in 2001.
NASA also plans a number of orbiting telescopes, such as the Chandra X-Ray Observatory, an X-ray astronomy telescope launched from the space shuttle in 1999. Another program, called Origins, is designed to use ground-based and space-borne telescopes to search for Earthlike planets orbiting other stars.
Space exploration experts have long hoped that as international tensions have eased, an increasing number of space activities could be undertaken on an international, cooperative basis. One example is the International Space Station. In 1998, however, countries and agencies such as Japan and the European Space Agency (ESA) began to reassess their commitments to space exploration because of economic uncertainty. The transportation system for this mission may involve Russian space hardware, such as the Soyuz spacecraft.
In addition to the economic savings that could result from nations pooling their resources to explore space, the new perspective gained by space voyages could be an important benefit to international relations. The Apollo astronauts have said the greatest discovery from our voyages to the Moon was the view of their own world as a precious island of life in the void. Ultimately that awareness could help to improve our lives on Earth.