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space exploration

space exploration, the investigation of physical conditions in space and on stars, planets, and other celestial bodies through the use of artificial satellites (spacecraft that orbit the earth), space probes (spacecraft that pass through the solar system and that may or may not orbit another celestial body), and spacecraft with human crews.

Satellites and Probes

Although studies from earth using optical and radio telescopes had accumulated much data on the nature of celestial bodies, it was not until after World War II that the development of powerful rockets made direct space exploration a technological possibility. The first artificial satellite, Sputnik I, was launched by the USSR (now Russia) on Oct. 4, 1957, and spurred the dormant U.S. program into action, leading to an international competition popularly known as the "space race." Explorer I, the first American satellite, was launched on Jan. 31, 1958. Although earth-orbiting satellites have by far accounted for the great majority of launches in the space program, even more information on the moon, other planets, and the sun has been acquired by space probes.

Lunar Probes

In the decade following Sputnik I, the United States and the USSR between them launched about 50 space probes to explore the moon. The first probes were intended either to pass very close to the moon (flyby) or to crash into it (hard landing). Later probes made soft landings with instruments intact and achieved stable orbits around the moon. Each of these four objectives required increasingly greater rocket power and more precise maneuvering; successive launches in the Soviet Luna series were the first to accomplish each objective. Luna 2 made a hard lunar landing in Sept., 1959, and Luna 3 took pictures of the moon's far side as the probe flew by in Nov., 1959. Luna 9 soft-landed in Feb., 1966, and Luna 10 orbited the moon in Apr., 1966; both sent back many television pictures to earth. Beginning with Luna 16, which was launched in Sept., 1970, the USSR sent a several probes to the moon that either returned lunar soil samples to earth or deployed Lunokhod rovers. In addition to the 24 lunar probes in the Luna program, the Soviets also launched five circumlunar probes in its Zond program.

Early American successes generally lagged behind Soviet accomplishments by several months but provided more detailed scientific information. The U.S. program did not bear fruit until 1964, when Rangers 7,8, and 9 transmitted thousands of pictures, many taken at altitudes of less than 1 mi (1.6 km) just before impact and showing craters only a few feet in diameter. Two years later, the Surveyor series began a program of soft landings on the moon. Surveyor 1 touched down in June, 1966; in addition to television cameras, it carried instruments to measure soil strength and composition. The Surveyor program established that the moon's surface was solid enough to support a spacecraft carrying astronauts.

In Aug., 1966, the United States successfully launched the first Lunar Orbiter, which took pictures of both sides of the moon as well as the first pictures of the earth from the moon's vicinity. The Orbiter's primary mission was to locate suitable landing sites for the Apollo Lunar Module, but in the process it also discovered the lunar mascons, regions of large concentration of mass on the moon's surface. Between May, 1966, and Nov., 1968, the United States launched seven Surveyors and five Lunar Orbiters. Clementine, launched in 1994, engaged in a systematic mapping of the lunar surface. In 1998, Lunar Prospector orbited the moon in a low polar orbit investigating possible polar ice deposits, but a controlled crash near the south pole detected no water. The U.S. Lunar Reconnaissance Orbiter, launched in 2009, was designed to collect data that can be used to prepare for future missions to the moon; information from it has been used to produce a relatively detailed, nearly complete topographic map of the moon.

China became the third nation to send a spacecraft to the moon when Chang'e 1, which was launched in 2007, orbited and mapped the moon until it was crash-landed on the lunar surface in 2009. Chang'e 2 also orbited and mapped the moon (2010–11) and later conducted a flyby of an asteroid (2012). In Dec., 2013, Chang'e 3 landed on the moon and deployed a rover, Yutu.

Interplanetary Probes

While the bulk of space exploration initially was directed at the earth-moon system, the focus gradually shifted to other members of the solar system. The U.S. Mariner program studied Venus and Mars, the two planets closest to the earth; the Soviet Venera series also studied Venus. From 1962 to 1971, these probes confirmed the high surface temperature and thick atmosphere of Venus, discovered signs of recent volcanism and possible water erosion on Mars, and investigated Mercury. Between 1971 and 1973 the Soviet Union launched six successful probes as part of its Mars program. Exploration of Mars continued with the U.S. Viking landings on the Martian surface. Two Viking spacecraft arrived on Mars in 1976. Their mechanical arms scooped up soil samples for automated tests that searched for photosynthesis, respiration, and metabolism by any microorganisms that might be present; one test suggested at least the possibility of organic activity. The Soviet Phobos 1 and 2 missions were unsuccessful in 1988. The U.S. Magellan spacecraft succeeded in orbiting Venus in 1990, returning a complete radar map of the planet's hidden surface. The Japanese probes Sakigake and Suisei and the European Space Agency's probe Giotto both rendezvoused with Halley's comet in 1986, and Giotto also came within 125 mi (200 km) of the nucleus of the comet Grigg-Skjellerup in 1992. The U.S. probe Ulysses returned data about the poles of the sun in 1994, and the ESA Solar and Heliospheric Observatory (SOHO) was put into orbit in 1995. Launched in 1996 to study asteroids and comets, the Near Earth Asteroid Rendezvous (NEAR) probe made flybys of the asteroids Mathilde (1997) and Eros (1999) and began orbiting the latter in 2000. The Mars Pathfinder and Mars Global Surveyor, both of which reached Mars in 1997, were highly successful, the former in analyzing the Martian surface and the latter in mapping it. The ESA Mars Express, launched in 2003, began orbiting Mars later that year, and although its Beagle 2 lander failed to establish contact, the orbiter has sent back data. Spirit and Opportunity, NASA rovers, landed successfully on Mars in 2004, as did the NASA rover Curiosity in 2012. Messenger, also launched by NASA, became the first space probe to orbit Mercury in 2011; its mission ended in 2015. In 2014 the ESA's Rosetta became the first probe to orbit a comet (Comet 67P); prior to that rendezvous the space probe had made flybys of Mars and two asteroids.

Space probes have also been aimed at the outer planets, with spectacular results. One such probe, Pioneer 10, passed through the asteroid belt in 1973, then became the first object made by human beings to move beyond the orbits of the planets. In 1974, Pioneer 11 photographed Jupiter's equatorial latitudes and its moons, and in 1979 it made the first direct observations of Saturn. Voyagers 1 and 2, which were launched in 1977, took advantage of a rare alignment of Jupiter, Saturn, Uranus, and Neptune to explore all four planets. Passing as close as 3,000 mi (4,800 km) to each planet's surface, the Voyagers discovered new rings, explored complex magnetic fields, and returned detailed photographs of the outer planets and their unique moons. They subsequently moved toward the heliopause, the boundary between the influence of the sun's magnetic field and the interstellar magnetic field, and in 2013 NASA reported that Voyager 1 most likely crossed the heliopause in 2012 and entered interstellar space, becoming the first spacecraft to do so.

Launched in 1989, the Galileo spacecraft followed a circuitous route that enabled it to return data about Venus (1990), the moon (1992), and the asteroids 951 Gaspra (1991) and 243 Ida (1993) before it orbited Jupiter (1995–2003); it also returned data about the Jupiter's atmosphere and its largest moons (Io, Ganymede, Europa, and Callisto). The joint U.S.-ESA Cassini mission, launched in 1997, began exploring Saturn, its rings, and some of its moons upon arriving in 2004. It deployed Huygens, which landed on the surface of Saturn's moom Titan in early 2005.

Human Space Exploration

Human spaceflight has progressed from the simple to the complex, starting with suborbital flights; subsequent highlights included the launching of a single astronaut in orbit, the launching of several astronauts in a single capsule, the rendezvous and docking of two spacecraft, the attainment of lunar orbit, and the televised landing of an astronaut on the moon. The first person in earth orbit was a Soviet cosmonaut, Yuri Gagarin, in Vostok 1 on Apr. 12, 1961. The American Mercury program had its first orbital success in Feb., 1962, when John Glenn circled the earth three times; a flight of 22 orbits was achieved by Mercury in May, 1963. In Oct., 1964, three Soviet cosmonauts were launched in a Voskhod spacecraft. During the second Voskhod flight in Mar., 1965, a cosmonaut left the capsule to make the first "walk in space."

The first launch of the Gemini program, carrying two American astronauts, occurred a few days after the Soviet spacewalk. The United States made its first spacewalk during Gemini 4, and subsequent flights established techniques for rendezvous and docking in space. The first actual docking of two craft in space was achieved in Mar., 1966, when Gemini 8 docked with a crewless vehicle. In Oct., 1967, two Soviet Cosmos spacecraft performed the first automatic crewless rendezvous and docking. Gemini and Voskhod were followed by the American Apollo and the Soviet Soyuz programs, respectively.

The Apollo Program

In 1961, President Kennedy had committed the United States to the goal of landing astronauts on the moon and bringing them safely back to earth by the end of the decade. The resulting Apollo program was the largest scientific and technological undertaking in history. Apollo 8 was the first craft to orbit both the earth and the moon (Dec., 1968); on July 20, 1969, astronauts Neil A. Armstrong and Edwin E. ( "Buzz" ) Aldrin, Jr., stepped out onto the moon, while a third astronaut, Michael Collins, orbited the moon in the command ship. In all, there were 17 Apollo missions and 6 lunar landings (1969–72). Apollo 15 marked the first use of the Lunar Rover, a jeeplike vehicle. The scientific mission of Apollo centered around an automated geophysical laboratory, ALSEP (Apollo Lunar Surface Experimental Package). Much was learned about the physical constitution and early history of the moon, including information about magnetic fields, heat flow, volcanism, and seismic activity. The total lunar rock sample returned to earth weighed nearly 900 lb (400 kg).

Apollo moon flights were launched by the three-stage Saturn V rocket, which developed 7.5 million lb (3.4 million kg) of thrust at liftoff. At launch, the total assembly stood 363 ft (110 m) high and weighed more than 3,000 tons. The Apollo spacecraft itself weighed 44 tons and stood nearly 60 ft (20 m) high. It was composed of three sections: the command, service, and lunar modules. In earth orbit, the lunar module (LM) was freed from its protective compartment and docked to the nose of the command module. Once in lunar orbit, two astronauts transferred to the LM, which then detached from the command module and descended to the lunar surface. After lunar exploration, the descent stage of the LM remained on the moon, while the ascent stage was jettisoned after returning the astronauts to the command module. The service module was jettisoned just before reentering the earth's atmosphere. Thus, of the huge craft that left the earth, only the cone-shaped command module returned.

The Soyuz Program

Until late 1969 it appeared that the USSR was also working toward landing cosmonauts on the moon. In Nov., 1968, a Soviet cosmonaut in Soyuz 3 participated in an automated rendezvous and manual approach sequence with the crewless Soyuz 2.Soyuz 4 and 5 docked in space in Jan., 1969, and two cosmonauts transferred from Soyuz 5 to Soyuz 4; it was the first transfer of crew members in space from separately launched vehicles. But in July, 1969, the rocket that was to power the lunar mission exploded, destroying an entire launch complex, and the USSR abandoned the goal of human lunar exploration to concentrate on orbital flights. The program suffered a further setback in June, 1971, when Soyuz 11 accidentally depressurized during reentry, killing all three cosmonauts. In July, 1975, the United States and the USSR carried out the first internationally crewed spaceflight, when an Apollo and a Soyuz spacecraft docked while in earth orbit. Later Soyuz spacecraft have been used to ferry crew members to and from Salyut,Mir, and the International Space Station.

Space Stations

After the geophysical exploration of the moon via the Apollo program was completed, the United States continued human space exploration with Skylab, an earth-orbiting space station that served as workshop and living quarters for three astronauts. The main capsule was launched by a booster; the crews arrived later in an Apollo-type craft that docked to the main capsule. Skylab had an operational lifetime of eight months, during which three three-astronaut crews remained in the space station for periods of about one month, two months, and three months. The first crew reached Skylab in May, 1972.

Skylab's scientific mission alternated between predominantly solar astrophysical research and study of the earth's natural resources; in addition, the crews evaluated their response to prolonged conditions of weightlessness. The solar observatory contained eight high-resolution telescopes, each designed to study a different part of the spectrum (e.g., visible, ultraviolet, X-ray, or infrared light). Particular attention was given to the study of solar flares (see sun). The earth applications, which involved remote sensing of natural resources, relied on visible and infrared light in a technique called multispectral scanning (see space science). The data collected helped scientists to forecast crop and timber yields, locate potentially productive land, detect insect infestation, map deserts, measure snow and ice cover, locate mineral deposits, trace marine and wildlife migrations, and detect the dispersal patterns of air and water pollution. In addition, radar studies yielded information about the surface roughness and electrical properties of the sea on a global basis. Skylab fell out of orbit in July, 1979; despite diligent efforts, several large pieces of debris fell on land.

After that time the only continuing presence of humans in earth orbit were the Soviet Salyut and Mir space stations, in which cosmonauts worked for periods ranging to more than 14 months. In addition to conducting remote sensing and gathering medical data, cosmonauts used their microgravity environment to produce electronic and medical artifacts impossible to create on earth. In preparation for the International Space Station (ISS)—a cooperative program of the United States, Russia, Japan, Canada, Brazil, and the ESA—astronauts and cosmonauts from Afghanistan, Austria, Britain, Bulgaria, France, Germany, Japan, Kazakhstan, Syria, and the United States worked on Mir alongside their Russian counterparts. Assembly of the ISS began in Dec., 1998, with the linking of an American and a Russian module (see space station) Once the ISS was manned in 2000, maintaining Mir in orbit was no longer necessary and it was made to decay out of orbit in Mar., 2001.

The Space Shuttle

After the Skylab space station fell out of orbit in 1979, the United States did not resume sending astronauts into space until 1981, when the space shuttle, capable of ferrying people and equipment into orbit and back to earth, was launched. The shuttle itself was a hypersonic delta-wing airplane about the size of a DC-9. Takeoff was powered by three liquid-fuel engines fed from an external tank and two solid-fuel engines; the last were recovered by parachute. The shuttle itself returned to earth in a controlled glide, landing either in California or in Florida.

The shuttle put a payload of up to 25 tons (22,700 kg) in earth orbit below 600 mi (970 km); the payload was then boosted into final orbit by its own attached rocket. The Galileo probe, designed to investigate Jupiter's upper atmosphere, was launched from the space shuttle. Astronauts also used the shuttle to retrieve and repair satellites, to experiment with construction techniques needed for a permanent space station, and to conduct scientific experiments during extended periods in space.

At first it was hoped that shuttle flights could operate on a monthly basis, but schedule pressures contributed to the explosion of the Challenger shuttle in 1986, when cold launch conditions led to the failure of a rubber O-ring, and the resulting flame ruptured the main fuel tank. The shuttle program was suspended for three years, while the entire system was redesigned. The shuttle fleet subsequently operated on approximately a bimonthly schedule. A second accident occurred in 2003, when Columbia was lost during reentry because damaged heat shielding on the left wing, which had been damaged by insulation shed from the external fuel tank, failed to prevent superheated gas from entering the wing; the hot gas structurally weakened the wing and caused the shuttle to break up. Shuttle flights resumed in July, 2005, but new problems with fuel tank insulation led NASA to suspend shuttle launches for a year. The last shuttle flight was in July, 2011.

In 2004, President George W. Bush called for a return to the moon by 2020 and the establishment of a base there that would be used to support the human exploration of Mars. The following year NASA unveiled a $104 billion plan for a lunar expedition that resembled that Apollo program in many respects, except that two rockets would be used to launch the crew and lunar lander separately.

In June, 2004, SpaceShipOne, a privately financed spacecraft utilizing a reusable vehicle somewhat similar in concept to the shuttle, was launched into suborbital flight from the Mojave Desert in California. Unlike the shuttle, SpaceShipOne was carried aloft by a reusable jet mothership (White Knight) to 46,000 ft (13.8 km), where it was released and fires its rocket engine. The spacecraft was designed by Bert Rutan and built by his company, SCALED Composites. The vehicle's 90-minute flight was the first successful nongovernmental spaceflight. SpaceShipTwo, based on SpaceShipOne, is being developed for commercial tourist flights; it made its first powered flight in 2013. Another spacecraft was privately developed by Space Exploration Technologies, or SpaceX, in coordination with NASA. The company's Falcon 9 rocket had its first successful launch, from Cape Canaveral, in June, 2010. In Dec., 2010, SpaceX launched the Dragon space capsule, using a Falcon 9 rocket, and successfully returned the capsule to earth after almost two orbits. In May, 2012, the Dragon made its first resupply trip to the space station, returning with experiments and other items. Orbital Sciences Corp. (OSC) also developed a cargo capsule, Cygnus, in cooperation with NASA. OSC's Antares rocket, which is used to launch Cygnus, had its first test in Apr., 2013, and Cygnus had its first resupply flight later that year.

The Chinese Space Program

China launched its first satellite in 1970 and then began the Shuguang program to put an astronaut into space, but the program was twice halted, ending in 1980. In the 1990s, however, China began a new program, and launched the crewless Shenzhou 1, based on the Soyuz, in 1999. The Shenzhou, like the Soyuz, is capable of carrying a crew of three. In Oct., 2003, Shenzhou 5 carried a single astronaut, Yang Liwei, on a 21-hr, 14-orbit flight, making China only the third nation to place a person in orbit. A second mission, involving two astronauts, occurred in Oct., 2005. China also launched an unmanned moon mission in Oct., 2007. In June, 2012, the three-person Shenzhou 9, which included China's first woman astronaut, manually docked with the Tiangong 1 laboratory module.


See T. Wolfe, The Right Stuff (repr. 1983); B. C. Murray, Journey into Space (repr. 1990); V. Neal, Where Next, Columbus?: The Future of Space Exploration (1994); J. Harford, Korolev: How One Man Masterminded the Soviet Drive to Beat America to the Moon (1997); T. A. Heppenheimer, Countdown: A History of Space Flight (1997); F. J. Hale, Introduction to Space Flight (1998); R. D. Launius, Frontiers of Space Exploration (1998); C. Nelson, Rocket Men: The Epic Story of the First Men on the Moon (2009); A. Chaikin with V. Kohl, Voices from the Moon (2009).

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Space Exploration

Space Exploration

On July 20, 1969, the people of Earth looked up and saw the Moon in a way they had never seen it before. It was the same Moon that humans had been observing in the sky since the dawn of their existence, but on that July evening, for the first time in history, two members of their own species walked on its surface. At that time it seemed that Neil Armstrong's "giant leap for mankind" would mark the beginning of a bold new era in the exploration of other worlds by humans.

In 1969, some people believed that human scientific colonies would be established on the lunar surface by the 1980s, and that a manned mission to Mars would surely be completed before the turn of the twenty-first century. By the end of 1972, a dozen humans would explore the surface of the Moon. However, 28 years later, as revelers around the world greeted the new millenium, the number of lunar visitors remained at twelve, and a manned mission to Mars seemed farther away than it did in the summer of 1969. How is it that the United States could arguably produce the greatest engineering feat in human history in less than a decade and then fail to follow through with what seemed to be the next logical steps? The answers are complex, but have mostly to do with the tremendous costs of human missions and the American public's willingness to pay for them.

The Space Race Fuels Space Exploration

The Apollo program, which sent humans to the Moon, was hugely expensive. At its peak in 1965, 0.8 percent of the Gross Domestic Product (GDP) of the United States was spent on the program. In 2000, the budget for all of the National Aeronautics and Space Administration (NASA) was about0.25 percent of GDP, with little public support for increased spending on space programs. The huge budgets for the Apollo program were made palatable to Americans in the 1960s because of ongoing competition between the United States and the Soviet Union. The United States had been humiliated by the early successes of the Soviet Union's space program, including the launch of Sputnik.

The Soviet launch of Sputnik, the first man-made satellite, in 1957, and the early failures of American rockets put the United States well behind its greatest rival in what is now commonly known as the "space race." In 1961, when Soviet cosmonaut Yuri Gagarin became the first human to ride a spacecraft into orbit around the Earth, the leaders of the United States felt the need to respond in a very dramatic way. In less than a year, then-President John F. Kennedy set a national goal of sending a man to the Moon and returning him safely to Earth by the end of the decade.

The American public, feeling increasingly anxious about perceived Soviet superiority in science and engineering, gave their support to gigantic spending increases for NASA to fund the race to the Moon. Neil Armstrong's first footprint in the lunar dust on July 20, 1969, achieved the first half of Kennedy's goal. Four days later, splashdown in the Pacific Ocean of the Apollo 11 capsule carrying Armstrong, fellow Moon-walker Edwin "Buzz" Aldrin, and command pilot Michael Collins completed the achievement.

Americans were jubilant over this success but growing increasingly weary of spending such huge sums of money on the space program when there were pressing needs at home. By 1972, with the United States mired in an unpopular war in Vietnam, the public's willingness to fund any more forays, whether to the Moon or southeast Asia, had been exhausted. On December 14, 1972, astronaut Eugene Cernan stepped onto the ladder of the lunar module Challenger, leaving the twentieth century's final human footprint on the lunar surface.

Robotic Exploration of Space

Some scientists contend that the same amount of scientific data could have been obtained from the Moon by robotic missions costing a tenth as much as the Apollo missions. As the twenty-first century dawns, the enormous expense and complexity of human space flight to other worlds has scientists relying almost solely on robotic space missions to explore the solar system and beyond. Many scientists believe that the robotic exploration of space will lead to more discoveries than human missions since many robotic missions may be launched for the cost of a single human mission. NASA's current estimate of the cost of a single human mission to Mars is about $55 billion, more than the cost of the entire Apollo program. In the early-twenty-first century, human space travel is limited to flights of NASA's fleet of space shuttles, which carry satellites, space telescopes, and other scientific instruments to be placed in Earth's orbit. The shuttles also ferry astronaut-scientists to and from the Earth-orbiting International Space Station.

Numerous robotic missions are either in progress or are being planned for launch through the year 2010. In October of 1997, the Cassini mission to Saturn was launched. Cassini flew by Jupiter in December of 2000 and will reach Saturn in July of 2004. Cassini is the best instrumented spacecraft ever sent to another planet. It carries the European built Huygens probe, which will be launched from Cassini to land on Saturn's giant moon, Titan. The probe carries an entire scientific laboratory, which will make measurements of the atmospheric structure and composition, wind and weather, energy flux, and surface characteristics of Titan. It will transmit its findings to the Cassini orbiter, which will relay them back to Earth. The orbiter will continue its four-year tour of Saturn and its moons, measuring the system's magnetosphere and its interaction with the moons, rings, and the solar wind ; the internal structure and atmosphere of Saturn; the rings; the atmosphere and surface of Titan; and the surface and internal structure of the other moons of Saturn.

In April, 2001, NASA launched the Mars 2001 Odyssey with a scheduled arrival in October of 2001. Odyssey carries instruments to study the mineralogy and structure of the Martian surface, the abundance of hydrogen, and the levels of radiation that may pose a danger to future human explorers of Mars. NASA's Genesis mission launched on August 8, 2001. This spacecraft was sent to study the solar wind, collecting data for two years. This information will help scientists understand more about the composition of the Sun. In July of 2002, Contour is scheduled to launch with the purpose of flying by and taking high-resolution pictures of the nucleus and coma of at least two comets as they make their periodic visits to the inner solar system.

In November of 2002, the Japanese Institute of Space and Aeronautical Science is scheduled to launch the MUSES-C spacecraft, which will bring back surface samples from an asteroid. The European Space Agency will launch Rosetta in January of 2003 for an eight-year journey to rendezvous with the comet 46 P/Wirtanen. Rosetta will spend two years studying the comet's nucleus and environment and will make observations from as close as one kilometer. Six international robotic missions to Mars, one to Mercury, one to Jupiter's moons Europa and Io, and one to the comet P/Tempel 1 are planned for the period between 20032004. Scheduled for an early 2007 launch will be the Solar Probe, which will fly through the Sun's corona sending back information about its structure and dynamics. The Solar Probe is the last launch currently scheduled for the decade 20012010, although this could change as scientists propose new missions and governments grant funding for these missions.

The Future of Space Exploration

Beyond the first decade of the twenty-first century, plans for the exploration of space remain in the "dream" stage. Scientists at the world's major space facilities, universities, and other scientific laboratories are working to find breakthroughs that will make space travel faster, less expensive, and more hospitable to humans. One technology that is being met with some enthusiasm in the space science community is the "solar sail," which works somewhat like the wind sail on a sailboat except that it gets its propulsion from continuous sunlight. A spacecraft propelled by a solar sail would need no rockets, engines, or fuel, making it significantly lighter than rocket-propelled crafts. It could be steered in space in the same manner as a sailboat is steered in water, by tilting the sail at the correct angle. The sail itself would be a thin aluminized sheet. Powered by continuous sunlight, the solar sailcraft could cruise to the outermost reaches of the solar system and even into interstellar space. Estimates are that this technology will not be fully developed until sometime near the end of 2010.

In January of 2001, scientists at Israel's Ben Gurion University reported that they had shown that the rare nuclear material americium-242m (Am-242m) could sustain a nuclear fission reaction as a thin metallic film less than a thousandth of a millimeter thick. If true, this could enable Am-242m to act as a nuclear fuel propellant for a relatively lightweight spacecraft that could reach Mars in as little as two weeks instead of the six to eight months required for current spacecraft. However, like the solar sail, this method of propulsion is also far from the implementation stage. Producing enough of it to power spacecraft will be difficult and expensive, and no designs for the type of reactor necessary for this type of nuclear fission have even been proposed. Other scientists are looking into the possibility that antimatter could be harnessed and used in jet propulsion, but the research is still in the very early stages. It may be that one or more of these technologies will propel spacecraft around the solar system in the decades ahead, but none of them hold the promise of reaching the holy grail of interstellar travel at near light speed.

Moving at the Speed of Light. The difficulty with interstellar space travel is the almost incomprehensible vastness of space. If, at some future time, scientists and engineers could build a spacecraft that could travel at light speed, it would still require more than four years to reach Proxima Centauri, the nearest star to our solar system. Consequently, to achieve the kind of travel among the stars made popular in science fiction works, such as Star Trek or Star Wars, an entirely different level of technology than any we have yet seen is required. The Pioneer 10 and Voyager 1 spacecraft launched in the 1970s have traveled more than 6.5 billion miles and are on their way out of our solar system, but at the speed they are traveling it would take them tens of thousands of years to reach Proxima Centauri.

Incremental increases in speed, while helpful within the solar system, will not enable interstellar travel. That will require a major breakthrough in science and technology. Einstein's relativity theory excludes bodies traveling at or beyond the speed of light. As a result, scientists are looking for ways to circumvent this barrier by distorting the fabric of spacetime itself to create wormholes, which are shortcuts in space-time, or by using warp drives, which are moving segments of space-time. These would require the expenditure of enormous amounts of energy. To study the feasibility of such seemingly far-fetched proposals, NASA, in 1996, established the Breakthrough Propulsion Physics Program. It may seem to the people of Earth, at the beginning of the twenty-first century, that these dreams of creating reality from science fiction are next to impossiblebut we should remember that sending humans to the Moon was nothing more than science fiction at the turn of the twentieth century.

see also Light Speed; Space, Growing Old in; Spaceflight, History of; Spaceflight, Mathematics of.

Stephen Robinson


Cowen, Roger. "Travelin' Light." Science News 156 (1999): 120.

Friedman, James. Star Sailing: Solar Cells and Interstellar Travel. New York: John Wiley & Sons, 1988.

Harrison, Albert A. Spacefaring: The Human Dimension. Berkeley: University of California Press, 2001.

Wagner, Richard, and Howard Cook. Designs on Space: Blueprints for 21st Century Space Exploration. New York: Simon and Shuster, 2000.

lnternet Resources

Millis, Mark. "The Big Mystery: Interstellar Travel." MSNBC.COM. <>.

Exotic Element Could Fuel Nuclear Fusion for Space Flight. <>.

Solar System Exploration: Missions: Saturn: Cassini. <>.

Solar System Exploration: News: Launch and Events Calendar. <>.

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Space Exploration

Space Exploration




The sociocultural status of space exploration has been contested for many years and remains uncertain. Although astronomy and related sciences have benefited greatly from the worlds space programs, space exploration was motivated not by scientific curiosity but by the romanticism of a social movement and by competition between prestige-conscious nations. By the end of the nineteenth century, astronomy possessed a rough picture of the solar system, including the knowledge that objects like the Moon and Mars were worlds somewhat comparable to the earth, but realistic means for space travel had not yet been imagined. Then, autonomous intellectuals independently developed the correct theories for multistage liquid-fuel rockets.


Konstantin Tsiolkovsky (18571935) was an impoverished schoolteacher in Russia who devoted many years of socially isolated work to developing fruitful ideas about spaceflight. American Robert H. Goddard (18821945) independently developed many of the same ideas, and possessing greater resources was actually able to build a working liquid-fuel rocket in 1926. Romanian-German Hermann Oberth (18941989) learned of the work of his colleagues just as he was about to publish his treatise, The Rocket into Planetary Space, in 1923. On the basis of the work of these pioneers, spaceflight societies were founded in Germany (1927), the United States (1930), Russia (1931), and Great Britain (1933). The German, U.S., and Russian groups independently duplicated Goddards working liquid-fuel rocket, although Goddard refused to cooperate with the others in the vain hope that he could develop unaided the technology to send an unmanned rocket to the moon. United only by publications and occasional visits, these groups formed an international social movement dedicated to space travel for transcendent motives that were neither economic nor political.

As the financial troubles of the Great Depression deepened, the space-travel movement struggled to survive. Especially in Germany, and later in the United States and Russia, the movement entered into a marriage of convenience with the military. The Treaty of Versailles ending World War I (19141918) had limited German artillery and aircraft but did not mention rockets. Members of the movement, notably Oberths young protégé Wernher von Braun (19121977), presented liquid-fuel rockets to the German army as effective weapons, although development of conventional solid-fuel rockets would have been a better choice for military purposes. Near the end of World War II (19391945), von Brauns team completed development of the 300-mile-range V-2 rocket, demonstrating the potential of liquid-fuel technology for spaceflight. Starting with the launch of Sputnik I in 1957, the Soviet Union and the United States competed for international prestige through aggressive space programs, until the landing of the Apollo 11 lunar module on the moon in 1969.

On the basis of a huge library of technical and scholarly publications, the facts of the history of space exploration to date are clear, but the social-scientific interpretation is hotly debated. The view around 1960 was that international propaganda competition was the main driver, as has been summarized by Vernon van Dyke (1964). Amitai Etzioni (1965) argued that the American space program was a useless extravagance through which the military-industrial complex looted the national treasury. Then, John Logsdon (1970) argued that President John F. Kennedys (19171963) decision to go to the moon was a means for reviving the political spirit of his New Frontier program after defeats in 1961 with the aborted Bay of Pigs invasion of Cuba and in a meeting with the Soviet leader Nikita Khrushchev (18941971). William Bainbridge (1976) took the argument one step further, suggesting that in Germany and the Soviet Union, as well as in the United States, leaders of the transcendental spaceflight movement had cleverly manipulated beleaguered political leaders to invest in space as a symbolic solution to their inferiority in competition with other leaders. Michael Neufeld (1996) has argued against this thesis in the case of Germany, asserting that technically competent military engineers possessed a correct estimation of the military potential of the technology. Walter McDougall (1985) argued against this view in the case of the Soviet Union, stating that Marxist ideology naturally supported visionary technological projects. Most recently, Logsdon (2006) has argued that the American space program has been trapped in a vicious circle, as members of the movement convince political leaders to undertake technically demanding projects, but the public is not willing to invest enough to make them successful.


Public opinion has long been generally favorable toward the space program but has never been a driving force in motivating development of the technology. In October 1947, a Gallup poll asked 1,500 Americans, How long do you think it will be before man will be able to fly to the moon? Only 21 percent guessed a particular year, 38 percent said never, and the remainder had nothing to say on the topic. Throughout the Apollo program, a majority tended to feel the project was not worth the cost. Americans enthusiasm for the actual moon landing faded fast after 1969, possibly accelerated by a general loss of confidence in science and technology that prevailed until the mid-1970s. Since then, majorities have tended to feel that the National Aeronautics and Space Administration (NASA) was doing a good job, but they give space exploration a low priority for funding. The responses of 1,400 Americans to the General Social Survey in 2004 were typical. Only 14.3 percent wanted funding increased, 43.4 percent wanted it kept at current levels, 36.8 percent wanted funding reduced, and the remaining 5.5 percent had no opinion.

Around 1970 there was considerable discussion of the potential terrestrial benefits of space, especially the second-order consequences from technology transfer, often called spin-offs. These were popularly conceptualized as distinct inventions made in the space program that found valuable uses in society. Many people count Tang powdered fruit drink, Teflon coatings on frying pans, and Velcro fasteners among these, but all existed before the space program. Real spin-offs actually are rare, but their stories fit popular misconceptions about how technological progress occurs, so they are legends that gain strength in the retelling. Far more important are the intended applications of space technology, the most prominent of which are communications satellites, navigation satellites (Global Positioning System), meteorology satellites, and military reconnaissance satellites. Difficult to measure, but probably of equal value, is the general stimulus to scientific and technological development achieved by the space program through increasing the technical expertise in the population, widely disseminating abstract technical ideas that may contribute to innovations far from their original sources, and inspiring young people to study science.

When Bainbridge (1991) asked two thousand students at Harvard University in 1986 to identify the possible goals for the space program, they came up with a list of 125 goals that could be clustered into groups that served different values. Some goals were technical and economic, including the benefits of satellites listed above, spin-offs, possible manufacturing in the vacuum and weightlessness of space, new knowledge for sciences like physics and astronomy, and preservation of the earths environment. A different set of goals stressed emotional and idealistic values, such as spiritual fulfillment, personal inspiration, artistic and aesthetic transcendence, satisfaction of curiosity, and the building of world harmony. A small group of items concerned national pride, defense, and military capabilities in space. Finally, a number of far-out but reasonably popular goals envisioned colonization of the solar system and the discovery of extraterrestrial life.

The early decades of the twenty-first century appear to be a transition period, in which predictions would be especially hazardous. China has launched men into orbit, thereby demonstrating the quality of its technology, especially to the propaganda disadvantage of Japan, which has pursued a half-hearted and largely unsuccessful space program. Both Russia and the European Union have well-established space launch capabilities but lack ambitious goals. After failing twice to develop a successor to the space shuttle in the National Aerospace Plane and the X-33, and running more than fifteen years overdue in completing the space station, the U.S. space program clearly required fundamental redirection. The initial phase of reorganization, announced in 2004, severely cut back scientific research and technological development in favor of very long-term plans for adventurous but poorly motivated human voyages to the Moon and Mars.

As any science fiction fan would be happy to explain to any social scientist willing to listen, the long-term social implications of space exploration could possibly be profound. Despite daunting technical and economic hurdles, the colonization of Mars and of several large satellites in the solar system could lead to a time when more humans lived off the Earth than on it. Some think we will transform ourselves radically to become better adapted to those alien environments and better prepared for interstellar travel. If so, space travel could bring about a new adaptive radiation event comparable to that which produced the human species five million years ago in East Africa, what the science fiction writer Alfred Bester (19131987) called arrival of the fittest in his novel, The Stars My Destination (1956). If the social scientist scoffed at such ideas, the science fiction fan might comment there must have been chimpanzees five million years ago who scoffed as well.

SEE ALSO Bay of Pigs; Industry; Mars; Science Fiction; World War II


Bainbridge, William Sims. 1976. The Spaceflight Revolution: A Sociological Study. New York: Wiley.

Bainbridge, William Sims. 1991. Goals in Space: American Values and the Future of Technology. Albany: State University of New York Press.

Bauer, Raymond. 1969. Second-Order Consequences: A Methodological Essay on the Impact of Technology. Cambridge, MA: MIT Press.

Etzioni, Amitai. 1964. The Moon-Doggle: Domestic and International Implications of the Space Race. Garden City, NY: Doubleday.

Ginzberg, Eli, James W. Kuhn, Jerome Schnee, and Boris Yavitz, eds. 1976. Economic Impact of Large Public Programs: The NASA Experience. Salt Lake City, UT: Olympus.

Launius, Roger D. 2003. Public Opinion Polls and Perceptions of U.S. Human Spaceflight. Space Policy 19: 163175.

Logsdon, John M. 1970. The Decision to Go to the Moon: Project Apollo and the National Interest. Cambridge, MA: MIT Press.

Logsdon, John M. 2006. A Failure of National Leadership: Why No Replacement for the Space Shuttle. In Critical Issues in the History of Spaceflight, ed. Steven J. Dick and Roger D. Launius, 269300. Washington, DC: NASA.

McDougall, Walter A. 1985. The Heavens and the Earth: A Political History of the Space Age. New York: Basic Books.

Neufeld, Michael, 1996. The Rocket and the Reich: Peenemunde and the Coming of the Ballistic Missile Era. Cambridge, MA: Harvard University Press.

Ordway, Frederick I., III., Carsbie C. Adams, and Mitchell R. Sharpe. 1971. Dividends from Space. New York: Crowell.

Roy, Stephanie A., Elaine C. Gresham, and Carissa Bryce Christensen. 2000. The Complex Fabric of Public Opinion on Space. Acta Astronautica 47: 665675.

Van Dyke, Vernon. 1964. Pride and Power: The Rationale of the Space Program. Urbana: University of Illinois Press.

William Sims Bainbridge

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space exploration

space exploration Using spacecraft to investigate outer space and heavenly bodies. On October 4, 1957, the Soviet Union launched the first artificial satellite, Sputnik 1, into Earth orbit. Soviet cosmonauts, and their American equivalents, astronauts, orbited the Earth soon after. Unmanned space probes crash-landed on the Moon, sending back pictures to Earth during the descent. Then came soft landings, and probes orbiting the Moon showed its hidden side for the first time. By 1968, Soviet scientists developed techniques for returning a Moon orbiter safely to the Earth. In 1969, the US Apollo 11 mission became the first to place a man on the Moon. By that time, the Soviet Union had sent probes to explore Mars and Venus. CHRONOLOGY: First probe to land on the Moon: Lunik 2, launched September 12, 1959. First manned spaceflight: Yuri Gagarin, April 12, 1961. First close-up pictures of Mars: Mariner 4, received July 14, 1965. First person to walk on the Moon, Neil Armstrong, July 20, 1969. First pictures from surface of another planet: Venera 9, received from Venus on October 22, 1975. First probes to land on Mars, Viking 1 and 2, July 1976. Fly-bys of Voyager 2: Jupiter (1979), Saturn (1981), Uranus (1986), Neptune (1989). First space shuttle: Columbia, launched April 12, 1981. Giotto probe, launched 1985, flew within 600km (380mi) of Halley's Comet, sending back photographs and data. Galileo project, launched October 1989, photographed the asteroids Gaspra (1991) and Ida (1993), dropped a sub-probe (1995) into Jupiter's atmosphere and provided detailed data on Galilean satellites; Clementine probe (1994), discovered what are thought to be water-ice deposits in craters of the Moon; Solar and Heliospheric Observatory (SOHO) probe, launched 1995, provided data on the Sun's interior until 2000; Mars Pathfinder mission, launched December 1996, landed on Mars to deploy a ‘microrover’ (Sojourner) to explore and collect rock samples; Cassini space probe (launched 1997) on a seven-year journey towards Saturn.

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