Manned spacecraft are vehicles that can transport human beings outside the Earth’s atmosphere. The word “manned,” though still used occasionally by the United States National Aeronautics and Space Administration (NASA), is often replaced today in discussions of space travel by the word “crewed,” in recognition of the fact that women also travel in space. The term human spaceflight is also used today instead of manned space missions.

In its earliest stages, crewed space flight was pursued primarily as a conspicuous demonstration of scientific and industrial might. The former Soviet Union (U.S.S.R.) and the United States, as rival superpowers, each claimed that their society was superior, and offered their space achievements as proof. The U.S.S.R. scored a tremendous and, to the United States, frightening propaganda victory by being the first to orbit a satellite of any kind: the Sputnik I, launched in 1957, which simply orbited Earth and transmitted a signature beep to an awestruck world. The Soviets were also the first, in

1961, to put a human being into orbit. They had already orbited a second cosmonaut (as the U.S.S.R. termed its astronauts) before the U.S. orbited astronaut John Herschel Glenn, Jr. (1921–) on February 20, 1962.

Scientists in both nations were also interested in collecting information about the moon, other planets in the solar system, and more distant astronomical objects, so crewed space flight—especially U.S. program—has, after the first spate of show-off flights, tended to be about research as well as about spectacle and romance. The Apollo 11 landing on the moon in 1969, for example, reaped a bounty of scientific data that clarified the development of the solar system. Today, the U.S. and the Russian Federation (inheritor of the now-defunct Soviet Union’s space program) continue crewed space flight mostly in association with the deployment and maintenance of scientific, military, and

commercial satellites and with the International Space Station (ISS), the bulk of whose research is devoted to problems of the long-term human habitation of space.

Ongoing debate: crewed vs. uncrewed flight

Since rockets first became capable of reaching space in the late 1950s, much debate has focused on the relative merits of crewed versus uncrewed space travel. Some experts have argued that scientists can learn almost all they want to know about the solar system and outer space by using uncrewed, mechanized space probes. Such probes can be designed to carry out most of the operations normally performed by humans at much less cost and with little or no risk to human life.

The enormous cost and complexity of crewed space flight—mandated by the tons of foolproof equipment needed to keep human beings alive in the utterly hostile space environment and to return them alive to Earth— is, these critics say, not justified by the modest additional benefits obtained by including human beings in a space vehicle. Other experts insist that there is no substitute in space exploration for human intelligence. Only human beings can deal with the unexpected.

To this debate about scientific efficacy have been several nonscientific elements, political and romantic. One is the emotional appeal of traveling in space, an appeal long promulgated by science fiction. Many people argue that it is human destiny to transcend the cradle of Earth and to colonize the planets or even the stars (which are many orders of magnitude harder to reach). For example, NASA planners are taking seriously the Mars exploration proposals of U.S. engineer Robert Zubrin (1956–), who argues that the psychological benefits of colonizing Mars would justify the high cost; U.S. society, Zubrin argues, can be reinvigorated by becoming a “frontier society” again, as during the opening of the American West. Another nonscientific motive for the exploration of space is, as mentioned above, national interest. This motive lessened for the U.S. and Soviet Union after the U.S. landed on the moon—by far the most spectacular goal within practical reach—but did not fade completely from the political scene. Even the cash-poor Russian Federation has maintained its space program, lest it suffer the humiliation of ceding space entirely to the United States. Furthermore, China placed astronauts into orbit in 2003, the third country to do so. Although China uses Soviet Salyut-style capsules from the 1960s, the boost to China’s international prestige will be substantial.

In the 2000s, however, much of the public captivation of space exploration has dissipated. Facing budgetary constraints and a weaker world economy, the world’s two space powers have begun to reassess the relative position of crewed versus uncrewed travel in some of their space programs. The loss of the space shuttle Columbia on February 1, 2003, has further spurred debate in the United States over whether crewed space exploration is cost-effective compared to the mechanized alternative.

There is, in fact, no debate about whether mechanized space probes such as Voyager, Pathfinder, Galileo, and Magellan produce more scientific knowledge per dollar than crewed space missions; what is at stake, ultimately, is intangible and nonquantifiable. Is it, as critics of crewed space travel argue, folly to spend trillions of dollars to put a few hundred human beings into space. Or is it, as crewed-space-flight advocates argue, folly not to make the human race a multi-planet species while humans can, with colony populations on at least the moon and Mars, thus no longer dependent on the fate of Earth for its long-term survival?

Overview

For the first four decades of the modern space era, two nations—the United States and the Soviet Union (now the Russian Federation)—have dominated crewed space travel. In 1987, the European Space Agency committed itself to participation in future crewed space programs, some operated independently and some in cooperation with the United States and Russia. Japan and Canada later made similar commitments. However, as of October 2006, no country other than the U.S., Russia, and China had yet demonstrated an ability to put its own crewed spacecraft into orbit with its own rockets. (Japan, India, and the European Union produce rockets that can loft uncrewed spacecraft into orbit and beyond; Japan has launched its own space probe to Mars.) As mentioned above, the U.S.-Russian monopoly on crewed space flight was over when the Chinese space program placed astronauts into orbit in 2003. China also has proclaimed its intention of eventually landing on the moon (around 2017) and Mars as well.

The history of crewed space programs in both Russia and the United States consist of a number of steps that led to the possibility of placing humans in orbit around Earth or on the moon. These steps were necessary in order to solve the many complex problems involved in keeping humans alive in outer space and bringing them back to Earth unharmed.

One-person crewed spacecraft

The first and simplest crewed spacecraft were designed to carry a single passenger. In the Soviet Union, these vehicles were designated by the code-name Vostok (East) and in the United States they were known as Mercury spacecraft. The first Vostok flight was piloted by Yuri A. Gargarin (1934–68) and was launched from the Tyuratam kosmodrome (space center) on April 12, 1961. In all, a total of six Vostok flights were completed over a period of just over two years. The last of these carried the first woman to fly in outer space, Valentina Tereshkova. Tereshkova spent three days in Vostok 6 between June 16 and 19, 1963.

The Vostok spacecraft was essentially a spherical cabin containing a single seat and all equipment necessary to support life and communicate with the Earth. It also held an ejection seat. The ejection seat activated at an altitude of about 23,000 ft (7,000 m), allowing the pilot to experience a soft parachute landing separately from his or her spacecraft.

The U.S. Mercury program followed a pattern similar to that of the Vostok series. In the first Mercury flight, American astronaut Alan B. Shephard traveled for 15 minutes in a suborbital flight—a long, parabolic arc over the Atlantic ocean—only three weeks after Yuri Gargarin’s trip. Nine months after Shepard’s flight, John Glenn became the first American to orbit Earth, in a space capsule he named Friendship 7 (referring to the first seven U.S. astronauts, who trained together). The Mercury spacecraft was a double-walled, bell-shaped capsule made of titanium and nickel alloy with an insulating ceramic outer coat and an ablative heat shield over the bottom of the bell to dissipate the friction of atmospheric reentry.

Two- and three-person spacecraft

The Mercury program came to a conclusion just a month before the end of the Vostok program and was followed by the U.S. two-person spacecraft, the Gemini. The Gemini cabin was not only larger than that of Mercury, it was also more sophisticated. The purpose of the Gemini program was to learn more about astronauts” ability to maneuver a spacecraft, to carry out extravehicular activities (EVAs, or, space walks), to rendezvous and dock with other spacecraft, and to perform other operations that would be necessary in the planned Apollo program, which would require such maneuvers to reach the moon.

Ten Gemini missions flew during 1965 and 1966. During one of these, Gemini 4, astronaut Edward White (1930–67) performed the first extravehicular activity (EVA), a space walk, by an American. White remained in space for a period of 21 minutes at the end of a 25 ft (7.5 m) umbilical cord connecting him to the main spacecraft.

The Soviets had decided to bypass two-person spacecraft entirely, and went directly to the development of a three-person vehicle. That program was code-named the Voskhod (Rising) series. On Voskhod 2, the space previously used for the third cosmonaut was replaced with a flexible airlock that allowed egress from the spacecraft. Cosmonaut Alexei Leonov (1934–) went EVA for over 23 minutes on March 18, 1965, the first time any human had walked in space.

Soyuz and Apollo

The Voskhod and Gemini programs each lasted for about two years, to be replaced, in turn, by spacecraft designed to carry humans to the moon. These programs were known as Soyuz (Union) in the Soviet Union and Apollo in the United States. At an early stage, the Soviets appear to have abandoned the goal of placing humans on the moon, and redesigned the Soyuz instead as an orbiting space station. The Soyuz spacecraft, a version of which is still used today by the Russian space program, consists of three primary components: the reentry vehicle, the orbital module, and the service module.

The reentry vehicle is designed to hold crew members during take-off, orbital flight, descent, and landing. It has an approximately bell-shaped appearance and contains the controls needed to maneuver the spacecraft. The orbital module contains the living and working quarters used by cosmonauts while the spacecraft is in orbit. A docking system is provided at the front end of the orbital module. The service module contains the fuel and engines needed for maneuvering the spacecraft while it is in orbit.

The first test of the Soyuz spacecraft took place in April, 1967, ending in disaster: cosmonaut V. M. Komarov (b. 1927) was killed when his parachute failed. A second Soyuz accident occurred on June 30, 1971, when a pressure valve in the vehicle apparently failed to close during descent. Air leaked out of the spacecraft and all three Soviet cosmonauts suffocated before their ship reached ground. Blame for this accident was later placed on the eagerness of Soviet politicians to put a three-man team into space before a vehicle suitable for such a flight was available. Because of crowded conditions in the Soyuz cabin, the three cosmonauts were unable to wear the space suits that would have prevented their deaths. For subsequent Soyuz flights, the spacecraft was redesigned to permit the wearing of space suits. The space needed for this modification meant, however, that the vehicle could carry only two passengers.

The Apollo spacecraft consisted of three main parts: the command module, the service module and the lunar module. The complete vehicle was designed with the objective of carrying three men to the moon— it was assumed without debate that the astronauts would be men—allowing one or more to walk on the moon’s surface and to carry out scientific experiments, then returning the crew to Earth.

The Apollo command module was a conical spacecraft in which the crew lived and worked. It was about 10 ft (3 m) high and nearly 13 ft (4 m) wide, with a total volume of about 210 cubic ft (6 cubic m). The service module had a cylindrical shape with the same diameter as the command module and roughly twice its length. The service module held the propulsion systems needed for maneuvering in orbit, electrical systems, and other subsystems needed to run the spacecraft in space.

The lunar module carried two astronauts from lunar orbit to the moon’s surface. One part of the lunar module, the descent stage, was used only during descent, and was left on the moon. The ascent stage of the lunar module rested on top of the descent stage and was used to carry the two astronauts back to the command module, which was waiting in orbit around the moon with one astronaut aboard, at the end of their stay on the moon.

The Apollo series involved a total of 11 crewed flights conducted over a period of four years between 1968 and 1972. Not all of these flights left Earth’s orbit or sought to land on the moon; several flew orbits around Earth or moon to test equipment. In December 1968, Apollo 8 became the first crewed spacecraft to travel to another world, orbiting the moon before returning to Earth. The climax of the Apollo mission series occurred on July 20, 1969, when astronauts Neil A. Armstrong (1930–) and Edwin E. (Buzz) Aldrin, Jr. (1930–), landed on the basaltic plain known as the Sea of Tranquility, walked on the moon’s surface, and collected samples of lunar soil and rock. Five more landings on the moon’s surface were accomplished before the Apollo program was ended, all placing two men on the surface while a thir dremained in or bit. Duringthelastthreelandingson the moon’s surface, astronauts were able to use a lunar roving vehicle (LRV) for moving about on the moon. The LRV was about 10 ft (3 m) long and 6 ft (1.8 m) wide, with an Earth weight of 460 lb (209 kg). It was carried to the moon inside the descent stage of the lunar module in a folded position and then unfolded for travel on the moon’s surface.

Like the Soviet space program, the American space effort has had accidents. The first of these occurred on January 27, 1967, during tests for the first Apollo flight. Fire broke out in the command module of the Apollo spacecraft, which had been filled with a pure oxygen atmosphere, and three astronauts—Roger Chaffee, Virgil “Gus” Grissom, and Edward White—died. This disaster caused a delay of 18 months in the Apollo program while engineers restudied and redesigned the Apollo spacecraft to improve its safety. The loss of the space shuttles Challengerand Columbia will be discussed later.

Space stations

Since the early 1970s, both the Soviet Union and the United States have sought to develop orbiting space stations. The emphasis in each nation has, however, been somewhat different.

The development of a successful space station requires two major feats: the construction of a habitat in space in which humans can live and work for long periods of time (months or years), and the development of a ferry system by which astronauts and materials can be transported from Earth to the space station and back. The Soviets have focused on the first of these two features and the Americans on the second.

Salyut and Mir

The first series of space stations developed by the Soviets was given the codename Salyut. Salyut space stations were 65 ft (19.8 m) long and 13 ft (4 m) wide, with a total weight of about 19 tons. Salyut 1 was launched on April 19, 1971, to be followed by six more vehicles of the same design. Each station was occupied by one or more host crews, each of whom spent many weeks or months in the spacecraft, and a number of visiting crews, who stayed in the spacecraft for no more than a few days. The visiting crews usually contained cosmonauts from nations friendly to the Soviet Union, such as Bulgaria, Cuba, Czechoslovakia, East Germany, Hungary, Poland, and Vietnam (another indication of the propaganda significance of space flight). Between February 8, 1984, and October 2, 1985, Soviet cosmonauts in Salyut 7 set an endurance record of 237 days.

A more advanced Soviet space station, code-named Mir (Peace), was launched on February 19, 1986. The Mir spacecraft was considerably more complex than its Salyut predecessor, with a central core 43 ft (13 m) long and 13.6 ft (4.1 m) wide. Six docking ports on this central core permit the attachment of four research laboratories, as well as the docking of two Soyuz spacecraft bringing new cosmonauts and additional materials and supplies. Living in Mir in 1994–1995, cosmonaut Valeriy Polyakov set the record for longest continuous period spent in space: 438 days. Mir was deorbited in 2001.

Skylab and the space shuttle

Prior to the beginning of construction work on the International Space Station in 1998 (a cooperative U.S.-Soviet-European effort, with the United States doing the greatest share of design, construction, and operation), the only craft comparable to Salyut or Mir was the U.S. space station Skylab, launched on May 14, 1973. The Skylab program consisted of two phases. First, the unoccupied orbital workshop itself was placed into orbit. Then, three separate crews of three astronauts each visited and worked in the space station.

The three crews spent a total of 28, 59, and 84 days in the summer and winter of 1973 and 1974. During their stays in Skylab, astronauts carried out a wide variety of experiments in the fields of solar and stellar astronomy, zero-gravity technology, geophysics and space physics, earth observation, and biomedical studies.

The United States space program has focused less on the construction of space stations, however, and more on the development of a spacecraft that will carry humans and materials to and from orbit. This program has been designated as the Space Transportation System (STS), otherwise known as the space shuttle. Space shuttles are designed to (usually) carry a crew of seven and payloads of up to 65,000 lb (30,000 kg). The spacecraft itself looks much like a blunt-nosed jet airplane with a length of 122 ft (37 m) and wingspan of 78 ft (24 m). Space shuttles are lifted into orbit in combination with a large external fuel tank to which are strapped twin solid rocket boosters; they return to Earth as unpowered gliders.

The first space shuttle, Enterprise, was flow in the atmosphere to prove the glider concept but was never equipped for space flight; it is now a museum piece. The shuttle Columbia was launched into orbit on April 12, 1981, and remained in orbit for three days. Later, four more shuttles—Challenger, Discovery, Endeavor, and Atlantis —were added to the STS fleet.

Challenger and Columbia were later lost in disasters. On January 28, 1986, 73 seconds after takeoff, Challenger exploded, killing all seven astronauts aboard. Research later showed that a failed O-ring gasket had allowed hot gases to escape from one of the shuttle’s solid fuel boosters, causing the large external fuel tank to explode. The Challenger disaster caused NASA to reconsider its ambitious program of 24 shuttle flights every year. Its plans were scaled back an average of 14 flights per year using four shuttle spacecraft. In order to complete this program of launches, the agency placed an order for a replacement for Challenger, named Endeavour, in July 1987.

Then, the Columbia disintegrated during reentry on February 1, 2003, killing seven astronauts, temporarily halting the shuttle program, and imperiling the International Space Station, which depends on fuel delivered by space shuttles to keep its orbit from decaying. Following the disaster, NASA scientists and engineers found that a hole punctured the leading edge of the left wing of Columbia. The hole was made when a piece of insulating foam from the external fuel tank ripped off during the launch. A coating of rigid foam insulation is used to keep the external fuel tank cool.

A Soviet space shuttle program comparable to the U.S. STS effort was codenamed Buran (Blizzard). The first (and last) Buran vehicle was launched on November 15, 1988. Buran closely resembled the U.S. shuttle, but lacked internal engines for launch. The Buran shuttles were intended to act as supply ferries for the Mir and later Russian space stations, but the program was discontinued and all the Burans dismantled.

Soviet-U.S. cooperation in space

For the first decade of space travel, Soviet and American space programs worked in competition. However, planners on both sides of that competition recognized early on the importance of eventually developing joint programs. This recognition led to the creation of the Apollo-Soyuz Test Project (ASTP). The purpose of this project was to make possible the docking of two crewed spacecraft, an Apollo and Soyuz vehicle, in orbit. Once again, the primary purpose of a crewed space-flight project was symbolic rather than scientific—but this time the goal was to demonstrate high-minded cooperativeness of political détente.

In July, 1975, the last Apollo flight docked with a Soyuz spacecraft for a total of 47 hours and 17 minutes. During that time, two Soviet cosmonauts and three American astronauts visited each others” spacecraft and conducted a series of scientific and technical experiments.

In spite of the success of the ASTP, it was nearly two decades before another such meeting occurred. Early in 1995, the American space shuttle Discovery docked with the Russian space station Mir. U.S. astronauts then entered the Russian vehicle and exchanged gifts with their Russian counterparts.

The future of crewed space flight

The ultimate goal of space programs in both the the former Soviet Union and the United States has been the construction of an Earth-orbiting space station, with vague hopes of establishing permanent moon bases, mounting there-and-back-again expeditions to Mars, or even colonizing Mars. Today, all these goals hang on the fate of the International Space Station.

Planning for the International Space Station began in 1984 as the result of a directive by then-president Ronald Reagan. According to original plans, construction of the space station was to have begun in 1995 and to have been completed four years later. However, budget problems in the United States and recessions in much of the rest of the world raised questions about the cost of the project. The space station design became much smaller when in 1993 president Bill Clinton called for it to be redesigned. The new plan considerably reduced the size of the station, and took on international partners in order to further reduce U.S. costs.

In December, 1993 the countries involved in the space station project invited Russia to join them. The Russians agreed. With Russian participation, the space station project underwent some major changes. Russia’s 20-plus years of experience with operating space stations paved the way for the space station to take on a new appearance. The first phase was a series of shuttle-Mir missions. There were nine docking missions between 1995 and 1998, testing the technologies needed to build the space station, examining the space environment, and conducting other experiments. From 1998, the second phase—construction and operation of the station—finally got under way.

The International Space Station (ISS) is NASA’s biggest project since Apollo. Since the mid-1990s, however, NASA has been faced with budget cuts and skepticism about the ISS from its European partners, while Russia, too, has struggled economically. To avoid delays and keep reductions in the scale of the space station project to a minimum, international cooperation has become more important than ever, with 16 countries participating. In January 1998, Russian President Boris Yeltsin (1931–2007) agreed to allocate more funds to the ISS project, easing fears among other project partner countries that Russia was losing enthusiasm for the project.

Construction of ISS began in November 1998 with the launch of the Zarya cargo block from Russia. In 1999, a total of 44 launches were projected to complete the facility in 2004. However, after the Columbia disaster of 2003, construction was halted. It resumed as of 2006. About 80% of the original hardware for the ISS will still be added to ISS. Between December 2006 and January 2010, 15 shuttle flights by NASA’s space shuttle fleet will deliver hardware to ISS. When finished, the ISS will contain about four times as much working space as the former Russian space station Mir (1986– 2001), the former record holder, and will have a mass of about 881,800 lb (400,000 kg) and a pressurized volume of about 35,300 cu ft (1,000 cu m). The ISS orbits at an average altitude of 220 mi (360 km) and at an average speed of 17,200 mi/hr (27,685 km/hr).

Technical requirements of crewed spacecraft

Many complex technical problems must be solved in the construction of spacecraft that can carry people into space. Most of these problems can be classified into three major categories: communications, environmental and support, and reentry.

Communications

Communications refers to the necessity of maintaining contact with members of a space mission, which includes monitoring both their health and the health of the spacecraft in which they are traveling. Direct communication between astronauts and cosmonauts can be accomplished by means of radio and television messages transmitted between a spacecraft and ground stations. To facilitate these communications, receiving stations at various locations around the earth have been established. Messages are received and transmitted to and from a space vehicle by means of large antennas located at these stations.

Various instruments are needed within a spacecraft to monitor cabin temperature, pressure, humidity, and other conditions as well as biological functions such as heartrate, body temperature, blood pressure, and other vital functions. Constant monitoring of spacecraft hardware is also necessary. Data obtained from these monitoring functions is converted to radio signals that are transmitted to Earth stations, allowing ground-based observers to maintain a constant check on the status of both the spacecraft and its human passengers.

Environmental controls

The fundamental requirement of a crewed spacecraft is, of course, to provide an environment in which humans can survive and carry out the tasks required of them. This means, first of all, providing the spacecraft with an Earth like atmosphere in which humans can breathe. Traditionally, the former Soviet Union has used a mixture of nitrogen and oxygen gases somewhat like that found in the Earth’s atmosphere. U.S. spacecraft have traditionally employed a pure oxygen atmosphere at about five pounds per square inch, roughly one-third the normal air pressure on the Earth’s surface; the space shuttles use a mixed nitrogen-oxygen atmosphere.

The level of carbon dioxide within a spacecraft must also be maintained at a healthy level. The most direct way of dealing with this problem is to provide the craft with a base, usually lithium hydroxide, which will absorb carbon dioxide exhaled by astronauts and cosmonauts. Humidity, temperature, odors, toxic gases, and sound levels are other factors that must be controlled at a level congenial to human existence.

Food and water provisions present additional problems. The space needed for the storage of conventional foodstuffs is prohibitive for spacecraft. Thus, one of the early challenges for space scientists was the development of dehydrated foods or foods prepared in other ways so that they would occupy as little space as possible. Space scientists have long recognized that food and water supplies present one of the most challenging problems of long-term space travel, as would be the case in a space station. Suggestions have been made, for example, for the purification and recycling of urine as drinking water and for the use of exhaled carbon dioxide in the growth of plants for foods in spacecraft that remain in orbit for long periods of time.

For hypothetical long-term flights such as a three-year round-trip journey to Mars, planners also worry about psychological factors. A group of astronauts would have to remain psychologically stable throughout such a flight, despite being cooped up in a very small environment with a small group of people for many months. There can be no guarantees in human behavior, but mission planners seek to understand group dynamics, privacy needs, and other factors to maximize the chances that such a long journey, if ever attempted, will not end in disaster because of human factors.

Power sources

An important aspect of spacecraft design is the provision for power sources needed to operate communication, environmental, and other instruments and devices within the vehicle. The earliest crewed spacecraft had fairly simple power systems. The Mercury series of vehicles, for example, were powered by six conventional batteries. As spacecraft increased in size and complexity, however, so did their power needs. The Gemini spacecraft required an additional conventional battery and two fuel cells, while the Apollo vehicles were provided with five batteries and three fuel cells each.

Physiological effects

One of the most serious on-going concerns of space scientists about crewed flights has been their potential effects on the human body. An important goal of nearly every space flight has been to determine how the human body reacts to a zero-gravity environment.

At this point, scientists have some answers to that question. For example, scientists know that one of the most serious dangers posed by extended space travel is the loss of calcium from bones. In addition, the absence of gravitational forces results in a space traveler’s blood collecting in the upper part of his or her body, especially in the left atrium. This knowledge has led to the development of special devices that modify the loss of gravitational effects during space travel.

Redundancy of systems

One of the challenges posed by crewed space flight is the need for redundancy in systems. Redundancy means that there must be two or three of every instrument, device, or spacecraft part that is needed for human survival. This level of redundancy is not necessary with uncrewed spacecraft where failure of a system may result in the loss of a space probe, but not the loss of a human life. It is crucial, however, when humans travel aboard a spacecraft.

An example of the role of redundancy was provided during the Apollo 13 mission. That mission’s plan of landing on the moon had to be aborted when one of the fuel cells in the service module exploded, eliminating a large part of the spacecraft’s power supply. A back-up fuel cell in the lunar module was brought on line, however, allowing the spacecraft to return to the Earth without loss of life.

Space suits

Space suits are designed to be worn by astronauts and cosmonauts during take-off and landing and during extravehicular activities (EVA). They are, in a sense, a space passenger’s own private space vehicle and present, in miniature, most of the same environmental problems as does the construction of the spacecraft itself. For example, a space suit must be able to protect the space traveler from marked changes in temperature, pressure, and humidity, and from exposure to radiation, unacceptable solar glare, and micro-meteorites. In addition, the space suit must allow the space traveler to move about with relative ease and to provide a means of communicating with fellow travelers in a spacecraft or with controllers on Earth’s surface. The removal and storage of human wastes is also a problem that must be solved for humans wearing a space suit.

Reentry problems and solutions

Ensuring that astronauts and cosmonauts are able to survive in space is only one of the problems facing space scientists. A spacecraft must also be able to return its human passengers safely to Earth’s surface. In the earliest crewed spacecraft, this problem was solved simply by allowing the vehicle to travel along a ballistic path back to Earth’s atmosphere and then to settle on land or sea by means of one or more large parachutes. Later, spacecraft were modified to allow

KEY TERMS

Crewed spacecraft —A vehicle designed to travel outside the Earth’s atmosphere carrying one or more humans.

Docking —The process by which two spacecraft join to each other while traveling in orbit.

EVA —Extravehicular activity, a term describing the movement of a human being outside an orbiting spacecraft.

LRV —Lunar roving vehicle, a car-like form of transportation used by astronauts in moving about on the Moon’s surface.

Module —A cabin-like space in a spacecraft, usually part of a larger system.

Orbital flight —The movement of a spacecraft around some astronomical body such as Earth or the Moon.

Redundancy —The process by which two or more identical items are included in a spacecraft to increase the safety of its human passengers.

Space shuttle —A crewed spacecraft used to carry humans and materials from Earth’s surface into space.

Space station —A manned artificial satellite in orbit about the Earth, intended as a base for space observation and exploration.

pilots some control over their reentry path. The space shuttles, for example, can be piloted back to Earth in the last stages of reentry in much the same way that a normal airplane is flown.

Perhaps the most serious single problem encountered during reentry is the heat that develops as the spacecraft returns to the earth’s atmosphere. Friction between vehicle and air produces temperatures that exceed 3,000°F (1,700°C). Most metals and alloys would melt or fail at these temperatures. To deal with this problem, spacecraft designers have developed a class of materials known as ablators that absorb and then radiate large amounts of heat in brief periods of time. Ablators have been made out of a variety of materials, including phenolic resins, epoxy compounds, and silicone rubbers.

Some are beginning to look beyond space shuttle flights and the International Space Station. While NASA’s main emphasis for some time will be unmanned probes and robots—in terms of spacecraft variety, not funding (most of which goes to the manned spaceflight program)—the most tempting target for a manned spacecraft will surely be Mars. Besides issues of long-term life support, any such mission will have to deal with long-term exposure to space radiation. Without sufficient protection, galactic cosmic rays would penetrate spacecraft and astronaut’s bodies, damaging their DNA (deoxyribonucleic acid) and perhaps disrupting nerve cells in their brains over the long-term. (Manned flights to the moon were protected from cosmic rays by the earth’s magnetosphere.) Shielding would be necessary, but it is always a trade-off between human protection and spacecraft weight. Moreover, estimates show it could add billions of dollars to the cost of any such flight.

Future of U.S.-manned missions

NASA expects to retire the space shuttle fleet in 2010 and replace it with Orion (previously known as crew exploration vehicle [CEV]), an Apollo-type vehicle. The Orion will be launched from a new complex at the Kennedy Space Center aboard a new Ares I crew launch vehicle. Orion will fly to the International Space Station on its first few flights, but will eventually be used for missions to the moon and Mars after 2015. When such missions occur, Orion will be teamed up with EDS (earth departure stage) and the LSAM (lunar surface access module) for missions to the moon.

See also Space probe.