Manned Spacecraft

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.

In its earliest stages, crewed space flight was pursued primarily as a conspicuous demonstration of scientific and industrial might. The former Soviet Union and the United States, as rival superpowers, each claimed that their society was superior, and offered their space achievements as proof. The Soviet Union scored a tremendous and, to the U.S., frightening propaganda victory by being the first to orbit a satellite of any kind: the Sputnik I, launched in 1957, which simply orbited the 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 USSR termed its astronauts) before the U.S. orbited John Glenn on February 20, 1962.

Scientists in both nations were also interested in collecting information about the Moon, other planets in our 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, the bulk of whose research is devoted to problems of the long-term human habitation of space.

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 U.S. 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 put 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 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 accidents. 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. 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.

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.

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 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 heart rate, 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 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.