SPACECRAFT

SPACECRAFT

SPACECRAFT, that do not have to carry humans may take a great variety of sizes, from a few centimeters to several meters in diameter, and many shapes, depending on the purposes for which they are designed. Automated spacecraft rely on two-way radio communication systems, both to relay information back to earth and to receive guidance from the control center.

A space vehicle designed for humans must provide air, food and water, navigation and guidance equipment, seating and sleeping accommodations, and communication equipment for sending and receiving information from the control center on earth. Another distinctive feature of human spaceflights is the heat shield that protects the vehicle as it reenters the atmosphere (see below).

The rocket engines that launch and propel spacecraft are of two main types: solid-propellant rockets, which use chemicals that burn in a fashion similar to gunpowder, and liquid-propellant rockets, which use liquid fuels and oxidizers carried in separate tanks. Most of the rockets that have launched American spacecraft have consisted of several separate rocket stages; each stage is separately powered with its own fuel. After the fuel in each stage is consumed, the empty stage drops away from the spacecraft. In 1996 NASA approved construction of the X-33, a new reusable launch vehicle (RLV).

Because the technology to build launch vehicles is closely akin to that for long-range ballistic missiles, the U.S. and the USSR were the only two countries that had the capability to launch satellites from 1957 to 1965. In subsequent years France, Japan, India, and China launched earth satellites of ever-increasing sophistication, and in May 1984 the 13-member European Space Agency began its own launch program from a space center at Kourou in French Guiana. The U.S. and the USSR, however, remained the only nations with launch vehicles capable of placing in orbit payloads (passengers, supplies, equipment, or cargo essential to the flight) of many tons—the prerequisite for human spaceflight.

A space vehicle is launched from a specially constructed launchpad, where the space vehicle and the rocket that propels it are set up and carefully inspected before launching. The operation is supervised by engineers and technicians in the nearby control center. When all preparations are complete, the rocket engines are fired and the rocket and spacecraft lift off.

Reentry is the name applied to the problem of slowing down a returning spacecraft so that it lands on earth without being destroyed by aerodynamic heating. The U.S. Mercury, Gemini, and Apollo programs overcame the problem of reentry by protecting the leading surface of the returning capsule with a specially developed heat shield, made of metals, plastics, and ceramic materials that melt and vaporize during reentry, thereby carrying off or dissipating the heat without damage to the capsule or its astronaut occupants. The heat shield developed to protect the space shuttle during reentry consists of a covering of ceramic tiles individually cemented to the shuttle’s hull. Prior to the development of the space shuttle, which lands on a runway (see Space Shuttle below), all American spacecraft designed to carry humans used the ocean to cushion the impact of landing; the astronauts and the capsules were retrieved quickly by helicopter and taken aboard waiting naval vessels. In the space programs of the Soviet Union and the Commonwealth of Independent States, cosmonauts have landed on solid ground in various sites in Siberia.

The orbit of a spacecraft around the earth may be in the shape of a circle or an ellipse. A satellite in a circular orbit travels at a constant speed. The higher the altitude, however, the lower the speed relative to the surface of the earth. Maintaining an altitude of 35,800 km (22,300 mi) over the equator, a satellite is geostationary. It moves in geosynchronous orbit, at exactly the same speed as the earth, so it remains in a fixed position over some particular spot on the equator. Most communications satellites are placed in such orbits.

In an elliptical orbit, the speed varies and is greatest at perigee (minimum altitude) and least at apogee (maximum altitude). Elliptical orbits can lie in any plane that passes through the earth’s center. A polar orbit lies in a plane passing through the North and South poles; in other words, it passes through the axis of rotation of the earth. An equatorial orbit is one that lies in a plane passing through the equator. The angle between the orbital plane and the equator plane is called the inclination of the orbit.

The earth rotates once every 24 hr under a satellite in a polar orbit. A polar-orbit weather satellite carrying television and infrared cameras, can thus observe meteorological conditions over the entire globe from pole to pole in a single day. An orbit at another inclination covers a smaller portion of the earth, omitting areas around the poles.

As long as the orbit of an object keeps it in the vacuum of space, the object will continue to orbit without propulsive power because no frictional force slows it down. If part or all of the orbit passes through the atmosphere of the earth, however, the body is slowed by aerodynamic friction with the air. This causes the orbit to decay gradually to lower and lower altitudes until the object has fully reentered the atmosphere and burns up, like a meteor.

Artificial Satellites and Space Probes

The long history of myths, dreams, fiction, science, and technology surrounding space travel culminated in the dramatic launching of the first artificial orbiting earth satellite, Sputnik 1, by the USSR on Oct. 4, 1957. Sputnik Zemli, meaning “traveling companion of the world,” is the Russian name for an artificial satellite, a companion of the earth as it travels around the sun. In the U.S. this name was abbreviated to Sputnik.

Early Artificial Satellites

Sputnik 1 was an aluminum sphere, 58 cm (23 in) in diameter, weighing 83 kg (184 lb). It orbited the earth in 96.2 min. The elliptic orbit of the satellite carried it to an apogee of 946 km (588 mi) and a perigee of 227 km (141 mi). The sphere contained instruments which, for 21 days, radioed data concerning cosmic rays, meteoroids, and the density and temperature of the upper atmosphere. At the end of 57 days the satellite reentered the atmosphere of the earth and was destroyed by aerodynamic frictional heat.

The second artificial earth satellite was also a Soviet space vehicle, called Sputnik 2. It was sent aloft on Nov. 3, 1957, with a dog named Laika aboard, and it relayed the first biomedical measurements in space. Sputnik 2 reentered the atmosphere of the earth and was destroyed after 162 days aloft.

While Sputnik 2 was still in orbit, the U.S. successfully launched its first earth satellite, Explorer 1, from Cape Canaveral (named Cape Kennedy 1963–73), Fla., on Jan. 31, 1958. The 14-kg (31-lb) cylindrical spacecraft, 15 cm (6 in) in diameter and 203 cm (80 in) long, transmitted measurements of cosmic rays and micrometeorites for 112 days and gave the first satellite-derived data leading to the discovery of the Van Allen radiation belts.

On March 17, 1958, the U.S. launched its second satellite, Vanguard 2; a precise study of variations of its orbit showed that the earth is slightly pear-shaped. Using solar power, the satellite transmitted signals for more than six years. Vanguard 2 was followed by the American satellite Explorer 3, launched on March 26, 1958, and by the Soviet satellite Sputnik 3, launched on May 15. The 1327-kg (2925-lb) Soviet spacecraft measured solar radiation, cosmic rays, magnetic fields, and other space phenomena until the craft’s orbit decayed in April 1960.

Lunar Probes

As the closest neighbor of the earth, the moon has been the objective of many space missions. In 1958 the first attempts by the U.S. and the USSR at lunar probes failed. The Russian Luna 2, launched Sept. 12, 1959, hit the moon 36 hr later. Since that date many moon shots have been made by both countries, with mixed results. The first photographs of the far side of the moon were taken by Luna 3, which was launched by the USSR on Oct. 4, 1959. One of the most dramatically successful moon shots was the mission accomplished by Ranger 6, launched by the U.S. on July 28, 1964. Just before hitting the side of the moon that faces the earth, it transmitted 4316 television pictures of the lunar surface from altitudes of about 1800 km (about 1120 mi) to about 300 m (about 1000 ft), giving earth-bound humans their first close-up view of the moon.

On Jan. 31, 1966, the USSR launched Luna 9, which made the first soft landing on the moon; that is, it landed without being destroyed. The U.S. followed with Surveyor 1 on May 30, which also made a soft landing on the lunar surface. It sent back to earth 11,150 close-up photographs of the moon.

Aside from the scientific information that was gathered, much of the interest of the lunar missions centered on the American program to land an astronaut on the moon. To this end a number of further automated moon flights were undertaken, among which were two soft landings made by Surveyor 3 and 5 in 1967. Both craft, after taking about two days for their journeys, sent back to earth a large number of television pictures of the lunar surface. Surveyor 3 picked up samples of lunar soil and examined them by television camera. Surveyor 5 chemically analyzed the lunar surface, using an alpha-particle scattering technique; this was the first on-site analysis of an extraterrestrial body.

Another spacecraft that contributed to future lunar landings was the Lunar Orbiter. In 1966 and 1967 five Lunar Orbiters circled the moon, relaying thousands of photographs to earth. From these photographs, landing sites were selected for the Apollo moon-landing program.

Two other automated lunar projects by the USSR are noteworthy. The Luna 16 spacecraft, launched Sept. 12, 1970, landed on the moon and placed about 113 gr (about 4 oz) of lunar soil in a sealed container that was then launched from the moon and recovered in the USSR. Luna 17, launched Nov. 10, 1970, softlanded an automated lunar-roving vehicle, Lunokhod 1, equipped with a television camera and solar batteries. During ten lunar days the vehicle, controlled from the earth, traveled 10.5 km (6.5 mi) on the moon, relaying television pictures and scientific data. Luna 21 in 1973 repeated this performance, placing Lunokhod 2 on the moon.

The U.S. spacecraft Lunar Prospector, launched in January 1998 on a mission to orbit the moon for at least a year, was designed to map the composition of the moon’s surface. NASA’s first lunar mission in 25 years, the probe detected water ice at the lunar poles, confirming an indication by the U.S. Defense Department’s Clementine probe in 1994.

Scientific Satellites

As space-launch vehicles (rocket boosters) and scientific measuring devices became more reliable, numerous types of satellites were developed to enable scientists to obtain data and make accurate studies of the sun, other stars, the earth, and space itself. The enveloping atmosphere of the earth prevents such data from being obtained from the earth’s surface except in a limited way through the use of high-altitude balloons.

In the U.S. many astronomical satellites have been launched. Beginning in 1962, for example, the Orbiting Solar Observatories (OSO) have studied the sun’s ultraviolet, X-ray, and gamma radiation. Pioneer satellites have studied cosmic radiation, the solar wind, and electromagnetic characteristics of space. The Orbiting Astronomical Observatories (OAO) have observed stellar radiation, and Orbiting Geophysical Observatories (OGO) have studied the relationships between the sun, the earth, and their space environment. The Infrared Astronomical Satellite (IRAS), an Anglo-American project launched in 1983, has probed the hidden reaches of our galaxy. Even richer scientific rewards were returned from the Hubble Space Telescope, launched by the space shuttle Discovery in 1990, after it was repaired in 1993.

Applications Satellites

Automatic communications, earth-survey, and navigation satellites are all classified as applications satellites. Earth-survey satellites observe the earth and transmit, or return to deliver, photographs for a variety of purposes. A weather satellite provides daily transmissions of temperatures and cloud patterns. One type is the Synchronous Meteorological Satellite (SMS). From stationary orbit, it sends pictures of a large area of the earth’s surface at 30-min intervals. Two SMS’s can cover the entire U.S. and adjacent ocean areas.

The U.S. Earth Resources Technology Satellites (ERTS), now called Landsats, observe the earth through multispectral optical filters and transmit the data to ground stations. Processed into color photographs, these pictures reveal data of great range and great potential value. Specific soil characteristics, water and ice quantities, coastal-water pollution, salinity, and insect blights of crops and forests are obtained. Even forest fires can be detected from earth orbit. Study of folds and fractures in the earth’s crust helps geologists to identify deposits of oil and minerals. SPOT (Système Probatoire d’Observation de la Terre), a French satellite launched in 1986, transmits images that show the earth in even greater detail than Landsats can.

Earth observation satellites are used by the U.S. and other countries to obtain photographs of military value, such as detection of nuclear explosions in the atmosphere and in space, ballistic-missile launch sites, and ship and troop movements. In the 1980s controversy was aroused by the U.S. proposal to develop a satellite antiballistic missile defense system making use of laser technology.

Navigation satellites provide an observation point orbiting the earth that, when observed by ships and submarines, can fix the vessel’s position within a few yards. The U.S. Navy Transit Satellite is available to commercial maritime fleets that can measure the Doppler shift of the satellite’s signal, and a complex global positioning system of Navstar satellites became fully operational in 1995 for military and commercial use.

Planetary Studies

Beyond the moon, spacecraft have landed on Mars and Venus, have flown by Mercury, Jupiter, Saturn, Uranus, and Neptune, and have made comet studies. In the U.S. space program, radio communications with interplanetary spacecraft are handled through NASA’s Deep Space Network, the world’s most precise radio navigation system.

Mars

The USSR launched Mars 2 and 3 in May 1971, two probes crash-landing on Mars but transmitting data briefly. In August 1973 the USSR launched Mars 4, 5, 6, and 7, but various technical malfunctions plagued all of the missions. In 1988 the USSR sent two probes, Phobos 1 and 2, to land on the Martian moon Phobos; the first was lost through human error, and the second dropped out of radio contact after relaying back some data and photographs.

In the U.S. program, Mariner 9 was launched in May 1971, orbited Mars from November 1971 to October 1972, and transmitted enough photographs for a nearly complete map of the planet. In August and September 1975 Viking 1 and 2 began an 11-month journey to Mars. Each spacecraft carried a lander equipped with life-detecting and chemical laboratories, two color television cameras, weather and seismographic instruments, and a 3-m (10-ft) retractable claw designed to be manipulated from the earth. Both functioned well for several years.

In September 1992, NASA launched Mars Observer, the first U.S. mission to Mars since the Viking program. Mission controllers lost contact with the spacecraft in August 1993 as it prepared to enter Mars orbit. Both the U.S. and Russia launched Mars probes in late 1996. Launched in November, the Russian spacecraft Mars 96, consisting of an orbiter and four landers, spun out of control and fell apart over the Pacific Ocean when one of its booster rockets failed.

The U.S. inaugurated a planned 10-year Mars exploration program with the successful launching in November 1996 of Mars Global Surveyor, which entered planetary orbit in September 1997; and of Mars Pathfinder in December, which descended to the surface in July 1997 (the first spacecraft to reach the surface of Mars without orbiting prior to landing) and released a robot rover to explore the Martian terrain. Pathfinder and the robotic explorer, a small vehicle named Sojourner that uses laser technology to navigate, gathered data that provided unprecedented details on the climate, atmosphere, and geology of Mars, including three-dimensional panoramic photographs. NASA released images of Mars’s rocky terrain, and announced the results of the first chemical analysis of a rock carried out on Mars by Sojourner.

Venus

The first flyby by U.S. spacecraft was in 1962 by Mariner 2 and followed by Mariner 5 in 1967 and Mariner 10 in 1974. The Soviet program to penetrate the dense, cloud-covered atmosphere of Venus met with great success. Venera 7 was launched in August 1970 and survived long enough to transmit 23 min of temperature data. Venera 8, launched in 1972, transmitted surface data that included soil analysis. In October 1975 Venera 9 and 10 placed landers on the surface; both survived for an hour and relayed the first photographs of the Venusian surface. In 1978 Venera 11 and 12 released probes that landed on Venus on December 25 and 21, respectively. Both probes recorded a pressure of 88 atmospheres and a surface temperature of 460° C (860° F). On March 1 and 5, 1982, Venera 13 and 14 landed on Venus. The craft relayed photographs of the planet’s surface and analyzed the chemical composition of the atmosphere and soil. On Oct. 10 and 14, 1983, Venera 15 and 16 entered orbit around Venus and returned radar images; and in June 1985, Vega 1 and 2, en route to Halley’s comet, released four probes into the Venusian atmosphere.

The U.S. Pioneer Venus 1, an orbiter, and 2, consisting of five atmospheric probes, were launched on May 20 and Aug. 8, 1978, and reached Venus on Dec. 5 and 9, 1978. The orbiter mapped nearly the entire surface of Venus, and the probes analyzed the composition and movement of the atmosphere and its interaction with the solar wind. The Magellan probe was launched toward Venus from a space shuttle in 1989 and began transmitting pictures of the planet in August 1990. The craft orbited Venus more than 15,000 times and used radar to map about 98 percent of the planet’s surface before it made a fiery descent into the planet’s atmosphere in October 1994.

Mercury

The planet nearest the sun came under scrutiny when the U.S. sent Mariner 10 on a journey through the inner solar system in October 1973. The spacecraft passed Venus in February 1974 and used its gravity to enter a solar orbit. In March it came within 692 km (430 mi) of Mercury, providing the first views of the planet’s moonlike cratered surface. On its second encounter with Mercury in September, the spacecraft detected a totally unsuspected magnetic field. On its third and final encounter in March 1975 Mariner 10 came within 317 km (197 mi) of the planet.

Jupiter and Saturn

The U.S. Pioneer 10 and 11 spacecraft, launched in 1972 and 1973, passed safely through the unexplored asteroid belt beyond the orbit of Mars and flew by Jupiter in December 1973 and December 1974. The two 258-kg (570-lb) spacecraft passed the planet at a distance of 130,400 km (81,000 mi) and 46,700 km (29,000 mi), and Pioneer 10 continued on its way out of the solar system, the first spacecraft ever targeted toward interstellar space; it had traveled 10.28 billion km (6.39 billion mi) when the mission was officially ended, because of weakening transmission, in March 1997. Pioneer 11 traveled by Saturn in September 1979, preparing the way for Voyager 1 and 2.

Launched in 1977, the spectacularly successful Voyager 1 and 2 encountered the Jovian system in March and July 1979 and took a variety of measurements and photographs. The spacecraft then flew by the Saturnian system in November 1980 and August 1981. By 1998, 21 years after launch, Voyager 1 had surpassed Pioneer 10 ’s distance record and was still transmitting data.

Galileo, the first spacecraft to orbit one of the outer planets, was launched in May 1989, flew by Venus in February 1991, and on its approach to Jupiter in July 1995 released a probe that reached the planet in December. As it plunged through the Jovian atmosphere, the probe relayed its observations back to earth via the main craft, which then went into orbit around Jupiter.

In October 1997 the Cassini spacecraft was launched by NASA, the European Space Agency, and several other partners on a mission to Saturn and its moon Titan. Cassini is expected to pass Venus (twice) and Jupiter before reaching Saturn in 2004; it is then scheduled to explore the Saturnian system for at least four years.

Uranus and Neptune

After flying past Saturn, Voyager 2 was directed toward Uranus. It passed within 80,000 km (50,000 mi) of the cloud-covered planet in January 1986, discovering four more rings as well as ten new moons. The spacecraft came even closer to one of the moons, Miranda, transmitting startling pictures of that icy body. Voyager 2 then headed for Neptune, flying within 5000 km (3100 mi) of the planet in August 1989 and discovering six additional Neptunian moons before continuing its journey toward interstellar space.