Space exploration

Space exploration may be defined as the investigation, by means of spacecraft, of all the reaches of the universe beyond the atmosphere of the Earth. Spacecraft, vehicles that operate above the Earth's atmosphere, include sounding rockets, Earth satellites, and lunar, planetary, and deep space probes.

On October 4, 1957, the Soviet Union launched the world's first artificial satellite, Sputnik 1, and set in motion a series of programs of space exploration by the United States and the Soviet Union. The first U.S. satellite, Explorer 1, was launched on January 31, 1958, not quite four months after Sputnik 1. Both nations participated during the next decades in a space race, with more than 5,000 successful launches of satellites and space probes of all varieties: scientific research, communications, meteorological, photographic reconnaissance, and navigation satellites, lunar and planetary probes, and manned space flights. The Soviet Union launched the first man into orbit around the Earth on April 12, 1961. On July 20, 1969, the United States landed two men on the surface of the Moon. On April 12, 1981, the 20th anniversary of manned space flight, the United States launched the first reusable manned vehicle, the Space Shuttle. 

Many of the spacecraft, such as manned and reconnaissance vehicles, are designed for recovery. Most operational satellites become inert after a few months or years of operation. The North American Air Defense Command (Norad) keeps a constant watch on the thousands of objects of human origin circling the Earth in a variety of orbital paths. Both radar and optical telescopes are used. In addition to satellite payloads is a much larger number of objects classified as debris--spent upper stages of launch vehicles, tether, cables, etc., that go into orbit along with functioning satellites, as well as fragments that result from in-space explosions. Eventually low-altitude debris reenters the Earth's atmosphere and usually burns up.

THE RAMIFICATIONS OF SPACE EXPLORATION

It is not yet possible to judge fully the value of the new knowledge of the Earth, planets, and solar system that has been gained through space endeavours. A wealth of new understanding and of new scientific puzzles has resulted. Many questions concerning the nature and origin of the cosmos have been answered, and many more have been raised. Until 1957, human beings were passive observers of the solar system. Now they have entered the realm of space and gathered firsthand knowledge of their planetary environment. Twelve men have been landed on the Moon, set up data-gathering experiments on the lunar surface, and brought back lunar rocks and soil for study and analysis. The hidden side of the Moon and the planets Mercury, Venus, Mars, Jupiter, and Saturn have been photographed and studied, and probes are scheduled to fly past both Uranus and Neptune by the late 1980s. From the vantage point of Earth orbit, the Earth and its weather have been studied and communications relays and navigation aids have been established. Space stations have been placed in orbit with crews to observe and perform experiments for up to seven months.

Doing these things required new materials and a major expansion of technology and manufacturing techniques. Reliability goals far greater than ever before were set and exceeded. The functional reliability demanded in launch vehicles having 6,000,000 component parts--a reliability greater than 99.9999+--had never before been required. Whole new industries based on these new technologies have become established.

There have been numerous other such "spin-offs," as, for example, the application in medical studies of techniques used for extracting data from photographic images of the Earth and the other planets. Other space technologies have been adopted by business and industry and converted into commercial products. These include freeze-dried foods originally developed for the U.S. manned Apollo missions and insulation for supertankers that transport liquefied natural gas derived from a material employed in the upper stages of the Saturn V launch vehicle to prevent liquid hydrogen from boiling. Yet another important spin-off is Emergency Management Computer-Aided Training (EMCAT), a system based on training techniques used in preparing spacecraft crews for all possible in-flight emergencies. The EMCAT system, which is designed to teach people how to make crucial decisions rapidly under pressure, has been used to train fire fighters by simulating situations that they would encounter during real fires.

PREHISTORY TO SPUTNIK 1

The dream of flight into space is as old as astronomy. Once people learned that the lights in the night sky were actual bodies a long way off, they felt an urge to travel to them. Tales of fanciful flight to the Earth's nearest celestial neighbour, the Moon, may be traced back to the 2nd century AD, when the Greek rhetorician Lucian wrote a satirical account of such a fantastic journey.

But hundreds of years were to pass before the motions of the planets about the Sun and the immense distances to be travelled were appreciated. Awareness of the universe evolved slowly until tools of observation were developed. In the 17th century Galileo's use of the telescope to study the Moon added greatly to contemporary knowledge. Within a few weeks he had mapped the major visible mountains and valleys of the Moon and concluded that it was a solid world. Turning his telescope to the planet Jupiter, he discovered four tiny specks that passed slowly across its face. Galileo deduced that these moving dots were moons of the planet. He postulated further that their small apparent size in comparison to that of the Earth's Moon was explained by the great distance to Jupiter. Thus for the first time a scale of magnitude was established.

Further observation with the telescope confirmed the pattern of the solar system described by Copernicus a hundred years earlier. Johannes Kepler calculated the elliptical orbits of the planets. Later in the 17th century, Isaac Newton formulated his "laws of motion," which at last placed physics and astronomy on a firm theoretical foundation. Not until nearly a hundred years later were the first free balloon ascents made and the journey upward and away from Earth begun.

Three brilliant men, destined to be called rocket pioneers, were the first scientists to conceive pragmatically of space flight: the Russian Konstantin Eduardovich Tsiolkovsky, the American Robert Hutchings Goddard, and the German Hermann Oberth. Technology in the early 20th century, however, was a long way from the level required for rocket-powered flight. Nonetheless, the theory and dynamics of such flights were rigorously studied. By the end of World War II, the German development of rocket propulsion for aircraft and guided missiles (notably the V-2) had reached a high level. With the German surrender in 1945, the United States as well as its Allies--Great Britain, France, and the Soviet Union--fell heir to the technical knowledge of rocket power developed by the Germans. The technical director of the German missile effort, Wernher von Braun, and some 150 of his top aides surrendered to U.S. troops. Most emigrated to the United States, where they assembled and launched V-2 missiles that had been captured and shipped there. The Soviet Union carried out an unpublicized but extensive program that must have been very similar; Britain and France conducted smaller programs.

In both the United States and the Soviet Union the development of military missile technology was essential to the achievement of satellite flight. Preparations for the International Geophysical Year (IGY, 1957-58) stimulated discussion of the possibility of launching artificial Earth satellites for scientific investigations. As a result, the planning committee for the IGY in 1954 passed a formal resolution calling attention to the desirability of using artificial satellites in the IGY program. Both the United States and the Soviet Union responded with announcements that they would prepare scientific satellites for launching during the IGY. While the United States was still developing a satellite launch vehicle, the Soviet Union startled the world by placing Sputnik 1 in orbit.

The launch of Sputnik 1 was followed a month later by Sputnik 2 carrying a live dog named Laika, catching millions by surprise. The failure by the United States to launch its small (two kilograms) payload on December 6, 1957, heightened that nation's political discomfiture in view of its supposed advanced status in science. Following debates on the necessity to achieve parity, the U.S. government established the National Aeronautics and Space Administration (NASA) in 1958. Since that time, NASA has conducted virtually all major aspects of the U.S. space program.

ELEMENTS OF SPACE FLIGHT

The environment of space.

Space, as considered here, may be defined as all the reaches of the universe beyond the atmosphere of the Earth. There is no definitive boundary of the atmosphere of the Earth. For convenience it may be considered to extend to an altitude of 160 kilometres above the Earth's surface. Such a distance is infinitesimal in comparison with the immensity of the universe. Even within the solar system, distances between planets are measured in tens of millions of kilometres. The distance to Pluto, the Sun's outermost planet, is more than 4,830,000,000 kilometres. While the planets and their satellites all travel at different speeds around the Sun, the solar system itself is travelling through our galaxy, the Milky Way. The Earth's nearest neighbouring stars lie more than four light-years (approximately 40,000,000,000,000 kilometres) away.

This apparent unimaginable void of space, however, is not empty. Throughout these vast reaches, matter (largely hydrogen) is scattered at the extremely low density of perhaps 100 particles per cubic centimetre in interplanetary space and 10 particles per cubic centimetre in interstellar space. This is, however, a much greater vacuum than has been achieved on Earth. Additionally, space is permeated by gravitational fields and a wide spectrum of electromagnetic radiation, by cosmic rays and magnetic fields of unknown intensities and distributions. Until 1946 all deductions about space had been made from observations through the distorting atmosphere of the Earth. With the advent of high-altitude sounding rockets after World War II and the instrumented satellites and probes of the space age, it has been possible to discover firsthand the great complexities of space phenomena.

Basic considerations in spacecraft design.

Spacecraft is a general term that includes sounding rockets, artificial satellites, and space probes. They are considered separately from the rocket-powered space launch vehicle, which gives escape velocity to the craft. A space probe is a spacecraft that is launched at higher than Earth orbital velocity and escapes the Earth's gravitational attraction.

Space probes may be classed as lunar, planetary, or deep-space. Other classifications of spacecraft are manned or unmanned, active or passive. A passive satellite transmits no radio signals. It may be tracked optically or with radar, and radio communications signals may be "bounced" off its surface. Active satellites send out radio signals to make tracking easier and to transmit data from their instruments to ground stations or other craft.

One other general differentiation of satellites is by function: scientific or applications. A scientific satellite carries instruments to obtain scientific data on magnetic fields, space radiation, the Sun or other stars, etc. Applications satellites have utilitarian tasks; examples are Earth survey, communications, and navigation satellites.

Spacecraft thus differ greatly in size, shape, complexity, and purpose. Because more than 5,000 spacecraft have been launched since 1957, it is convenient to group them into program families--e.g., the Soviet Sputnik, Vostok, Soyuz, and Venera; and the U.S. Explorer, Intelsat, Apollo, Voyager, and Space Shuttle.

Lightness of weight and functional reliability are primary features of spacecraft design. Depending upon their mission, spacecraft may spend minutes, days, months, or years in the environment of space. Mission functions must be performed while exposed to high vacuum, extreme variations in temperature, and radiation.

There are nine general categories of subsystems found on most spacecraft. They are (1) power supply; (2) on-board propulsion; (3) communications; (4) attitude control (i.e., maintaining a spacecraft's orientation toward a specific direction and pointing precisely at selected targets); (5) environmental control (e.g., regulation of temperature and pressure and removal of toxic substances); (6) guidance and velocity control; (7) computer and auxiliary hardware; (8) structure (skeleton framework of the spacecraft that physically supports all other subsystems); and (9) engineering instruments that monitor the status of the spacecraft.

Launching into space.

Gravity.

The gravitational attraction of the Earth was one of the major obstacles to space flight. Rocket pioneers understood the principles of space flight based upon the observations and calculations of Copernicus, Galileo, and Kepler and on Newton's universal laws of motion; but the use of these principles had to await the development of rocket power to project a spacecraft to the required altitude above the Earth and to give it the velocity necessary to accomplish its mission.

The spacecraft at the forward end of a multistaged launch vehicle is projected upward by reaction to the high-speed jet of combustion gases produced in the rocket motor. If the lifting force, or thrust, of a rocket-powered launch vehicle is twice the weight of the vehicle at lift-off, then the spacecraft assembly will rise at an initial acceleration of one g, or 9.8 metres per second, each second. Because the propellant mass is being consumed and ejected from the rocket motor, the vehicle is continually lightened and acceleration increases.

The gravitational pull of the Earth on the upward (and outward) travelling spacecraft subsides slowly. At an altitude of 160 kilometres, the gravitational attraction is just 5 percent less than at the Earth's surface, and at 2,700 kilometres it is halved (4.9 metres per second per second). For the purposes of space flight, the gravitational pull of the Earth becomes negligible only at distances of several million kilometres, except when a spacecraft approaches the Moon and lunar gravity (one-sixth that of Earth) exerts effect.

Staging.

This term is used to describe a technique by which several rocket propulsion systems are mounted one on top of the other. The lowest, or first, stage ignites and lifts the vehicle at increasing velocity until its propellants are exhausted. At that point the stage drops off and a second stage is ignited. The empty propellant tankage and vehicle structure are cast off, thus lightening the weight of the launch vehicle. The second stage of lower thrust then begins to accelerate the launch vehicle and spacecraft, commencing at the velocity reached under first-stage power. Most space launch vehicles have three stages.

Acceleration rates.

In general, the longer it takes a space vehicle to leave the Earth's atmosphere and achieve required velocity, the less economical the procedure becomes. At low accelerations the launch vehicle wastes great quantities of rocket propellant because it is losing, in effect, about 10 metres per second of velocity each second of travel. An upper limit of acceleration is governed by the maxima of accelerative stress permissible upon the vehicle structure and the spacecraft payload. In manned flight a gravity pull of six g is considered the maximum tolerable when the human body is positioned transversely with respect to the acceleration force (g-loading); i.e., with the head and heart at the same level.

Flight trajectories.

There are four general types of trajectories: sounding rocket, Earth orbit, Earth escape, and planetary.

Sounding rockets.

The sounding rockets first launched for upper atmosphere studies in 1945 were fired nearly vertically. Generally single-staged, they reached speeds of 4,800 to 8,000 kilometres per hour. Burnout, the completion of rocket motor firing, usually occurred at altitudes of 16 to 32 kilometres, at which point the rocket coasted upward, slowly losing speed because of gravity. Velocity upward dropped to zero at peak altitude, and the rocket began its fall, picking up speed until it crashed into desert or ocean. Instrumentation carried aboard for data gathering was sometimes ejected to descend by parachute, but increasingly the data were radioed (telemetered) to a tracking station at the launch site. Altitudes of 160 kilometres were reached with single-stage rockets.

Earth orbit.

Earth orbital flight is achieved by launching vertically and then tilting the trajectory so that flight is parallel to the Earth's surface at the time that orbital velocity at the desired altitude is reached. At this precise point, the rocket engine is cut off. A spacecraft attached to the final-stage rocket is then in free-fall about the Earth, the centrifugal pull on the spacecraft being equal to the Earth's pull of gravity. At an altitude of 200 kilometres, Earth orbital velocity is about 29,000 kilometres per hour. Since this 200-kilometre altitude is above most of the atmosphere, aerodynamic drag is not great, and the spacecraft will continue to orbit for an extended time.

The length of time required for the satellite to make one complete revolution is known as the period of orbit. At 200 kilometres this is about 90 minutes. At higher altitudes than this above the Earth, the velocity of a satellite decreases and the orbital period increases.

For example, at an altitude of 1,730 kilometres the orbital velocity is 25,400 kilometres per hour and the period is two hours. At 35,700 kilometres the velocity is 11,300 kilometres per hour and the period 24 hours. Because this particular period is equal to the time the Earth rotates once, such a satellite travels at the same angular velocity as the surface of the Earth and appears to be stationary in the sky. This particular orbit, called geostationary (or geosynchronous), has special value to communications and meteorological satellites. Finally, the Moon travels in an orbit of about 386,000 kilometres. Lunar velocity about the Earth is approximately 3,700 kilometres per hour, with a period of about 28 days. 

The above applies only to circular orbit, often ideal, but difficult to achieve. Usually a satellite's orbit is an ellipse with a perigee altitude and an apogee altitude. (Perigee and apogee are the points of the orbit nearest and farthest from the body orbited.) Orbits may be made more nearly circular if thrust is available by reducing the velocity at perigee (apogee is lowered) or by increasing the velocity at apogee (perigee is raised). Rocket power in such instances is applied along the axis of flight paths.

In projecting a satellite into Earth orbit, the launch vehicle is tilted after lift-off in an easterly direction. Launching to the east is done to take advantage of the Earth's eastward surface velocity. This rotational surface velocity is about 450 metres per second at the Equator and 400 metres per second at the latitude of Cape Canaveral, Florida. It would be possible to launch a satellite on a westerly orbit, but an additional velocity of 600 metres per second would be required to achieve an orbit of the same altitude compared with an easterly orbit.

If the satellite is launched in a northerly or southerly direction, a polar orbit is obtained. The easterly surface velocity launch advantage is lost, but there are other advantages. As the Earth turns on its polar axis, the satellite travels over all parts of the globe every few revolutions. This ground track, the path around the Earth directly under the satellite, thus varies according to the orbit, which is chosen according to the desired characteristics of a particular mission. Ground-tracking and data-receiving stations must be on or near the ground track.

Earth escape.

In order to escape completely from the Earth's gravity, a spacecraft launch velocity of about 40,000 kilometres per hour is required. At that speed the spacecraft is able to reach distances effectively beyond the Earth's gravitational field. If the spacecraft does not come under the gravitational influence of other celestial bodies, it will go into an orbit about the Sun like a tiny planetoid. With precise timing it is possible for the spacecraft to enter a trajectory that will carry it near the Moon. In the case of eight Apollo flights, the spacecraft was placed in a trajectory calculated to pass ahead of the Moon and, under the influence of lunar gravity, to swing around the far side. If no manoeuvre was made the spacecraft would loop around the Moon and return on a trajectory toward the Earth. By reducing flight speed on the far side of the Moon, the craft was placed in lunar orbit held by lunar gravity.

The so-called three-body problem of mechanics (the relative motions of the Earth, the spacecraft, and the Moon) is extremely complex. Although equations expressing the relative motions can be written, no general solution was possible before the development of the high-speed digital computer for ballistics calculations for long-range missile trajectories. The computer integrates the complicated equations of motion numerically upon command, provides direct readout of the complete translational motions of the spacecraft, and compares the actual to the preplanned flight path at any point in time.

Planetary flights.

Because of the elliptical nature of planetary orbits, distances to Earth's nearest neighbours, Venus and Mars, vary. A so-called favourable launch opportunity occurs about every two years. Flights can be made at other times, but the velocity required is greater and length of time is longer, or, for the same launch vehicle, the payload carried must be lighter in weight. 

The trajectory from the Earth to Venus or Mars may be planned to take advantage of the changing orbital relationships of the planets for the most economical flight. Such advantageous paths were calculated in the late 1920s. Although these transfer trajectories require the least velocity, they are of long duration, as long as 260 days to Mars. Thus a compromise trajectory is used, as in the case of Mariners 6 and 7 in 1969. Launched on February 25, 1969, Mariner 6 passed within 3,410 kilometres of the planet 157 days later, when Mars was 92,800,000 kilometres distant from the Earth. This is typical of simple Hohmann transfers. Some trajectories use the fall into a planet's gravitational field to transfer momentum from the planet to the spacecraft, thereby increasing its velocity. This technique was used by various U.S. planetary probes launched during the 1970s: Mariner 10 to fly past Mercury, and Pioneer 11 and Voyagers 1 and 2 to travel from Jupiter to Saturn.  

Navigation, docking, and recovery.

Navigation.

Travelling from point A to point B in space is almost never in a straight line or at constant velocity because of the many influences on the body in motion. The basis for space navigation is inertial guidance--i.e., guidance based on the inertia of a spinning gyroscope, irrespective of any external forces and without reference to the Sun or stars. By the use of three gyroscopes and accelerometers, precise measurements may be made of any change of velocity (acceleration), either positive or negative, along any or all of the three axes. By use of a memory system in the computer it is possible to determine where the vehicle is at any time along a trajectory. By changing attitude (rotation about one or more axes) and firing a small jet or rocket motor, corrections may be made to the trajectory. Preprogrammed computers, both on the ground and in larger spacecraft, automatically or on command "keep an eye on" where the spacecraft is, where it was, and where it is supposed to be going. Direct readouts of such data from the computers are displayed to the crew in manned spacecraft.

During the launch phase, corrections to deviations in planned flight path are usually made at once by special small thrust motors, by deflection of the rocket exhaust jet, or by swinging one or more of the rocket engines in a gimbal mount.

In the case of rendezvous between two spacecraft, radar sightings fed to the computer tell the crew the corrections required along each of the three axes in metres per second. In the case of the Apollo lunar missions, deviations encountered were stored in a computer memory system until requested. One or more midcourse manoeuvres might be made. With the implementation of the Navstar Global Positioning System in the 1980s, it became possible for spacecraft to verify their locations to within a few metres, and their speeds to within a few metres per second.

The above description of navigation techniques is oversimplified; the systems used are complex, powerful tools without which manned or unmanned spacecraft would not be feasible.

Rendezvous and docking.

Rendezvous is the action of one spacecraft approaching another spacecraft. Rendezvous in space operations refers to a match of orbital trajectories and the action of one spacecraft arriving in proximity to another. Ideally, the orbit planes should be the same. The term is relative but, generally speaking, rendezvous distance should be within 100 metres. To achieve rendezvous, the launch of the second spacecraft is timed within a fraction of a second. Since the orbiting spacecraft already has a high velocity and the second spacecraft is at rest (zero velocity) before launch, the second spacecraft is launched well before the first spacecraft passes overhead. The aim is for a coplanar orbit just below the first spacecraft. When this is done, the first spacecraft is travelling at a slower speed, being at a higher orbit; thus the second spacecraft will overtake the first. When it is slightly ahead of the first spacecraft, application of rocket thrust causes the second spacecraft to rise in orbit and thus to slow down, matching the first spacecraft's orbital altitude and velocity. Radar systems and on-board computers are necessary for such operations. Gemini 6 and 7 were the first spacecraft to rendezvous with each other. In Apollo lunar landing flights the ascent stage of the Lunar Module rose from the surface of the Moon to rendezvous and dock with the orbiting Command Module.

Docking, the meeting and mating of two spacecraft, was an essential element of the Apollo lunar landing (lunar-orbital rendezvous) technique and is crucial for various future manned missions, such as a permanent Earth-orbiting space station. Significant in docking manoeuvres, from the starting point of rendezvous, are precise radar data (distance and closing velocity rate), attitude control, and computer subsystems. Whereas the United States has relied on on-board crew guidance for close rendezvous and docking, the Soviet Union has demonstrated the ability to perform this task automatically.

The concept of building a large space station out of component parts brought to orbit in successive stages or, similarly, assembling a deep-space mission as for a manned trip to Mars requires a reliable rendezvous and docking technique. The entire vehicle and propellants cannot be launched into Earth orbit at one time because of limitations on space launch vehicle payload. Further, rotation of space crews and emergency rescue missions require rendezvous and docking capability.

Reentry and recovery.

Reentry means the return of a spacecraft into the Earth's atmosphere. This blanket of relatively dense gas surrounding Earth is useful as a braking, or retarding, force resulting from aerodynamic drag. A concomitant effect, however, is rapid and severe frictional heating caused by rapid flow of nitrogen and oxygen molecules along the blunt forward profile of the spacecraft. Initially heatshields were made of ablative materials that carried the heat of reentry, but the Space Shuttle introduced refractory materials--silica tiles and a reinforced carbon-carbon material--that withstand the heat directly.

Inherent in safe reentry of a spacecraft is precise control of the angle of reentry. This angle of trajectory with respect to the Earth's horizon is -6.2 and is held within limits of +/-1 . A returning Apollo Command Module approached the Earth at nearly 40,000 kilometres per hour. If the angle is too shallow, the spacecraft will skip or bounce off the atmosphere out into space probe trajectory. If the angle of reentry is too great, the heat shield will not survive the extreme heating rates nor the crew and spacecraft the high g-forces. Even so, the heat shield of the Apollo Command Module was subjected to temperatures approaching 3,000 C.

The U.S. Air Force recovers small spacecraft by aircraft while the spacecraft is still descending to Earth by parachute. This air-snatch technique eliminates the problem of search and recovery of spacecraft after landing.

During the final phases of descent, manned spacecraft, as, for example, the Soviet Soyuz craft, may also deploy parachutes, which lower the vehicle to a soft landing. The Apollo Command Module employed this technique, but, unlike the Soyuz spacecraft that soft lands on the ground (the Siberian plains), the Apollo craft parachuted to a splashdown in the ocean (see photograph). The reentry procedure of the Space Shuttle differs markedly: the vehicle descends by gliding and lands on a runway like an ordinary jet airplane.

SPACE PROGRAMS

Considering the two categories of space programs, manned and unmanned, a few generalizations may be made. Spacecraft without a human being aboard have invariably pioneered explorations. They are smaller, can operate for months or years, and offer no hazard to human life. Experiments and measurements, however, are limited by the need for preplanning. In manned flights, the range of experiments is greater because judgment can be exercised in observations, instrumentation can be adjusted, and, perhaps, repairs can be made and equipment maintained.

Manned space flight is much more expensive because of the added weight of vehicle and equipment required to provide a habitable atmosphere and controls. This extra weight requires a much larger launch vehicle. Backup systems are often provided in manned spacecraft to provide for high reliability and safety of personnel.

Space launch vehicles.

Although sounding rockets may reach altitudes above the atmosphere of the Earth, the term space launch vehicle is applied usually to those rocket boosters designed to place satellites in orbit or to impart Earth-escape velocity to spacecraft.

By about 1950 the technology of rocket propulsion had reached a level at which consideration of a project to launch an Earth satellite became feasible. Worldwide scientific studies during the IGY of 1957-58 provided the basis for funding. In 1955 both the United States and the Soviet Union announced satellite programs as part of their national effort in the IGY.

When Sputnik 1 and 2 were launched in 1957 the Soviet Union released no details of their launch vehicles. In May 1958 Sputnik 3, weighing nearly 1,360 kilograms, was launched. It was not until 1967 that the basic Soviet launch vehicle was displayed. It was a 2 1/2-stage vehicle of the "A" series (in this case, "A-1"): two stages with four drop-away booster pods. Each booster pod contained four rocket engines (totalling 16) with propellant tankage, and the central core had four engines. Propellants were conventional liquid oxygen and kerosene.

The United States launched its early satellites with two different vehicles, the Jupiter-C and Vanguard. Jupiter-C was a modified Redstone liquid-propellant ballistic weapon of medium range to which were added more tankage length and three upper stages of clustered solid-propellant rockets. The modification was originally designed to achieve a velocity of six kilometres per second to test a nose cone (reentry vehicle). The desired velocity was obtained with two upper stages, one a cluster of four solid-propellant rockets and the other a single rocket. It was obvious that by increasing the final velocity 1.5 kilometres per second to the required 7.5 kilometres per second, satellite velocity could be obtained for a small scientific payload. The additional velocity was obtained by adding another stage with a cluster of solid-propellant rockets so that the upper stages consisted of 11, three, and finally one rocket carrying a payload weighing 8.2 kilograms. In 1954 the Army Ballistic Missile Agency and the Office of Naval Research jointly proposed this scheme, known as Project Orbiter, but a newly designed Vanguard launch vehicle was selected. Failures in early attempts to launch Vanguard, however, resulted in eventual approval of the Project Orbiter approach. Thus the first U.S. satellite, Explorer 1, was launched by a Jupiter-C on January 31, 1958.

The Vanguard launch vehicle was a three-stage booster approximately equal in length (about 22 metres) to the Jupiter-C but much lighter in takeoff weight (10,250 kilograms compared to 29,000 kilograms). Vanguard launched its first satellite (1.4 kilograms) into high orbit on March 17, 1958. After a few more flights, the Jupiter-C was retired in 1958 and the Vanguard in 1959.

During the 1960s the United States developed a series of standard launch vehicles. The Air Force modified a Titan II intercontinental ballistic missile (ICBM) for space launch purposes by strapping two solid-propellant booster rockets, three metres in diameter, to the liquid-propellant core vehicle. The Titan IIIC was used for large military satellites. Then NASA increased performance of the obsolete Thor intermediate-range ballistic missile (IRBM) by adding solid-propellant boosters. A liquid oxygen/liquid hydrogen upper stage, Centaur, was used on obsolete Atlas ICBM's and Titan III ICBM's to launch large spacecraft.

The Saturn series of NASA launch vehicles was developed specifically for the Apollo lunar mission program. The two operational Saturn models were the two-stage Saturn IB and three-stage Saturn V. The Saturn IB was used for Earth orbital developmental missions of Apollo, while the Saturn V was employed for lunar missions. Saturn V stood 110.6 metres high and weighed over 2,700,000 kilograms at launch. It could place 104,000 kilograms in orbit and send 45,000 kilograms to escape velocity.

For some years the launching of spacecraft was limited to the United States and the Soviet Union. The reason was that the rocket-powered launch vehicles were based on long-range ballistic missiles, which only these countries had developed. France was the third nation to launch a satellite (1965), followed by Japan (1970), the People's Republic of China (1970), and the United Kingdom (1971). Under the auspices of the European Space Agency (ESA), the nations of western Europe developed the Ariane expendable launcher during the 1970s to assure themselves of independent launch capability. This action was taken in response to the U.S. refusal to guarantee flights for communications satellites that might compete with U.S. telecommunications carriers. A three-stage vehicle that burns stored solid propellants in its first two stages and employs a cryogenic engine in its third, Ariane has become a formidable competitor for the U.S. Space Shuttle. It is capable of launching two satellites of the U.S. Delta class (an Earth-orbit payload of 1,770 kilograms) at one time or one Atlas-Centaur-class satellite (an Earth-orbit payload of 4,670 kilograms). With lengthened stages and the addition of solid boosters, Ariane is approaching payload weights that only the Shuttle can handle.

Unmanned programs.

The unmanned space exploration program includes sounding rockets, scientific satellites and probes, and lunar and planetary programs.

Sounding rockets.

The first program to use sounding rockets in exploration of the upper atmosphere was carried out from 1946 to 1951, when the U.S. Army fired about 60 V-2s in near-vertical trajectories in New Mexico. The one-ton warhead was replaced by a payload of scientific instruments. The highest altitude obtained in these firings was 214 kilometres. The Aerobee and Viking sounding rocket programs were similar projects conducted at White Sands by the U.S. Navy, reaching altitudes of about 320 kilometres.

The development of the Nike series of surface-to-air guided missiles provided a new family of relatively low-cost sounding rockets. Except for sounding rockets based on the liquid-propellant Aerobee design, all U.S. sounding rockets by 1971 used solid propellants. During the IGY (1957-58), there were many cooperative programs around the world in which sounding rockets were fired for scientific purposes. The United States launched 210 rockets from widely separated locations. Meanwhile, the Soviet Union and many other countries also developed and launched sounding rockets.

Less glamorous than their larger cousins that launch satellites and space probes, sounding rockets are workhorses valuable to continuing scientific studies. Although the period of their measurements is brief, usually five to 10 minutes, they provide the only feasible way of taking such measurements between 40 and 160 kilometres altitude above the Earth--above the range of aircraft and below minimum continuous satellite orbital height.

Balloons.

Balloons, the first vehicles by which man approached the edge of space, have continued to play a major role on the frontier. During the early years of space exploration, balloons carried, as did sounding rockets, the forerunners of many important instruments later developed for use on orbiting satellites. Though balloons have the disadvantage of lower altitudes, they enable investigators to make observations for hours or even days with instruments too heavy for rockets to carry.

Scientific satellites and space probes.

This category of programs includes those spacecraft whose purpose is to make measurements of natural phenomena in space environment (e.g., solar and other cosmic radiation and magnetic fields). Such data are important in developing and verifying theories of the evolution of the solar system and the fundamental natural processes and forces that affect Earth and people. These natural processes directly affect living conditions, agriculture, and economy. This is the underlying logic behind space research.

The United States has a number of programs in this category. The Orbiting Solar Observatory (OSO), first launched in 1962, was the first of a series of these observatories that point toward the Sun and record and transmit a variety of data such as the frequency and energy of solar electromagnetic radiation in ultraviolet, X-ray, and gamma-ray regions of the spectrum. An OSO carries instruments for eight to 10 experiments. Although not a part of the OSO series, the U.S. Navy's Solwind was almost identical, being built from the backup for the advanced OSO 8. Two major solar observatories launched after the OSO series were the Apollo Telescope Mount carried by the Skylab space station in 1973-74 and the Solar Maximum Mission (SMM) satellite launched in 1980 (repaired and returned to service in 1984 by Space Shuttle astronauts). The SMM satellite carries eight powerful instruments to view the Sun in white light through the gamma-ray region and to monitor its total energy output.

The next major solar observatory consisted of the solar telescopes aboard the Spacelab 2/Shuttle mission in 1985. (Spacelab is the name of a compact manned space laboratory built by ESA that is carried by the Shuttle on certain missions [see below Space Shuttle or Space Transportation System (STS)].)

A Pioneer series of small spacecraft operates in deep space in solar orbit, typically carrying out experiments involving measurement of magnetic fields, plasma analysis, and cosmic-ray and solar-wind studies. Advanced in situ studies have been carried out by the German-built Helios spacecraft, which were placed in orbit in 1975 at about 0.7 astronomical unit (one astronomical unit [a.u.] equals about 150,000,000 kilometres). In 1980 the International Sun-Earth Explorer 3 (ISEE 3) spacecraft was placed in a "halo orbit," in which Sun-Earth gravitational forces are balanced, and was held 1,600,000 kilometres in front of the Earth where the solar wind could be monitored before it hit the magnetosphere.

An Explorer family of spacecraft launched by the United States has collected a widely variant series of data. Explorer 1, the first American satellite, was launched by the U.S. Army. The program was taken over by NASA after its organization in 1958. Explorer satellites have made measurements of the magnetosphere, Van Allen radiation belts, micrometeoroids, energetic particles, cosmic radiation, solar wind, and ionosphere. Some Explorer satellites, called Interplanetary Monitoring Platforms, are placed in eccentric orbits to take measurements at maximum altitudes. Another subseries of Explorer satellites is the Small Astronomy Satellite (SAS), the first of which was launched in late 1970 and was designed to scan the celestial sphere and catalog stellar X-ray sources. This led to the development of the High Energy Astronomy Observatory (HEAO) series, which operated during 1977-81. HEAO 1 carried instruments to make X-ray scans of the skies and generate a catalog more detailed than the one SAS had provided. HEAO 2 had a 0.5-metre reflector telescope that studied the details of various objects in soft X rays (i.e., those of lower energy or longer wavelengths), and HEAO 3 carried cosmic-ray and gamma-ray detectors.

The Orbiting Astronomical Observatory is a large satellite equipped with a precisely stabilized guidance platform. Its purpose is to map the entire electromagnetic spectrum that Earth-bound astronomers cannot see because of the absorptive and distorting features of the atmosphere. One such device, launched in 1968, carried 11 star telescopes and studied ultraviolet, gamma-ray, X-ray, and infrared radiation emissions from precise locations in the celestial sphere. Another, launched in 1972, carried a single large 81-centimetre telescope. The International Ultraviolet Explorer, placed in geostationary orbit in 1978, provided detailed spectra of hot, ultraviolet sources for more than seven years. The largest and most powerful observatory so far placed in Earth orbit is the Hubble Space Telescope (HST). Deployed by the Space Shuttle in 1990, this optical-ultraviolet telescope is equipped with a 2.4-metre primary mirror and was designed to produce images with a resolution seven to 10 times higher than those from ground-based reflectors. The HST is used to observe such cosmic objects as gaseous nebulas, galaxies, and quasars and to monitor atmospheric and surface phenomena of various planets of the solar system. A major flaw was discovered in the HST's primary mirror after it had been placed in orbit. Various measures have been undertaken to compensate for the problem, including computer enhancement of its images.

Other orbiting telescopic systems have been launched to survey in greater detail objects emitting radiation from different regions of the electromagnetic spectrum. Sometimes referred to as "great observatories," they include the Compton Gamma Ray Observatory, the Extreme Ultraviolet Explorer (EUVE), the Röntgenstrahlen Satellite (ROSAT), equipped with an X-ray telescope, and the Cosmic Background Explorer (with mainly infrared sensors).

Orbiting geophysical observatories are a class of large Earth-orbiting satellites weighing more than 450 kilograms and carrying instrumentation for 20 to 30 experiments designed to study the relationship between Sun, Earth, and space environment. Areas of investigation include cosmic rays, energetic particles, magnetic fields, solar radiation, solar plasma, aurora, airglow, micrometeoroids, atmospheric composition, and solar flares. Some of these satellites are placed into a low polar orbit, others into very eccentric orbits ranging out to nearly 160,000 kilometres. Studies of the type mentioned above were conducted during the 1970s and '80s by three Atmosphere Explorer satellites, which dipped into regions of the upper atmosphere that other spacecraft cannot touch and maintain stable orbits; three International Sun-Earth Explorers (ISEE; one in halo orbit [see above] and two in polar orbits); and two Dynamics Explorers. In 1984-85 the Active Magnetospheric Particle Tracer Explorers--spacecraft built by the United States in collaboration with Britain and West Germany--created a series of artificial comets, using barium, lithium, and other chemicals, which traced the flow of ionized matter in the magnetosphere and across its boundaries. Increasing concern about the environment has given new impetus to Earth studies. Both the United States and the ESA have launched satellites to gather information on atmospheric chemistry, deforestation, and even plankton populations. U.S. researchers envision a larger project known as the Earth Observing System (EOS), which is scheduled to begin in the late 1990s and to last at least 15 years.

Biosatellite was the name of a family of recoverable scientific satellites flown to test the effects on biological specimens of weightlessness, radiation, and absence of the Earth's 24-hour day-night rhythm. The period of test was three days. Experiment specimens included bacteria, mold, seedlings, frog eggs, and amoebas. Retrieval in midair was by aircraft.

The United Kingdom built two Ariel satellites, which were launched by NASA to carry out experiments focussed on ionosphere and atmosphere research. Prospero, a technology satellite using the British Black Arrow, was launched in 1971. The European Space Research Organization (now European Space Agency) had two satellites, called Iris and Heos, launched by NASA. Iris carried seven experiments to study solar and cosmic radiation in the lower Van Allen radiation belt. Heos carried experiments to measure radiation and magnetic fields.

Lunar programs.

An early target for space exploration was the Moon, the nearest extraterrestrial neighbour. By 1978 the Soviet Union and the United States were still the only countries with space launch vehicles capable of sending payloads to the Moon.

The Soviet Luna 3 first photographed the far side of the Moon in 1959. Other Luna-series vehicles orbited the Moon or soft landed, transmitted television images of the surface, and tested the load-bearing characteristics of the lunar soil. Luna 16, in September 1970, soft landed, obtained a core sample of soil, and returned it to Earth. A significant development was Luna 17, which in November 1970 landed an automated roving vehicle. Remotely controlled from Earth, this television-equipped craft explored several kilometres of the lunar surface.

Another Soviet series of lunar probes used the so-called Zond vehicles. A number of these made circumlunar flights carrying biological specimens and returned photographs to Earth.

Three U.S. programs studied the Moon with spacecraft: Ranger, Surveyor, and Lunar Orbiter. Ranger spacecraft transmitted television pictures of the Moon from as close as 3,000 metres before crashing on the surface. The Surveyor spacecraft was a soft lander, using retro-rockets to reduce trajectory velocity to zero. It transmitted to Earth thousands of television images of the lunar surface and data on lunar soil characteristics and chemical composition. Lunar Orbiter obtained high-quality television pictures of the Moon's surface in selected areas in preparation for Project Apollo manned landings.

Planetary programs.

Both the United States and the Soviet Union sent spacecraft to Mars and Venus, the Soviets concentrating most effort on Venus and the Americans on Mars. In addition, the United States has sent spacecraft to Jupiter, Mercury, and Saturn.

The Soviets made numerous attempts to launch spacecraft to Mars and Venus and achieved success beginning with Venera 4 in 1967. In this and subsequent missions, an instrumented capsule was dropped by parachute into the hot, dense Venusian atmosphere. In each case, data were incomplete, though Venera 7 (1970) and 8 (1972) survived the landing and returned useful data from the surface. Venera 9 and 10 (1975) provided further data and the first photographs taken on the surface of another planet.

In May 1971 the Soviets launched two spacecraft to Mars. Arriving six months later, both craft ejected landing capsules. The capsules were equipped with a television transmitter, but the signals ceased shortly after landing. The Mars 2 and 3 craft continued to orbit Mars in highly elliptical orbits and transmit scientific data.

The U.S. Mariner program consisted of interplanetary probes designed to fly by Mars, Venus, and Mercury. Mariners 2 in 1962 and 5 in 1967 passed Venus within about 35,000 and 7,600 kilometres and made temperature and atmospheric density measurements. Mariners 4 in 1964 and 6 and 7 in 1969 obtained photographs of the Martian surface and made significant analyses of the atmosphere. Mariners 6 and 7 made thermal maps of Mars by means of infrared radiometers.

In May 1971 the United States launched Mariner 9 toward Mars. It arrived and was placed in eccentric orbit in November, while the planet was shrouded in a global dust storm with winds perhaps of 160 kilometres per hour. After several weeks the planet's thin atmosphere cleared, and the orbiter's two television cameras transmitted clear images of the surface, revealing many new details and formations. Other sensors measured atmospheric pressure, temperature, the ionosphere, and the gravitational field. Large areas of Mars were mapped, and the planet's two moons, Phobos and Deimos, were photographed.

Mariner 10 was launched in 1973 to explore Mercury. Swinging by Venus, utilizing Venusian gravity to increase velocity, the spacecraft flew by Mercury and obtained the first close-up photographs of this planet.

One of the most exciting planetary missions in the history of the U.S. space program began with the launching of Vikings 1 and 2 (1975). Each spacecraft consisted of two sections, one that would remain in orbit around Mars while the other completed a soft landing on the surface. Both Viking 1 and 2 landers made successful descents to the surface in 1976, where they conducted the first long-term in situ analysis of the surface material, atmosphere, and possible life-forms of another planet.

The first spacecraft to fly by Jupiter were Pioneers 10 and 11, launched by the United States in 1972 and 1973. Both returned physical data and hundreds of revealing photographs of the planet. Pioneer 11 (also called Pioneer Saturn) passed close to the rings of Saturn in 1979. On June 13, 1983, Pioneer 10 became the first man-made object to pass beyond the known limits of the solar system.

Voyagers 1 and 2 were launched by NASA in August and September 1977 on a flyby mission to Jupiter and Saturn. The twin probes reached the vicinity of Jupiter in 1979. Voyager 1 then sped by Saturn in 1980, and Voyager 2 made its rendezvous with the planet the following year. The two spacecraft continued to transmit data on conditions in deep space. Voyager 1 made the first measurements above the plane of the ecliptic as it flew north on a trajectory reshaped by its encounter with Saturn. Voyager 2 passed by Uranus in January 1986 and revealed theretofore unsuspected features of the planet and its moons. Voyager 2 passed within 5,000 kilometres of Neptune's cloud tops in August 1989. It provided a wealth of data on and close-up images of Neptune and its largest satellite, Triton. The probe also sighted six additional moons and a system of four rings encircling the planet. Voyager 2 is expected to continue operating well into the 21st century and periodically to transmit information from beyond the outer edge of the solar system.

While the United States redirected much of its resources from the planetary program to Shuttle development and application in the late 1970s and early 1980s, other nations engaged in space exploration forged ahead in planetary studies. In 1981 the Soviet Union launched the Venera 13 and 14 probes, and in 1983 Venera 15 and 16. Each of these craft deposited landers on the surface of Venus. In the case of Venera 15 and 16, the carrier spacecraft were also equipped with imaging radar. In 1985 the Soviets launched Vega 1 and 2, which ejected balloon-borne instrument packages into the Venusian atmosphere and then proceeded on for a flyby of Halley's Comet in 1986 at its closest approach to Earth. ESA sent up the cometary probe Giotto in 1985. Equipped with high-resolution cameras, Giotto produced detailed images of the comet's coma and nucleus. Also in 1985 Japan launched two probes, one of which passed by Halley's Comet at a distance of only about 100,000 kilometres.

Although the United States did not send a probe to Halley's Comet, it did divert the ISEE 3 spacecraft, which had been placed in Earth orbit in 1983, to fly past Comet Giacobini-Zinner in September 1985.

Planetary exploration by the United States was resumed with the launching of the Magellan probe in 1989. Placed in orbit around Venus in August 1990, the spacecraft secured high-resolution radar images of over 90 percent of the planet's cloud-enshrouded surface. In addition, Magellan, equipped with a radar altimeter system, produced the first high-quality map of Venus' topography. Galileo, another planetary probe launched in 1989, is scheduled to reach Jupiter in December 1995. Unfortunately a malfunction in the craft's main antenna system is expected to significantly reduce the amount of data and images that can be transmitted back to Earth. Two other probes, Ulysses and the Mars Observer (both launched in the early 1990s), are designed to study solar phenomena and the Martian surface and atmosphere, respectively.

Manned programs.

By the 1990s, only the United States and the Soviet Union had launched manned spacecraft.

Vostok and Mercury.

Both the United States and the Soviet Union early made manned flight a major goal in their space programs. The Soviet intercontinental ballistic missile was much larger than the U.S. counterpart, having been designed to carry a heavier warhead. The first Soviet manned spacecraft, Vostok 1, weighed more than 4,500 kilograms, while the U.S. Mercury weighed only about a third as much.

The Soviet Union launched Yury Alekseyevich Gagarin into a single Earth orbit on April 12, 1961. In August 1962 the Soviet Union launched two manned spacecraft in nearly identical orbits one day apart. In June 1963 a similar set of launches was made, one containing the first woman cosmonaut, Valentina Tereshkova.

At that time the United States had made only four orbital manned flights, and Soviet cosmonauts had logged nearly eight times as many man-hours in space as had U.S. astronauts. By May 1978, however, the United States had accumulated 22,503 hours of flight time compared with 19,823 hours by the Soviet Union.

Gemini and Voskhod.

While the Vostok and Mercury flights had demonstrated that humans could function while weightless and in space, neither spacecraft had operational manoeuvrability. The U.S. Gemini program was designed to develop objectives of rendezvous with a target vehicle and of docking, a capacity vital to the Apollo Moon-landing program. The dynamics of rendezvous with a target vehicle launched a day earlier called for extreme precision in tracking, timing of launch, and placement of spacecraft in almost identical orbits. A delay of one second in launch would allow the target vehicle to fly eight kilometres beyond the rendezvous point. The Soviet Union meanwhile advanced to a three-man flight with Voskhod 1 on October 12-13, 1964. On March 18, 1965, a two-man flight was made in which the cosmonaut Aleksey Leonov passed through an airlock to become the first man to float free in space. On Gemini 4, in June 1965, the U.S. astronaut Edward H. White II duplicated the feat. Called extravehicular activity (EVA), this manoeuvrability demonstrated the increasing ability of humans to function in space. It also proved that humans could adapt to weightlessness for as long as two weeks and readapt to the gravity of Earth.

Soyuz.

In April 1967 the Soviet Union launched Soyuz 1, called the largest, most complex spacecraft yet flown. V.M. Komarov, its cosmonaut, was killed in an apparent failure of the parachute recovery system. In October 1968 Soyuz 3 made rendezvous several times with the unmanned Soyuz 2, sent aloft a day earlier. Early in 1969 Soyuz 4 and Soyuz 5, launched on successive days, made rendezvous and docked; two cosmonauts left Soyuz 5 and transferred to Soyuz 4, undocked, reentered, and landed. In October 1969, Soyuz 6, 7, and 8 were launched on successive days; welding experiments were performed, and the spacecraft landed with their combined crew of seven after five days in orbit.

In June 1970 Soyuz 9 established an endurance record for manned space flight of nearly 18 days. The cosmonauts reportedly required nearly two days to regain coordination and normal sleeping habits upon return to Earth. Soyuz 10, in April 1971, rendezvoused with a previously orbited "space station," Salyut. The flight lasted only two days, however, and it has been presumed that operational difficulties occurred. In June 1971, Soyuz 11 carried three cosmonauts to rendezvous with Salyut. Twenty-seven hours after launch, the cosmonauts transferred to the 14-metre-long Salyut space laboratory. This successful flight ended in tragedy when a minor malfunction of a door seal during reentry caused loss of the air in the spacecraft and the death of all three cosmonauts. The Soviet space-station program took three years to rebound. The Salyut 2 station broke up shortly after launch; not until Salyut 3 (1974) did the Soviets commence a successful program of long-term occupancy. Two distinct classes of missions soon emerged: those involving Soyuz spacecraft on solitary missions and those in support of the Salyut series of space stations. Through data gleaned by Western observers from Soviet and other reports, it appears that Salyuts 3 and 5 (1974 and 1976) had primarily military missions. Salyut 4 (1974) established the basic design for civilian space stations.

More advanced models, Salyuts 6 and 7 (1977 and 1982), represent the second generation of Soviet stations. These have docking ports fore and aft, a refuelling system, better habitation features, and a number of other improvements. Another significant development was the introduction of the improved Soyuz T spacecraft, which was more reliable and could carry three crewmen in pressure suits (instead of only two, or three without suits), and of the unmanned Progress resupply vehicle. A large extension module to the station, dubbed Cosmos 929, roughly doubled the internal volume of the station. With Salyuts 6 and 7, the Soviets steadily increased the length of time that crews could live and work on board the station, eventually reaching a record 211 days. These extended stays were made tolerable by visits from other crews, which often included "guest" cosmonauts from Communist-bloc nations as well as from France and India.

In February 1986 the Soviets launched a new space station designed to serve as the core of a permanent manned orbiting facility. The station, known as Mir, is extensively automated and has six docking ports for cargo transports, visiting manned spacecraft, and expansion modules, which can be used as living quarters and research facilities. These modules make it possible to house a crew of up to six aboard the station.

The Soviet Union developed a reusable spacecraft known as "Buran." It closely resembled the U.S. Space Shuttle, including its use of a separate booster and unpowered reentry. The Russian Federation assumed administration of the "Buran" project after the dissolution of the Soviet Union in 1991.

Apollo program.

In May 1961 Pres. John F. Kennedy committed the United States to landing a manned spacecraft on the Moon "before this decade is out." The name given to the program was Apollo.

Several techniques for the lunar mission appeared to be feasible and were studied. A method called the lunar-orbit rendezvous technique was selected. This method would employ a single Saturn V launch vehicle to send the three-man Apollo spacecraft, weighing 45,000 kilograms, on a trajectory toward the Moon. Approximately 2 1/2 days later, as the craft approached the Moon, its own propulsion system would place it in an orbit around the Moon. In lunar orbit one element of the spacecraft, the Lunar Module (LM), with two men aboard, would separate from the mother ship and be flown toward the Moon, while the third man circled the Moon in the other section of the spacecraft, the Command Module (CM). After a soft landing on the lunar surface, the two explorers would make scientific observations and collect geological samples. After about one day, they would take off from the surface and fly toward a rendezvous with the Command Module in lunar orbit. Spacecraft propulsion would then eject the craft from lunar orbit and send it back toward Earth. Precise guidance and control on the return trip would allow the craft to enter within the limits of the narrow reentry corridor (see above). Within the atmosphere the CM would have limited manoeuvrability.

The Apollo spacecraft consisted of three modules: command, service, and lunar. The Command Module was the spacecraft's control centre and housed the crew of three astronauts. It was conical in shape, 3.7 metres high and four metres in diameter, and weighed more than 4,500 kilograms. Below the Command Module was the Service Module, 3.9 metres in diameter, 6.7 metres long, and containing the propulsion system for mid-course corrections, retrofire to achieve lunar orbit, and thrust to return from lunar orbit into Earth trajectory. Between the Service Module and the Saturn V launch vehicle was the Lunar Module adapter, designed to carry two astronauts from lunar orbit to the surface of the Moon and return them to the Command Module. The LM was about 6.4 metres high and 3.4 metres in diameter, weighing 13,600 kilograms. There were two engines, one with controllable thrust for descent to the surface of the Moon and the other with a fixed thrust of 15,900 newtons (one newton = 0.225 pound of force) for ascent to rendezvous with the Command Module in lunar orbit.

The first manned Apollo flight, in Earth orbit, scheduled for February 1967, was delayed when on January 27, 1967, during a countdown rehearsal, a fire broke out inside the spacecraft cabin and spread rapidly in the concentrated oxygen environment at higher-than-sea-level pressure. Three astronauts--Virgil I. ("Gus") Grissom, Edward H. White, and Roger B. Chaffee--lost their lives.

In November 1967 a Saturn V placed an unmanned Apollo spacecraft in orbit. Two more unmanned Apollo flights were made successfully, and on October 11, 1968, Apollo 7, with three astronauts aboard, tested the Command Module in a 163-orbit flight. Apollo 8, launched on December 21, 1968, after an initial Earth orbit was injected into a translunar trajectory, inserted into lunar orbit for 10 orbits, and then returned safely to Earth. Live television broadcasts were made during the mission.

Apollo 9, launched on March 3, 1969, on a 10-day trip, checked out the Lunar Module operation in Earth orbit. Apollo 10 lifted off on May 18. Paralleling Apollo 8's flight into lunar orbit, two astronauts transferred to the LM and lowered their orbit to 14,300 metres above the Moon.

Apollo 11, launched July 16, 1969, manned by Neil A. Armstrong, Edwin E. Aldrin, Jr., and Michael Collins, repeated Apollo 10's flight to lunar orbit. Armstrong and Aldrin transferred to the Lunar Module, and descent and landing on the Moon were made July 20 at 8:17 PM Greenwich Mean Time, at 23.26 E and 0.41 N. Armstrong stepped onto the Moon's surface at 2:56 AM GMT. His first words were, "That's one small step for [a] man, one giant leap for mankind." He reported sinking about three millimetres into the fine, powdery surface material. Aldrin joined Armstrong, and together they spent some two hours taking photographs, collecting about 21.7 kilograms of lunar soil (including two core samples), planting a U.S. flag, and deploying a solar-wind experiment, a seismic experiment package, and a laser-beam reflecting device. They were seen on Earth television via a camera erected some distance from the Lunar Module.

The astronauts made their lunar ascent stage takeoff on July 21, concluding a total lunar stay time of 21 hours 36 minutes. Transferring to the Command Module, they relanded on Earth in the Pacific on July 24. Because of possible contamination by organisms with unknown effect upon terrestrial life, decontamination and quarantine were employed.

Apollo 12, launched November 14, followed a flight plan similar to that of Apollo 11. The Lunar Module landed on the Moon on November 19 in the Ocean of Storms. Two Moon walks were made; a nuclear isotope-powered surface experiments package was emplaced, and 34 kilograms of lunar samples were collected.

Apollo 13, launched April 11, 1970, failed to complete its lunar landing mission owing to an explosion in the Service Module that damaged the main power supply and cut off the chief source of oxygen for the crew. Emergency procedures were quickly devised, with the crew using the Lunar Module as a "lifeboat" until just before reentry into the Earth's atmosphere. At that point, the three astronauts moved into the Command Module--the only part of the Apollo craft capable of safe reentry--and splashed down in the Pacific near the recovery site.

Apollo 14, launched January 31, 1971, continued exploration of the Moon. Landing within 18 metres of the target area, the astronauts made two surface explorations totalling more than nine hours. A surface experiments package was emplaced, and 43 kilograms of lunar rocks were brought back.

Apollo 15 was launched July 26, 1971, and returned August 7, 1971. Landing near Hadley Rille and the Apennine Mountains, it carried a four-wheeled, battery-powered Lunar Roving Vehicle that permitted extensive exploration of the Moon's surface by crew members. The weight of lunar material returned was 77 kilograms.

On April 16, 1972, Apollo 16 was launched. Landing in the Descartes region, it brought back to Earth some 98 kilograms of lunar rocks and soil. Apollo 17, the last U.S. mission to the Moon, launched December 7, 1972, broke previous records of lunar exploration time, distance travelled, and quantity of lunar samples returned (116 kilograms from the Taurus-Littrow region).

Skylab and ASTP.

Several additional manned missions to the Moon had been planned initially. When these missions were cancelled, the Saturn IB (Earth orbital) and Saturn V (lunar) launch vehicles, as well as Apollo Command and Service modules, became excess equipment. This equipment was used instead for Skylab and the Apollo-Soyuz Test Project (ASTP). 

Skylab, the first U.S. space station, was launched on May 14, 1973. Damage sustained during launching required extensive repairs, and these were carried out by the first two crews to visit Skylab. The station consisted of four basic sections. The Orbital Workshop, a converted third stage (Saturn IVB) of Saturn V, included a hydrogen tank divided into two floors to provide the basic living and work area for the crew. Astronauts dressed in protective space suits were able to exit and reenter the Skylab for work in space through the Airlock Module. A Multiple Docking Adapter was provided for docking the Apollo spacecraft to the Skylab, while an Apollo Telescope Mount (ATM) was an uninhabitable area containing experiments.

Between May and November 1973, three crews of three astronauts each lived and worked aboard Skylab for 171 days 13 hours. These missions contributed to understanding of the space environment and of the biological implications of long-term space habitation. In July 1979 Skylab, abandoned for more than five years, reentered the Earth's atmosphere and disintegrated, showering debris over the Indian Ocean and western Australia.

The Apollo-Soyuz Test Project was the only cooperative U.S.-Soviet manned space mission. Launched on July 15, 1975, from their respective launch complexes, the three-man Apollo and the two-man Soyuz 19 spacecraft rendezvoused and docked in orbit two days later by means of a U.S.-built docking module. The spacecraft remained linked for two days while the astronauts and cosmonauts conducted joint Earth survey, astronomical, medical, and technical experiments. At the conclusion of the two-day joint mission, the Soyuz craft returned to Earth while the Apollo crew remained in orbit for another five days. The ASTP demonstrated the possibilities of international cooperation in space as well as the feasibility of using the docking module as a rescue device.

Space Shuttle or Space Transportation System (STS).

The U.S. space program entered a new era on April 12, 1981, with the initial launch of the Space Shuttle, the first manned spacecraft designed for reuse. The program was started in the late 1960s when NASA turned its attention to a comprehensive program that featured a permanent manned space station, a reusable Earth-to-orbit transport, and reusable chemical- and nuclear-powered space tugs. Because of tightening budgets, only the Earth-to-orbit vehicle, the Shuttle, survived, and as a "stage-and-a-half" vehicle rather than the fully reusable, two-stage craft originally planned. The design was chosen to reduce development costs and at the same time to hold down the operational costs of manned space flight.

In its present form, the Shuttle consists of a winged orbiter, a drop tank, and twin solid rockets. Altogether it stands 56 metres high and weighs 2,000,000 kilograms at lift-off. The orbiter carries the cargo and crew and contains virtually all of the craft's computer and electronic hardware, as well as its three main engines. At the centre of the orbiter is the payload bay, which measures 4.6 metres in diameter and 20 metres in length and can carry 29,500 kilograms into space and 14,500 back to Earth. Putting all of the main engines' liquid hydrogen (fuel) and liquid oxygen (oxidizer) in an external tank that is discarded after engine cutoff makes the size of the orbiter smaller and the cost much lower than if the propellants were carried internally. Twin solid boosters were chosen for the first stage as a low-risk concept that would make up for any shortcomings in the design of the orbiter and tank. This was the first time that a manned space vehicle used solid rockets as a major propulsion element.

During launch, the solid boosters and liquid engines of the Shuttle fire together, generating 31,000,000 newtons of thrust until roughly two minutes after lift-off, when the boosters burn out and are jettisoned by parachute into the ocean for retrieval. The solid boosters are used 20 times each. The main engines continue for another 6 1/2 minutes, giving the Shuttle 99-percent orbital velocity. At this point, the external tank is jettisoned (it disintegrates upon entering the atmosphere), and two manoeuvring engines give the Shuttle the final push into orbit. For reentry, these engines again fire as retro-rockets. Tiles made of high-grade silica protect the aluminum airframe from the intense heat produced by friction as the Shuttle uses the atmosphere as a brake. As the Shuttle slows and the atmosphere becomes thicker, lift is generated over the wings and the vehicle becomes a hypersonic glider that makes an unpowered landing on a runway.

While in orbit--usually for about a week, though 10 days is possible--the Shuttle can carry out a variety of tasks, which may range from the crew deploying satellites to playing with toys to demonstrate basic physical laws. Each mission has a primary goal, but a number of secondary tasks are added to make full use of the crew.

Because the Shuttle was designed only to reach low Earth orbit (less than 480 kilometres on most missions), a number of "third stages" had to be developed to carry payload satellites to higher altitudes. Initially this was to be done by a space tug, but budget cuts made it necessary to devise an Interim Upper Stage (now called Inertial Upper Stage [IUS]) to accommodate that task. The IUS weighs about 20,000 kilograms and can place 2,500 kilograms of payload into geostationary orbit. Because most communications satellites weigh less than that, Payload Assist Modules were developed for smaller loads. Also, a wide-body version of the Centaur rocket (used on the Atlas booster rocket) was designed to boost deep-space probes when the IUS encountered developmental problems.

The two-level cabin of the orbiter is too crowded for carrying out large-scale experiments. In an effort to participate in the Shuttle program, ESA offered to build Spacelab, a set of modules and pallets that could be assembled in several configurations for experiments in all of the space science disciplines. Carried in the orbiter's cargo bay, Spacelab serves as a kind of space station, providing the room and facilities needed for conducting research in space.

Problems in developing the Shuttle's main engines (which are not only reusable but also more throttleable than earlier engines) and in bonding the heat-shield tiles to the first orbiter, "Columbia," and cutbacks in annual budgets delayed the initial launch by three years. The first four missions were considered developmental flights and carried a crew of only two astronauts. The first flight's cargo consisted of developmental instrumentation. The second through fourth flights carried cargo designed to demonstrate the Shuttle's suitability for making Earth observations and for conducting space science and military experiments; a robot arm for manipulating payloads outside the Shuttle was also added. On its fifth mission, in November 1982, the Shuttle was, for the first time, employed for commercial operations: it launched two communications satellites, the Telesat Canada Anik C and the SBS-3. The Spacelab 1 mission on STS-9 demonstrated that the Shuttle and the ESA-built space laboratory could handle experiments in all fields of space science.

Besides providing a means for conducting scientific experiments and deploying satellites in space, the Shuttle has been used to repair and retrieve inoperative satellites already in orbit around the Earth. The objective of the 11th mission, in April 1984, was to repair the Solar Maximum Mission satellite (see above). The Shuttle crew succeeded in grappling the crippled satellite with the aid of the orbiter's robot arm and made the necessary repairs to restore the craft to normal operation. On a subsequent mission in November 1984, astronauts used a device called the Manned Maneuvering Unit (MMU) to retrieve two communications satellites for reuse. The MMU, introduced in February 1984 during the 10th Shuttle flight, enables astronauts to perform extravehicular activities far more complicated than any undertaken in the past. Essentially a jet-equipped backpack, the MMU can propel the wearer up to 98 metres from the orbiter without being tethered by lifelines to the spacecraft.

The Shuttle program suffered a serious setback in 1986. On January 28 of that year the orbiter "Challenger" exploded 73 seconds after liftoff, killing its entire seven-member crew (including a high-school teacher, the first private citizen to fly aboard the craft). The accident, determined to have been caused by flawed booster rocket seams, occurred on the 25th Shuttle mission and resulted in the suspension of flights so that the design problem could be corrected. Shuttle flights were resumed in 1988 after corrective measures were satisfactorily completed and new safety systems installed in the three remaining orbiters--"Discovery," "Columbia," and "Atlantis."

The Shuttle makes possible missions that cannot be carried out with conventional spacecraft. Among these may be the assemblage and resupply of a permanent manned space station. NASA has studied the feasibility of such a space station project since the 1960s but has always been diverted by other programs or budgetary problems. In 1984, however, the U.S. government committed NASA to developing a space station during the 1990s. The United States is developing the space station "Freedom" for deployment in the late 1990s. Its size and funding remain the subject of debate.

Applications satellites.

The search for scientific knowledge, exploration of the unknown, and establishment of man's capability in space have been, and remain, primary goals of national space programs. Increasingly, however, Earth-oriented satellites (called applications satellites) of direct economic benefit are being placed in operation.

There are three general classifications of such satellites: communications, Earth survey, and navigation.

Communications satellites.

In the decade following the passive reflector balloon-satellite Echo 1, communications satellites became a significant part of global communications. The Intelsat 4 satellite, for example, launched in January 1971, was capable of handling 3,000-9,000 telephone circuits or 12 colour television channels or a combination of both. In addition, domestic communications satellites, such as Comstar, Satcom, and Westar in the United States and Anik in Canada, meet the specific needs of particular nations. Voice, television, and facsimile or high-speed data (or both) are relayed.

Other specialized communications satellites in service since 1976 include three Maritime Satellite systems (Marisats), designed to provide greatly improved ship-to-shore communications. Previously, 90 percent of ship-to-shore communications was by hand-operated radiotelegraphy, and ionospheric disturbances in poor weather often limited communications. Marisat provides high-fidelity voice, data, Telex, and facsimile services.

Two developmental communications satellites called Symphonie--financed cooperatively by France and West Germany--were launched in 1974 and 1975 from Cape Canaveral, Florida, and manoeuvred into geosynchronous orbit off the coast of West Africa to provide telephone, television, and data service between western Europe, the Americas, and Africa.

The U.S. Department of Defense has a "secure" satellite communications system, using both synchronous and lower altitude orbits. The United Kingdom has a military Skynet comsat system launched by the United States.

The Soviet-launched Molniya comsats revolve in highly eccentric orbits with a 480-kilometre perigee over the Southern Hemisphere and a 39,420-kilometre apogee over the Northern Hemisphere; the orbits are inclined at an angle of 65 from the plane of the Equator. This arrangement provides a period of eight to nine hours each day in which each satellite is visible from Soviet ground terminals. Thus, three of these satellites, properly spaced, make possible 24-hour coverage.

Applications Technology satellites (ATS) are a series of geosynchronous orbit satellites launched by the United States, their purpose being to test new instrumentation and advanced communications techniques such as higher transmission frequencies and improved transmission antennas. ATS 6, placed in orbit in 1974, explored the possibilities of direct broadcast communications from space through the use of inexpensive ground receivers.

Earth survey satellites.

This broad category of satellites has progressed from relatively simple photography to include many forms of observation of the Earth. Meteorological, or weather, satellites operate globally. From a polar orbit, photographs of the Earth's surface are taken by television scanning and transmitted to Earth receiver stations. In addition, meteorological satellites in geostationary orbit have the advantage of continuous watch over large regions of the Earth. Seventy percent of the Earth is covered by water, and there are few observation stations in desolate and jungle areas. Global observation of weather and its movement by satellites has become important to all nations. Tracking of hurricane formation and movement has permitted advance warning, saved numberless lives, and minimized property damage.

The first weather satellite was TIROS--Television and Infra-Red Observation Satellite. TIROS satellites transmitted cloud-cover pictures enabling meteorologists to track, forecast, and analyze storms. In 1970 the first operational "second generation" weather satellite was launched. Twice the size and weight of TIROS, it had an important new instrument, scanning radiometers to obtain direct-readout and stored images of the Earth's night side. It provides global coverage at 12-hour rather than 24-hour intervals.

The world's first Synchronous Meteorological Satellite, SMS 1, was launched by the United States in May 1974. Following the launching of SMS 2 in February 1975, a Geostationary Operational Environmental Satellite (GOES) was placed in orbit in October to provide 24-hour coverage of U.S. weather. In addition, the SMS and GOES craft collect and relay information transmitted by unmanned, ground- or ocean-based sensor stations in isolated locations. They also contain instruments to study solar activity, X-radiation, and the Earth's magnetic field. The SMS test satellites and the operational GOES take part in the Global Atmospheric Research Program (GARP), an international project aimed at achieving improved understanding of the mechanisms that govern the world's weather. Other elements in this worldwide system include the Japanese Geostationary Meteorological Satellite (GMS) and the ESA's Meteosat, launched in 1977. Soviet weather satellites, called Meteor, began to distribute weather photographs internationally in 1969.

Earth satellites offer a method of improving maps. If the altitude of a satellite is known, the angles of simultaneous widely separated observations may be used to calculate a base line between the two points of observation. Satellite observations have made possible the "tying together" of landmasses of the world not hitherto possible.

NASA launched its first Geodynamics Experimental Ocean Satellite (GEOS 1) in 1965; it then sent up GEOS 2 in 1968. In 1970, 43 stations in 23 countries made observations. The aim was to provide a unified three-dimensional framework connecting all landmasses of the world within an accuracy of 10 metres. In 1971 an International Satellite Geodesy Experiment program using GEOS 2 included most major nations.

By the use of laser beam techniques, extremely precise measurements may make it possible to record Earth movements associated with earthquakes. Lageos (Laser Geodynamic Satellite), launched in 1976, represented the first attempt at this method developed to advance earthquake prediction. A completely passive satellite, Lageos is essentially a brass core encased in an aluminum sphere covered with 426 retroflectors that allow ground-based laser ranging stations to take measurements for the study of continental drift, tides, and fault motions. Lageos was placed in a very stable orbit 5,793 kilometres high, where it is expected to remain for some 8,000,000 years.

The Seasat spacecraft, which operated for only 100 days in 1978, used imaging radar to produce maps of the ocean surface and currents with unprecedented detail, revealing previously unknown features of the ocean bottom. The U.S. Navy's Geosat satellite, launched in 1985, was developed to obtain radar images of the same kind.

Other benefits may be derived from Earth survey satellites. Revealing images are obtained by viewing the Earth's surface in different portions of the radiant spectrum.

The United States launched the first Earth Resources Technology Satellite (ERTS) in 1972. In polar orbit at an altitude of about 910 kilometres, it transmitted to the Earth multispectral images that provided data to hundreds of scientific investigators in many disciplines: agriculture, forestry, mineral and land resources, land use, and water and marine resources. ERTS 1 was renamed Landsat 1, and in January 1975 Landsat 2 was orbited successfully. The two spacecraft are 180 apart in orbit, thereby providing a view of the same local area with the same sun angle every nine days. In 1982 the new Landsat D series was introduced with the launch of Landsat 4 and in 1984 with Landsat 5. These satellites retained the multispectral scanner used on the first three Landsats but replaced the black-and-white cameras with a thematic mapper that viewed the Earth in several bands of the deep infrared spectrum.

Other Earth observations were made by the Shuttle imaging radar, demonstrating the ability of radar to map terrain below vegetation. As noted earlier, radar even made possible the detection of ancient riverbeds below desert sands in the Sahara.

Some of the most valuable uses of Earth surveys made by orbiting spacecraft have been estimating crop acreage, monitoring urban development and planning future land use, locating air and water pollution, locating geologic formations that may indicate the presence of minerals and petroleum, updating maps and navigation charts, and studying flood hazards and managing water resources.

The United States, the Soviet Union, and its primary successor, Russia, have been the only nations with both the technological ability and the financial resources to utilize satellites for photographic and electronic monitoring for military purposes. Such programs have been conducted by both nations since about 1959; in fact, military reconnaissance is the oldest use of the applications satellites. Although there has never been official disclosure of such programs by either country, the use of reconnaissance satellites continues by tacit agreement. Such satellites were credited with being the main technological base in nuclear-arms control under the terms of the Strategic Arms Limitations Talks (SALT). 

In the 1960s the United States began using three other types of military detection satellites. The Vela series carried gamma-ray detectors capable of warning of nuclear tests in space, which would be a violation of existing treaties. They were supplanted in 1984 by more sensitive detectors carried on board Navstar navigation satellites. The Defense Support Program, initiated in the late 1960s and improved in the 1980s, always maintains at least two satellites in geostationary orbit to monitor land-based missile fields in eastern Europe and Central Asia. The infrared telescopes carried by these satellites are designed to give the U.S. manned bomber fleet warning of a massive missile strike within two minutes, thereby assuring its survival. Also in use are electronic "ferret" satellites that eavesdrop on foreign radio and telephonic transmissions.

Space-based detection and weapons systems are integral to the Strategic Defense Initiative (SDI) program instituted by the U.S. government in 1984. The SDI, popularly referred to as the "Star Wars" program, was initiated to promote research on a wide array of nonnuclear defense technologies for providing protection against incoming intercontinental ballistic missiles. Among the various systems under investigation or development were detection systems and combat-control computers that rely on airborne infrared tracking telescopes and short-wavelength radar for locating and identifying targets; high-energy lasers capable of destroying approaching missiles; particle accelerators that would fire beams of charged or neutral atomic particles at the electronic guidance systems of nuclear warheads; and homing interceptor vehicles designed to collide with targets by following heat emitted from them.

Navigation.

This specialized class of Earth-oriented satellites, first launched by the U.S. Navy in 1960, was originally classified secret. The main purpose of the Transit series of satellites was to enable nuclear submarines to fix their position accurately under all surface weather conditions. It made star sighting unnecessary. Basically a beacon in the sky, the system was revealed in 1967. Operationally the system is based on observation of the Doppler shift of the satellite's transmitted signal. A major military advantage is the fact that a ship does not have to emit an electromagnetic signal. Use of this navigation system by merchant ships is limited because of the high cost of the equipment.

The newest field of space applications is microgravity materials science (formerly called materials processing in space), which makes use of weightlessness to produce materials free of the defects common to terrestrial manufacture. Initial tests were carried out on Apollo, Skylab, and Apollo-Soyuz missions, the results of which produced greater enthusiasm than was warranted. A mature research program, however, has evolved. Concerned with a broad range of fundamental questions, it involves systematic experimentation on the Space Shuttle. While results are still pending, it appears certain that it will at least be possible to produce materials that are closer to theoretical perfection and so can serve as models for improving terrestrial production. At best, it is expected that several new product lines will be opened, including the manufacture of several pharmaceuticals by means of electrophoretic separation and the production of gallium-arsenide substrate crystals for advanced military computers. The first "made in space" materials became commercially available in 1985, when the U.S. National Bureau of Standards began marketing latex microspheres manufactured aboard the Shuttle. These microspheres are larger than those made on Earth and have had broad applications in biomedical research and other fields.

CONTRIBUTIONS TO THE PHYSICAL SCIENCES

Discoveries about the Earth.

Before the advent of artificial satellites, it was accepted that the shape of the Earth was, except for some high mountains and deep valleys, an ellipsoid--i.e., a sphere, slightly flattened. Accurate observation of variations of the orbital path of Vanguard 1 proved that, on the contrary, the Earth is slightly pearshaped, the distance from the Earth's centre to the North Pole being greater than the distance to the South Pole. Further observations of anomalies in the orbits of satellites have shown that the Equator is elliptical and that the surface gravity of the Earth has several "hills" and "valleys" varying as much as 30 to 90 metres from the average.

Originally it was thought that cloud patterns and weather systems were relatively small, perhaps a few hundred kilometres across. Photographs from space have shown that the Earth's cloud cover is completely organized globally. Coherent cloud-cover systems have been found to extend for thousands of kilometres and to be related to other systems of similar dimensions. It was discovered that weather systems could be directly identified by their cloud structure, and thus it was possible immediately to locate important atmospheric phenomena such as fronts and storms and to chart their courses daily with accuracy.

Atmospheric density and variation with altitude above the Earth had been studied for many years by balloons and to some extent by sounding rockets. Observations of atmospheric drag on satellites and resultant lowering of orbits, however, revealed that variations in atmospheric density are related directly to solar flares and the impact of electrically charged particles about 26 hours after marked solar activity. Further, solar flares have been found to increase the temperature of the atmosphere by interaction of short-wavelength radiation atoms and molecules in the atmosphere. Recombination of the ionized particles is accompanied by the liberation of heat.

Although it had been postulated that some charged particles might be trapped in the Earth's magnetic field, Explorers 1 and 4 revealed a major phenomenon in geophysics: the existence of two permanent doughnut-shaped belts of high-energy radiation caused by trapped charged particles, 3,000-16,000 kilometres above the Earth. 

The earlier view of the ionosphere was that from 60 to 600 kilometres distinct layers of ionized atoms and molecules, known as the D, E, and F layers, existed and were necessary for radio communications. Evidence from space exploration is that ionization in the atmosphere is actually continuous but that peaks of concentration of electrons occur at altitudes where the existence of layers had formerly been postulated. Still higher, ionization is practically complete and extends to great altitudes.

It was once believed that the Earth's magnetic field was similar in shape and symmetry to a short bar magnet, with the magnetic field extending outward into space. It has been learned from satellites that the Earth's magnetic field on the side toward the Sun is sharply bounded because of the solar wind (a constant outpouring of high-speed particles from the Sun's corona). On the dark side of the Earth, in the direction away from the Sun, the field streams out in a long, thin tail beyond the Moon's orbit. The Dynamics Explorers and the U.S. Air Force's High Latitude (HiLat) satellite produced images of the entire auroral oval seen from space during night and day and revealed data on how it is "pumped" by events in the magnetotail. The Dynamics Explorers also discovered fountainlike jets of nitrogen and oxygen ions propelled upward from the magnetic poles along the field lines.

Data on solar phenomena.

Since ultraviolet rays and X rays are absorbed in the atmosphere, measurements of these radiations from the Sun were made by sounding rockets in some of their first flights. Solar spectra have been refined by spectrographic cameras carried aboard many satellites. Variation in measurements correlated with observed solar activity is providing new knowledge of the processes occurring within the Sun. Although the solar radiation in the visible portion of the spectrum is constant, the ultraviolet and X-radiation are variable. From the data gathered by space exploration it also has been learned that the Sun radiates in the manner of an incandescent gas, that ultraviolet and X-radiation are generated not in the region of the solar disk but higher in the solar atmosphere, in the chromosphere and the corona. Skylab's solar telescopes discovered "holes" (i.e., cool regions) in the corona, which are now believed to be the source of the solar wind. The Solar Maximum Mission satellite showed variations in the Sun's total energy output--as much as 0.2 percent in two days--though these averaged out to produce a much slighter downward trend that could have implications for climate over the centuries.

Discoveries about interplanetary phenomena.

The earlier view was that interplanetary space was empty of matter except for that thrown out from the Sun and for the clouds of particles that became visible as meteoroids on striking the Earth's atmosphere at night. From space exploration it has been learned that the solar wind flows continuously and at such velocities (from 400 to 500 kilometres per second) that the particles escape the solar system. 

Earlier it was believed that the existence of electric fields in space was unlikely. Evidence based on measurements in space indicates that such fields do exist.

The earlier view of the number of meteoroids in space was based mainly on conjecture. Measurements by Explorer 16, Pegasus, and other spacecraft have provided data indicating that the number of meteoroids is much smaller than previously thought and that the hazards they pose for spacecraft are very slight. In addition, the passage of the Pioneer and Voyager spacecraft through the asteroid belts demonstrated that such objects present less of a threat than had been anticipated.

Exploration of the Moon and planets.

Detailed photographs of the Moon have made possible a nearly complete map, including the hidden far side. Soft-landing vehicles have televised views of the surface by remotely controlled cameras, determined the physical and load-bearing characteristics of the soil, and performed chemical analysis by isotope backscatter technique. Seismometers, laser reflectors, solar wind and ion detectors, charged particle measuring devices, magnetometers, and other instruments have been set up. Six Apollo missions brought back 382 kilograms of selected lunar rocks and soil for study.

The physical nature of the Moon has been revealed by space exploration in a way impossible before. The list of discoveries is long and detailed. Perhaps the most significant is that the age of the surface material studied has been determined to be 4,600,000,000 years. For much of that time the Moon has not drastically changed, but lunar history appears to have been complex. Because it has changed so little, a record of the Sun can be traced on the lunar surface. This type of data has many implications for Earth history.

Other findings about the Moon are: there is no life or water; lunar rocks are similar to Earth rocks but have different compositions; seismometers have recorded numerous "moonquakes" about the time the Moon is closest to the Earth each month; there is a lunar magnetic field, but it is much weaker than the Earth's; the solar wind is not disturbed as it approaches the Moon, signifying that there is no layer of charged particles similar to the Earth's ionosphere; laser reflection makes it possible to measure Earth- Moon distance to within about 23 centimetres.

Through Soviet and U.S. spacecraft travelling to the vicinity of Venus, much greater understanding has been obtained of this planet. The Soviet technique of parachuting specially protected capsules through the dense Venusian atmosphere confirmed cloud-covered surface temperatures of some 460 C. The thick atmosphere consists primarily of carbon dioxide, more than 96 percent, and nitrogen, about 3.5 percent.

The Pioneer Venus spacecraft revealed the planet to be one of the few objects in the solar system known to be subject to active volcanism. Analysis of data from the Pioneer Venus probes that descended through the atmosphere and from the orbiter that scanned it revealed a large quantity of sulfur dioxide in the atmosphere, which declined after a period. This finding indicated that the sulfur dioxide had been ejected by a volcano and subsequently settled to lower levels and was scavenged by chemical reactions. Radar altimeter observations from the Magellan probe showed that much of the surface of Venus consists of flat rolling plains with two enormous highland areas, Ishtar Terra in the northern hemisphere and Aphrodite Terra along the equator. A large portion of Ishtar's interior consists of a high plateau; it is bounded to the east by Maxwell Montes, a huge volcano that exceeds Mt. Everest in height. There are other tectonic features on Venus, the most spectacular of which are the great rift valleys that lie atop such broad, elevated areas as Beta Regio.

Prior to 1964 it generally had been accepted that the atmospheric pressure on Mars was about 85 millibars. (Terrestrial atmospheric pressure is about 1,000 millibars.) Mariner 4 data showed that Martian atmosphere is about one-tenth this pressure, an important factor in the design of Martian landers.

U.S. Mariner spacecraft flew within 1,380 kilometres of Mars, photographing the polar caps and revealing some widespread cratering, not unlike that of the Moon. There were, however, flat featureless plains covered by dust. The polar caps appear to be crusted with frozen carbon dioxide, possibly to a thickness of 0.6 metre. The Martian atmosphere is composed chiefly of carbon dioxide with only traces of water. Unlike that of the Earth, Mars's atmosphere does not shield it from ultraviolet radiation, and most of it reaches the surface.

Scientists continue to study data returned from Vikings 1 and 2, which sent landing vehicles to the Martian surface. These spacecraft have enormously expanded scientific knowledge of conditions on Mars. The atmosphere of the planet is primarily composed of carbon dioxide, but traces of nitrogen, argon, krypton, xenon, and oxygen have also been identified. Humidity varies significantly, the summer hemisphere being more humid than the winter. The Viking biological experiments failed to uncover any evidence of life on the surface, and no organic chemicals were detected. Some test results have indicated the possibility of unusual soil chemistry.

The Voyager spacecraft radically altered scientists' views of the giant outer planets. A thin ring system was discovered encircling Jupiter at an altitude of 128,000 kilometres. Lightning was detected in the clouds covering the planet, as were auroras similar to those of the Earth. Jupiter's most prominent feature, the Great Red Spot, was found to be a massive hurricane rotating every six days at more than 360 kilometres per hour along its edge. Moreover, Voyager data and photographs resulted in the discovery of three additional Jovian moons and volcanism on Io. Photographs of the latter revealed fountains of molten sulfur, presumably heated by internal tidal forces, erupting to recoat its surface. It was also discovered that sulfur, together with sodium and oxygen, from Io forms a plasma torus around Jupiter. 

The cloud tops of Saturn proved more muted than those of Jupiter, but its rings more than made up the difference. The latter were found to be composed of a complex of ringlets numbering thousands within the three major rings long known to astronomers. "Shepherd" satellites were discovered at the outer edges of the rings where they apparently keep material from drifting out. Dark spokes appear to rotate as a solid body through the rings. Because no solid structure can exist in this manner, it is believed to be an electromagnetic effect of some kind. Scientists also learned that Titan, the only satellite in the solar system known to have a dense atmosphere, is as cloudy as Venus but with a surface pressure 1.5 times that of the Earth's. Titan's atmosphere is thought to consist primarily of nitrogen gas, though some argon may be present as well.

Voyager 2's discovery in 1986 of a comparatively strong magnetic field around Uranus enabled astronomers to make accurate estimates of its rotation period. An analysis of field data (e.g., radio emissions coming from charged particles trapped in the field) indicated that Uranus rotates once every 17.24 hours. The probe also uncovered 10 small satellites inside the orbit of Miranda, the innermost of the five major Uranian moons, and a previously undetected ring.

Voyager 2's flyby of Neptune yielded an similar wealth of information about the planet and its satellites. Images and data from the probe revealed that Neptune is surrounded by a turbulent atmosphere. Winds as high as 700 metres per second blow westward around the planet near latitude 20 S, and two enormous storm systems (the so-called Great Dark Spot and the Small Dark Spot) occur in the southern hemisphere. Voyager 2 observations led to the detection of six additional satellites around Neptune besides Triton and Nereid and a system of four narrow rings composed largely of dust-size particles. High-resolution images transmitted by the Voyager cameras showed that Triton, the largest of the planet's moons, has a very unusual landscape, one that has been likened to the rind of a cantaloupe. Huge canyons and narrow ridges cut across one another along the equator. There are vast expanses of ice flows, presumably of frozen methane and nitrogen.

Observations of astronomical phenomena.

Astronomers once felt that the temperatures of stars could be calculated from the visible radiations that penetrate to the Earth's surface. Space research suggests that inclusion of the ultraviolet radiation causes significant changes in the calculated temperatures of individual stars. Some stars have been found to be cooler than previously thought, and supergiant stars are hypothesized to be losing mass into the interstellar medium at a greater rate than was expected.

The earlier view was that much of the mass of the Milky Way Galaxy, which contains the solar system, was molecular hydrogen. The evidence obtained from spacecraft is that the quantity of molecular hydrogen is much less than was thought and undetectable with available instruments.

It was originally hypothesized that stars radiated most of their energy in the visible regions of their spectra. The evidence from spacecraft is that in the case of some stars the energy output in the visible spectrum is exceeded by that in the ultraviolet, radio, or infrared, or in all three. Such tremendous outpourings of energy have caused theories of stellar evolution to be reconsidered.

Since many of the stellar distances are dependent on determining the intensity of stellar sources, these discoveries of much greater total energy outputs may cause astronomers to double the estimated physical size of the known universe. Previously unknown types of stellar sources are being discovered at a rapid rate. The Extreme Ultraviolet Explorer spacecraft has detected a galaxy 2,000,000,000 light-years away that radiates as much energy as a trillion suns. The Hubble Space Telescope has provided much clearer pictures of many interstellar formations than were previously available and uncovered evidence that supports the prevailing theory of black holes. In like manner, the Infrared Astronomy Satellite (IRAS), which was expected to observe only a few thousand objects, discovered almost 246,000 infrared sources as well as various diffuse clouds of cold dust and stars around which planetary systems appear to be forming.

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Bibliography

All aspects of space exploration and discovery are covered in MICHAEL RYCROFT (ed.), The Cambridge Encyclopedia of Space (1990), with an excellent bibliography; ANTHONY R. CURTIS, Space Almanac, 2nd ed. (1992); and KENNETH GATLAND, The Illustrated Encyclopedia of Space Technology, 2nd ed. (1984). Historical treatments of the conquest of space include WILLY LEY, Rockets, Missiles, and Men in Space, newly rev. and expanded ed. (1968); WERNHER VON BRAUN and FREDERICK I. ORDWAY III, History of Rocketry & Space Travel, 3rd rev. ed. (1975), and Space Travel: A History, 4th ed. rev. in collaboration with DAVE DOOLING (1985); HOMER E. NEWELL, Beyond the Atmosphere: Early Years of Space Science (1980); DAVID BAKER, The History of Manned Space Flight, new ed., updated and enlarged (1985); PHILLIP CLARK, The Soviet Manned Space Program (1988); and PATRICK MOORE, Mission to the Planets (1990). Current information is available in Interavia Space Directory (annual), which includes detailed, illustrated coverage of programs by country and Earth observations; and Space Year (annual), a chronology of the year's major space events, including manned, unmanned, and planetary missions. Earlier missions are covered in ANDREW WILSON, Solar System Log (1987), covering mid-1958 to mid-1985; and TIM FURNISS, Manned Spaceflight Log, new ed. (1986), covering early 1961 to early 1986. Other aspects of space exploration are considered by WILLIAM SIMS BAINBRIDGE, The Spaceflight Revolution: A Sociological Study (1976, reprinted with a new preface 1983); and WALTER A. McDOUGALL, The Heavens and the Earth: A Political History of the Space Age (1985).

Coverage of specific programs can be found in the general works cited above and in the following: LOYD S. SWENSON, JR., JAMES M. GRIMWOOD, and CHARLES C. ALEXANDER, This New Ocean: A History of Project Mercury (1966); on Skylab, HENRY S.F. COOPER, JR., A House in Space (1976); and W. DAVID COMPTON and CHARLES D. BENSON, Living and Working in Space (1983); RICHARD S. LEWIS, The Voyages of Columbia (1984); DAVID SHAPLAND and MICHAEL RYCROFT, Spacelab (1984), an authoritative account of the project from design to results; DOUGLAS R. LORD, Spacelab (1987), a history of the program's development; ROBERT W. SMITH, The Space Telescope (1989), a history up to the completion of its construction; ERIC J. CHAISSON, "Early Results from the Hubble Space Telescope," Scientific American, 266(6):44-51 (June 1992); and, on several of the more recent unmanned flights, RICHARD O. FIMMEL, LAWRENCE COLIN, and ERIC BURGESS, Pioneer Venus (1983); RICHARD O. FIMMEL, JAMES VAN ALLEN, and ERIC BURGESS, Pioneer: First to Jupiter, Saturn, and Beyond (1980); and BRADFORD A. SMITH, "The Voyager Encounters," in J. KELLY BEATTY and ANDREW CHAIKIN (eds.), The New Solar System, 3rd ed. (1990), pp. 107-130, a profusely illustrated account. Further books on the space probes may be found in the bibliography to the article SOLAR SYSTEM, THE. Collections of colour photographs of the Earth taken from spacecraft include ORAN W. NICKS (ed.), This Island Earth (1970); NICHOLAS M. SHORT et al., Mission to Earth: Landsat Views the World (1976); and LYNDON B. JOHNSON SPACE CENTER, Skylab Explores the Earth (1977).

The Apollo lunar landings are treated in considerable detail by NEIL ARMSTRONG et al., First on the Moon (1970); EDGAR M. CORTRIGHT (ed.), Apollo Expeditions to the Moon (1975); and CHARLES MURRAY and CATHERINE BLY COX, Apollo: The Race to the Moon (1989).

Speculative treatments of space exploration include HARRY L. SHIPMAN, Humans in Space: 21st Century Frontiers (1989); JOSEPH F. BAUGHER, On Civilized Stars: The Search for Intelligent Life in Outer Space (1985); DONALD GOLDSMITH and TOBIAS OWEN, The Search for Life in the Universe, 2nd ed. (1992); THEODORE R. SIMPSON (ed.), The Space Station (1985); IVAN BEKEY and DANIEL HERMAN (eds.), Space Stations and Space Platforms (1985); and W.W. MENDELL (ed.), Lunar Bases and Space Activities of the 21st Century (1985).