SPACEEXPLORE

SPACE EXPLORATION

SPACE EXPLORATION, science and engineering of spacecraft and space probes used for investigation of physical conditions in space and in celestial bodies (stars, and planets and their moons). In a large sense, space exploration, or astronautics, is interdisciplinary in that it draws upon the findings of such fields as physics, astronomy, mathematics, chemistry, biology, medicine, electronics, and meteorology.

Automated space probes and human spaceflight have provided a wealth of scientific data on the nature and origin of the solar system and the universe; earth-orbiting satellites have improved global communications, weather forecasting, navigational aids, and reconnaissance of the earth’s surface for the location of mineral resources and for military purposes.

The space age and practical astronautics commenced with the launching of Sputnik 1 by the Soviet Union in October 1957 and of Explorer 1 by the U.S. in January 1958. In October 1958 the National Aeronautics and Space Administration (NASA) was created in the U.S. During the next four decades, more than 4600 spacecraft of all varieties were launched, mostly in earth orbit. The overwhelming majority of these were launched by the Soviet Union and the U.S., but other countries also carried out successful launches. More than 300 individuals flew in space, and 12 men walked on the moon’s surface and returned to earth. In the late 1990s many thousands of objects—mostly spent, upper stages of space-launch vehicles and inert spacecraft—were circling the earth.

The Physics of Space

The boundary between the atmosphere of the earth and space is diffuse rather than sharp. Because the density of air diminishes gradually with increasing altitude, the air in the upper atmosphere is so thin that it merges almost imperceptibly with space. The barometric pressure, which is a measure of atmospheric density, is 760 torrs at sea level. (One torr is defined as the pressure caused by the weight of a column of mercury 1 mm/0.039 in. high at sea level.) At 30 km (19 mi) above sea level, the barometric pressure is 9.5 torrs; at 60 km (37 mi), 0.21 torr; at 90 km (56 mi), 0.0019 torr. Even at an altitude of 200 km (124 mi), sufficient residual atmosphere remains to slow down artificial satellites by aerodynamic drag; thus, long-duration satellites must have a higher orbital altitude.

Radiation in Space

By ordinary standards, space is a vacuum. Space, however, does contain very minute quantities of gases such as hydrogen and small quantities of meteorites and meteoric dust. X rays, ultraviolet radiation, visible light, and infrared radiation from the sun all traverse space. Cosmic rays, consisting mainly of protons, alpha particles, and heavy nuclei, are also present.

Gravitation

The law of universal gravitation states that every particle of matter in the universe attracts every other particle with a force directly proportional to the products of their masses and inversely proportional to the square of the distance between them. Consequently, the gravitational pull exerted by the earth upon all other bodies (including spacecraft) diminishes with distance from the earth. The gravitational field, however, extends to an infinite distance; gravity does not cease to act at any altitude. A spacecraft is said to be weightless when it is in orbit around the earth (or around any other celestial body) because the centrifugal effect (which acts away from the center) is then equal and opposite to the force of gravity. Under these conditions, objects in a spacecraft seem to float in space. In the same way, the moon does not fall toward the earth because of the centrifugal effect that balances the force of gravity.

Aerodynamic forces on the lifting surfaces (for example, the wings) of an aircraft keep it aloft against the force of gravity, but a space vehicle cannot stay aloft in this way because of the absence of air in space. The spacecraft, therefore, must orbit if it is to remain in space. Aircraft flying in the earth’s atmosphere can use propellers and winged surfaces for propulsion and maneuvering, but spacecraft cannot do so because of the lack of air. A space vehicle must rely on the reaction of rockets for propulsion and maneuvers, based on Newton’s laws of motion. When a spacecraft fires a rocket blast in one direction, reaction against the rocket exhaust imparts momentum to the spacecraft in the opposite direction.

Humans In Space

Space is a hostile environment for humans in a number of ways. It contains neither air nor oxygen, so human beings are unable to breathe. The vacuum of space can destroy an unprotected human body in a few seconds by explosive decompression. Temperatures in space in the shadow of a planet approach absolute zero; on the other hand, temperatures can become fatally high under direct solar radiation. Energetic solar and cosmic radiations in space also may be fatal to an unshielded person who is not protected by the atmosphere of the earth. These environmental conditions may also affect the instruments and devices used in spacecraft, so the design and construction of these materials are dictated by the space environment. Experiments in weightlessness for long periods of time have been studied intensively to discover what adverse effects this condition will have on humans in space.

Humans are protected against the space environment in several ways. They are enclosed inside a hermetically sealed cabin or space suit, with a supply of pressurized air or oxygen to approximate conditions on earth. Air conditioning controls the temperature and humidity inside the cabin or space suit. Absorbing and reflecting surfaces on the outside of the spacecraft regulate the amount of heat radiation affecting the craft. Furthermore, space journeys are carefully planned to avoid the intense radiation belts around the earth. On long interplanetary voyages of the future, heavy shielding may be necessary to protect against solar radiation storms, or crews might be sheltered in a central position within the spacecraft with supplies and equipment to surround and shield them. For lengthy space journeys, or for prolonged stays in an earth-orbiting satellite, the effects of weightlessness are reduced by spinning the craft so that the centrifugal effect provides artificial gravity.

History

People dreamed of spaceflight for millennia before it became reality. Evidence of the dream exists in myth and fiction as far back as Babylonian texts of 4000 bc. The ancient Greek myths of Daedalus and Icarus also reflect the universal desire to fly. As early as the 2d century ad the Greek satirist Lucian wrote about an imaginary voyage to the moon. In the early 17th century the German astronomer Johannes Kepler wrote Somnium (Sleep), which might be called a scientific satire of a journey to the moon. The French writer and philosopher Voltaire, in Micromégas (1752), told of the travels of certain inhabitants of Sirius and Saturn; and in 1865 the French author Jules Verne depicted space travel in his popular novel From the Earth to the Moon. The dream of flight into space continued unabated into the 20th century, notably in the works of the British writer H. G. Wells, who published The War of the Worlds in 1898 and The First Men in the Moon in 1901. Fantasies of spaceflight continue to be nourished by science fiction.

Early Developments

During the centuries when space travel was only a fantasy, researchers in the sciences of astronomy, chemistry, mathematics, meteorology, and physics developed an understanding of the solar system, the stellar universe, the atmosphere of the earth, and the probable environment in space. In the 7th and 6th centuries bc, the Greek philosophers Thales and Pythagoras noted that the earth is a sphere; in the 3d century bc the astronomer Aristarchus of Samos asserted that the earth moved around the sun. Hipparchus, another Greek, prepared information about stars and the motions of the moon in the 2d century bc. In the 2d century ad Ptolemy of Alexandria placed the earth at the center of the solar system in the Ptolemaic system.

Scientific Discoveries

Not until some 1400 years later did the Polish astronomer Nicolaus Copernicus systematically explain that the planets, including the earth, revolve about the sun. Later in the 16th century the observations of the Danish astronomer Tycho Brahe greatly influenced the laws of planetary motion set forth by Kepler. Galileo, Edmund Halley, Sir William Herschel, and Sir James Jeans were other astronomers who made contributions pertinent to astronautics.

Physicists and mathematicians also helped to lay the foundations of astronautics. In 1654 the German physicist Otto von Guericke proved that a vacuum could be maintained, refuting the old theory that nature “abhors” a vacuum. In the late 17th century Newton formulated the laws of universal gravitation and motion. Newton’s laws of motion established the basic principles governing the propulsion and orbital motion of modern spacecraft.

Despite the scientific foundations laid in earlier ages, however, space travel did not become possible until the advances of the 20th century provided the actual means of rocket propulsion, guidance, and control for space vehicles.

Rocket Propulsion

The techniques of rocket propulsion also originated long ago. Ancient rockets used gunpowder as fuel, very much as in fireworks today. In ad 1232 in China the city of Kaifeng was reportedly defended against the Mongols by the use of rockets. From the Renaissance onward, references were made to the proposed or actual military use of rockets in European warfare. As early as 1804 the British army established a rocket corps equipped with rockets that had a range of about 1830 m (about 6000 ft).

In the U.S. the foremost pioneer in rocket propulsion was Robert Goddard, a professor of physics at Clark College (now Clark University). He began experimenting with liquid fuels for rocketry in the early 1920s. He launched the first successful liquid-propelled rocket on March 16, 1926. During the same general period, studies on spaceships and rocket propulsion were being conducted in several parts of the world. About 1890 Herman Ganswindt (1856–1934), a German law student, conceived of a solid-propellant spaceship that demonstrated a marked awareness of the stability problem. Konstantin Tsiolkovsky, a Russian schoolteacher, published in 1903 A Rocket into Cosmic Space, which proposed the use of liquid propellants for spaceships. In 1923 a German mathematician and physicist, Hermann Oberth (1894–1989), published his prophetic work, Die Rakete zu den Planetenräumen (The Rocket into Interplanetary Space). The book was supplemented by Walter Hohmann (1880–1941), a German architect, who published in 1925 Die Erreichbarkeit der Himmelskörper (The Possibility of Reaching Celestial Bodies), which contained the first detailed calculation of interplanetary orbits.

World War II provided the impetus and motivation for the development of long-range suborbital rockets. The U.S., the Soviet Union, Great Britain, and Germany simultaneously developed rockets for military purposes. The most successful were the Germans, who developed the V-2 (a liquid-propellant rocket used in the bombardment of London) at Peenemünde, a village near the Baltic coast. At the close of the war, the U.S. Army brought back a number of the V-2s, which were then used in the U.S. for experimental research in vertical flights. Some German engineers went to the USSR after the war, but the leading rocket experts went to the U.S., including Walter Dornberger (1895–1980), and Wernher von Braun.