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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 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.
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.
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.
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.
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.
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.
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.
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.