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 2nd 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. More recently, fantasies
of spaceflight have been nourished by science fiction.
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- 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 3rd century BC the astronomer Aristarchus of Sámos
asserted that the earth moved around the sun. Hipparchus, another
Greek, prepared information about stars and the motions of the
moon in the 2nd century BC. In the 2nd century AD Ptolemy of
Alexandria placed the earth at the center of the solar system
in the Ptolemaic system.
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- 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 German astronomer Johannes Kepler.
Italian scientist Galileo and British scientists Edmund Halley,
Sir William Herschel, and Sir James Jeans were other astronomers
who made contributions pertinent to astronautics.
Celestial
Models by Ptolemy and Copernicus
Currently, most people
consider it obvious that the sun is at the center of the solar
system, but the sun-centered (heliocentric) concept was slow
to evolve. In the 2nd century AD, Claudius Ptolemy proposed a
model of the universe with the earth at the center (geocentric).
His model (shown left) depicts the earth as stationary with the
planets, moon, and sun moving around it in small, circular orbits
called epicycles. Ptolemy's system was accepted by astronomers
and religious thinkers alike for several hundred years. It was
not until the 16th century that Nicolaus Copernicus developed
a model for the universe in which the sun was at the center instead
of the earth. The new model was rejected by the church, but it
gradually gained popular acceptance because it provided better
explanations for observed phenomena. Ironically, Copernicus'
initial measurements were no more accurate than Ptolemy's, they
just made more sense.
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- 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 English mathematician
and physicist Sir Isaac 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.
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- 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).
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- In the United States,
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, 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,
published Die Rakete zu den Planetenräumen (The Rocket
into Interplanetary Space). The book was supplemented by
Walter Hohmann, 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.
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- World War II (1939-1945)
provided the impetus and motivation for the development of long-range
suborbital rockets. The United States, the USSR, 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 United States for experimental research
in vertical flights. Some German engineers went to the USSR after
the war, but the leading rocket experts went to the United States,
including Walter Dornberger, and Wernher von Braun.
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- Spacecraft
Spacecraft that do not
have to carry humans can be of a great variety of sizes, from
a few centimeters to several meters in diameter, and many shapes,
depending on the purposes for which they are designed. Spacecraft
that do not carry a crew have radio-transmitting equipment, both
to relay information back to earth and to signal the position
of the spacecraft.
Saturn
5 Rocket
A Saturn 5 rocket rises
slowly from its launch pad in the early stages of the Apollo
17 mission. More than 110 m (363 ft) tall, the multistage rockets
are fueled by kerosene and liquid hydrogen. In addition to being
used extensively in the Apollo Space Program, one of the massive
Saturn 5 rockets was used to launch NASA's Skylab in 1973.
- Manned spacecraft
must fulfill more exacting requirements than unmanned vehicles
because of the needs of the human occupants. A manned space vehicle
is designed to provide air for the astronauts, food and water,
navigation and guidance equipment, seating and sleeping accommodations,
and communication equipment so the astronauts can send and receive
information from the control center on earth. A distinctive feature
of manned spacecraft is the heat shield that protects the vehicle
as it reenters the atmosphere
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- Propulsion
The rocket engines
that launch and propel spacecraft are of two main types: solid-propellant
rockets, which use chemicals that burn in a fashion similar to
gunpowder, and liquid-propellant rockets, which use liquid fuels
and oxidizers carried in separate. Most of the rockets that have
launched American spacecraft have had several separate rocket
stages; each stage is separately powered with its own fuel. After
the fuel in each stage is consumed, the empty stage drops away
from the spacecraft.
- Because the technology
to build space-launch vehicles is closely akin to that for long-range
ballistic missiles, the United States and the USSR were the only
two countries that had the ability to launch satellites from
1957 to 1965. In subsequent years France, Japan, India, and China
launched earth satellites of ever-increasing sophistication,
and in May 1984 the 13-member European Space Agency began its
own launch program from a space center at Kourou in French Guiana.
The United States and the USSR, however, remained the only nations
with launch vehicles capable of placing in orbit payloads of
many tons-the prerequisite for manned spaceflight.
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- Launching and Reentry
A space vehicle
is launched from a specially constructed launchpad, where the
space vehicle and the rocket that propels it are set up and carefully
inspected before launching. The operation is supervised by engineers
and technicians in the nearby control center. When all preparations
are complete, the rocket engines are fired and the rocket and
spacecraft lift off.
Reentry is the name applied
to the problem of slowing down a returning spacecraft so that
it lands on earth without being destroyed by the intense heat
caused by friction with the earth's atmosphere. The spaceflights
of the U.S. Mercury, Gemini, and Apollo programs overcame the
problem of reentry by protecting the leading surface of the returning
capsule with a specially developed heat shield, made of metals,
plastics, and ceramic materials that melt and vaporize during
reentry, thereby carrying off or dissipating the heat without
damage to the capsule or the astronauts. The heat shield developed
to protect the space shuttle during reentry consists of a covering
of ceramic tiles individually cemented to the shuttle's hull.
Prior to the development of the space shuttle, which lands on
a runway, all American manned spacecraft used the ocean to cushion
the impact of landing; the astronauts and the capsules were retrieved
quickly by helicopter and taken aboard waiting naval vessels.
Soviet cosmonauts have landed on solid ground in various sites
in Siberia.
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- Orbiting the Earth
The orbit of a
spacecraft around the earth can be in the shape of a circle or
an ellipse. A satellite in a circular orbit travels at a constant
speed. The higher the altitude, however, the lower the speed
relative to the surface of the earth. Maintaining an altitude
of 35,800 km (22,300 mi) over the equator, a satellite is geostationary.
It moves in geosynchronous orbit, at exactly the same speed as
the earth, so it remains in a fixed position over some particular
spot on the equator. Most communications satellites. are placed
in such orbits.
Polar Orbiting
Artificial Satellite
Nimbus, an environmental
satellite, takes photos as it circles the earth several times
in a day in an orbit that passes over the North and South Poles.
Because the earth rotates, each pass produces a new set of images
and the entire earth can be photographed every day. Information
about the earth's atmosphere and oceans is relayed back to the
surface.
GOES Weather
Satellite
Broadcasters use data
from meteorological satellites to predict weather and broadcast
storm warnings when necessary. Satellites like the GOES (Geostationary
Operational Environmental Satellite) collect meteorological and
infrared information about the atmosphere and the ocean. A camera
on the GOES is continuously pointed at the earth, broadcasting
satellite images of cloud patterns both day and night. Here,
the GOES-C satellite is being encapsulated inside its payload
fairing aboard a Delta rocket.
Solar Maximum
Mission Satellite
The Solar Maximum Mission
Satellite was a scientific satellite designed to study solar
radiation. Launched in early 1980, the craft failed later in
the year. It was repaired and relaunched by the space shuttle
in 1984, collecting information until 1989, when it was destroyed
by a solar flare. Information collected by the satellite indicated
that the corona displays an unexpectedly high amount of violent
activity related to sunspot cycling. Data also showed that sunspots
reduce the amount of solar energy reaching the earth's atmosphere.
- In an elliptical orbit,
the speed varies and is greatest at perigee (minimum altitude)
and least at apogee (maximum altitude). Elliptical orbits
can lie in any plane that passes through the earth's center.
A polar orbit lies in a plane passing through the North and South
poles-that is, it passes through the axis of rotation of the
earth. An equatorial orbit is one that lies in a plane passing
through the equator. The angle between the orbital plane and
the equatorial plane is called the inclination of the orbit.
- The earth rotates
once every 24 hours under a satellite in a polar orbit. A polar-orbit
weather satellite, carrying television and infrared cameras,
can thus observe meteorological conditions over the entire globe
from pole to pole in a single day. An orbit at another inclination
covers a smaller portion of the earth, omitting areas around
the poles.
- As long as the orbit
of an object keeps it in the vacuum of space, the object will
continue to orbit without propulsive power because no frictional
force slows it down. If part or all of the orbit passes through
the atmosphere of the earth, however, the body is slowed by aerodynamic
friction with the air. This causes the orbit to decay gradually
to lower and lower altitudes until the object has fully reentered
the atmosphere and burns up, like a meteor.
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- Space Programs-Unmanned
The long history of myths,
dreams, fiction, science, and technology culminated in the dramatic
launching of the first artificial orbiting earth satellite, Sputnik
1, by the USSR on October 4, 1957. Sputnik Zemli,
meaning "traveling companion of the world" is the Russian
name for an artificial satellite, a companion of the earth as
it travels around the sun. In the United States, this name was
abbreviated to Sputnik.
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