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 Samos 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.
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 ve