Earth-orbiting satellites have improved global communications, weather forecasting, navigational aids, and watching the earth's surface for the location of mineral resources and for military purposes.

The space age and practical astronautics started with the launching of Sputnik 1 by the USSR in October 1957 and of Explorer 1 by the United States in January 1958. In October 1958 the National Aeronautics and Space Administration (NASA) was created in the United States. During the next three decades, thousands of spacecraft of all varieties were launched, mostly in earth orbit. Twelve men walked on the moon's surface and returned to earth. By 1995 about 8000 artificial objects—mostly spent, upper stages of space-launch vehicles and inert spacecraft—larger than 10 cm (4 in) in diameter were circling the earth.

At 30 km above sea level, the pressure is 9.5 torrs; at 60 km, 0.21 torr; at 90 km, 0.0019. Even at an altitude of 200 km, air remains to slow down artificial satellites by aerodynamic drag; so long-duration satellites must have a higher orbital altitude.

For long stays in an earth-orbiting satellite, the effects of weightlessness might be reduced by spinning the craft so that the centrifugal effect provides artificial gravity. For this purpose, the spacecraft might be shaped like a large wheel that spins slowly around its own axis, or it might be built like a dumbbell, both ends of which rotate around the centre of gravity of the dumbbell.

Because the technology to build space-launch vehicles is related to long-range 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 programme from a space centre 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 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 over the equator, a satellite is geostationary. It moves 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.

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 centre. 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 friction with the air. This causes the orbit to decay gradually to lower and lower altitudes until the object has fully entered the atmosphere and burns up, like a meteor.

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.

Early Artificial Satellites 

Sputnik 1 was an aluminium sphere, 58 cm in diameter, weighing 83 kg. It orbited the earth in 96.2 minutes. The elliptical orbit of the satellite carried it to an apogee of 946 km and a perigee of 227 km. The sphere contained instruments which, for 21 days, radioed data concerning cosmic rays, meteoroids, and the density and temperature of the upper atmosphere. At the end of 57 days the satellite entered the atmosphere of the earth and was destroyed by aerodynamic frictional heat.

The second artificial earth satellite was also a Soviet space vehicle, called Sputnik 2. It was sent on November 3, 1957, with a dog named Laika aboard, and it relayed the first biomedical measurements in space. Sputnik 2 entered the atmosphere of the earth and destroyed itself after 162 days.

While Sputnik 2 was still in orbit, the United States successfully launched its first earth satellite, Explorer 1, from Cape Canaveral, Florida, on January 31, 1958. The 14 kg cylindrical spacecraft, 15 cm in diameter and 203 cm long, transmitted measurements of cosmic rays and micro meteorites for 112 days and gave the first satellite-derived data leading to the discovery of the Van Allen radiation belts.

On March 17, 1958, the United States launched its second satellite, Vanguard 2; a precise study of variations of its orbit showed that the earth is slightly pear-shaped. Using solar power, the satellite transmitted signals for more than six years. Vanguard 2 was followed by the American satellite Explorer 3, launched on March 26, 1958, and by the Soviet satellite Sputnik 3, launched on May 15. The 1327 kg Soviet spacecraft measured solar radiation, cosmic rays, magnetic fields, and other space phenomena until the craft's orbit decayed in April 1960.