Artificial Satellite

Artificial Satellites are man-made objects that are placed into orbit around the Earth or other astronomical body. The first artificial satellite named Sputnik I, was launched by the Soviet Union on 1957 October 4. Satellites are generally classified as military or civil, although there is considerable overlap. About 25% of current launches are for military purposes, such as photoreconnaissance, communications, electronic listening and navigation.

All satellites move in elliptical orbits governed by Newton’s laws of motion. However, the orbit is modified continuously by external forces, such as friction with the upper atmosphere and variations in the gravitational pull of the Earth. The rates of change depend on the height of the satellite and the inclination of the orbital plane to the equator.

Changes in the density of the atmosphere, due principally to solar activity, cause variable drag on a satellite. The overall effect is to change the eccentricity, with the orbit becoming more and more circular. Eventually, the satellite will spiral inwards and experience increasing drag until it finally plunges into the denser regions of the atmosphere and burns up. Other factors affecting a satellite’s orbit include atmospheric tides and winds. Solar radiation pressure has a major effect on satellites with a large area/mass ratio, such as those with large solar arrays. The overall result is that no satellite can have a stable orbit below a height of about 160 km (100 mi). At this height the orbital period is about 88 minutes.

Photographic, visual and laser techniques are all used to track satellites. However, most satellites are tracked through their telemetry, by measuring the phase difference between a signal transmitted from the ground and a return signal from the satellite. This method can determine an Earth orbiting satellite’s distance to within 10 m (33 ft).

If the sky is clear, satellites can be seen after dusk or just before dawn, when the sky is dark but the satellite is still illuminated by the Sun. The length of these visibility periods depends on the time of the year, the latitude of the observer and the orbit of the satellite. The satellite’s brightness depends on the nature and curvature of the reflecting surface, the phase angle and its distance and altitude from the observer. Some objects, such as the international space station, rival the brightest planets, but others can only be seen through medium-sized telescopes.

The rate at which satellites cross the sky depends on their height above the Earth. Low satellites may cross the sky in about 2 minutes, but those at heights of about 2000 km (1200 mi) may take half an hour. Satellites at heights of about 36,000 km (22,400 mi) take 24 hours to complete one revolution. If the orbital inclination is 0º, the satellite appears motionless in the sky. Many meteorological and communication satellites have been launched into these geostationary orbits.

Roughly two-thirds of all the satellites launched have been from the former Soviet Union.

By 2002 there had been almost 5000 successful launches, with an annual launch rate of around 90. Most of these carried single satellites, but it is increasingly common for two or more to be carried by one launch vehicle. At the same time, other objects such as the upper rocket stage and protective nose cone may also be released, creating a cloud of artificial debris – an undesirable class of satellites – around Earth.

Questions to Ponder

• What makes a satellite geosynchronise itself with Earth's orbit?
• What are the differences between Polar & Geosynchronous satellites?
• Find out the possible applications of Polar & Geosynchronous satellites.