by: Gary Davis
Depending on the mission of a satellite, it will eventually be no longer useful. Some satellites end their useful life when falling out of orbit and burning up in the atmosphere. Others continue to orbit the earth long after the mission has ended.
Depending on the altitude of the orbit, a satellite will either stay in orbit or fall back to earth. Below 1000 km (620 miles) satellites experience friction from the atmosphere which slows them down and because of that the altitude of the orbit decreases until eventually the satellite falls out of orbit and plunges back to earth where it burns up in the atmosphere. Heavy but small satellites maintain their orbits longer than light, but big satellites. A higher mass has more kinetic energy than a small mass, so more friction is needed to slow the satellite down. (it is like a big heavy cargo train, it just stops much slower than a normal car).
The First Satellites
In 1957 the Soviet Union launched successfully the first satellite (Sputnik) that orbited the earth. Sputnik had an elliptical orbit with a minimum altitude of 225 km (140 miles) and a maximum altitude of 900 km (559 miles). It stayed in orbit for about 3 months after which is fell back to earth and burned up in the atmosphere.
The USA successfully launched its first satellite (Explorer 1) in 1958. The orbit was very much more elliptical with a minimum altitude of 360 km (224 miles) and a maximum altitude of 2500 km (1553 miles). During this first mission scientists discovered the Van Allen Radiation Belts; zones in space around the earth, and the planets Jupiter and Saturn, containing high-energy protons and electrons.
In 1960 the USA launched a satellite that brought with it the first artificial object that was ever recovered from space. The satellite rejected a capsule which fell back to earth and was later recovered. This mission led the way to photos of the earth taken from a satellite to be recovered.
Current Count of Satellites
Because of the many launched of satellites (over 4800) the earth is now surrounded by artificial debris. This debris ranges from small objects like astronauts gloves and tools to very big objects like obsolete satellites and used rocket parts. There have been over 25.000 individual objects cataloged of which over 8000 are still in orbit. The rest fell back to earth and burned up in the atmosphere.
There is a good reason for cataloging all space debris. When a satellite launch is planned the path of the satellite is known and is checked against the catalog of space debris. You can imagine that a collision with an old part of a previously launched rocket would be disastrous. But even collisions with relatively small debris can cause huge problems. A screwdriver in orbit can have a speed of 27.000 km (16.777 miles) per hour. If this tiny screwdriver hits the space shuttle for instance it will blast a hole in the shuttle, killing all people inside. Space debris is actually becoming a problem especially for missions in low earth orbit.
By Gary Davis
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