Because NASA Space Shuttles did not visit the Space Station for over two years after the demise of Space Shuttle Columbia, its orbit has been steadily declining due to atmospheric drag. Even though the Space Station is often said to be in outer space, a small amount of the Earth’s atmosphere still exists at its normal orbit of about 172.6 nautical mile (319.6 kilometers) of perigee (closest point to the Earth in its orbit) and 187.3 nautical mile (346.9 kilometers) of apogee (furthest point)

In fact, the atmosphere of the Earth consists of many gaseous particles (what we call air) that can cause drag to the Space Station. These particles include nitrogen, oxygen, argon, carbon dioxide, and other smaller amounts of gases.

There is no definite boundary between the Earth’s atmosphere and outer space (sometimes simply called space), which is the relatively empty regions of the universe (thus, containing few particles with mass) outside the atmospheres of celestial bodies. The altitude of about 75 miles (120 kilometers) is generally considered the boundary where atmospheric effects (like drag) becomes noticeable for spacecraft de-orbiting and re-entering the Earth’s atmosphere.

With this said, the International Space Station (ISS) is at an average altitude of around 207 miles (332 kilometers). It is losing about 300 feet (90 meters) in altitude each day. According to NASA data, this height is the lowest for the ISS in its nine years orbiting around the Earth.

The station is in no danger of de-orbiting and plunging into the Earth, however, a major boost from a NASA Space Shuttle is needed for the station to regain a much higher orbit. Russian spacecraft, such as the Progress cargo craft, have been maintaining the station’s height, but they are unable to give it the heavy boost that only a NASA Shuttle can perform.

The first time that a Space Shuttle will be able to give the station its heavy booster thrust will be sometime later next year. Until that time, the Russian’s have scheduled numerous small reboost firings over the next year or so to keep the station at its current orbital height and to prepare for further personnel changes at the Space Station. One such firing just occurred on Thursday, March 15, 2007, when the Progress 23 supply ship’s engines were fired. The 12 minutes, 32 second burn raised the orbit by about 3 miles (4.8 kilometers).

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