Atmospheric Drag

When a spacecraft in orbit slows down, gravity pulls it closer to Earth and it descends into thicker air, which slows it down even more. This keeps happening until the spacecraft plunges deeper into the atmosphere. This process is called orbital decay, and it is caused by atmos­pheric drag.

The Space Shuttle usually orbits the Earth at an altitude of about 185 to 250 miles (about 300 to 400 kilometers). The atmosphere here is about a million times thinner than the atmosphere at sea level, but there is still enough air to slow the spacecraft down. The Space Shuttle is in space for only a week or two, however, and orbital decay is not a problem dur­ing such a short time.

The International Space Station stays in space all the time. It orbits Earth at a height of 200-250 miles (320-400 kilometers). Atmospheric drag lowers its orbit by a few feet per day, or about 1.24 miles (2 kilometers) a year, so it has to be propelled up to a higher orbit every few months.

The amount of atmospheric drag changes from day to day and season to season because of the effect of the Sun on the atmosphere. The Sun heats the atmosphere on the daylight side of the Earth and makes it expand. Spacecraft in orbit experience more drag (and increased orbital decay) from the thicker air expanding upward from below.

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The air-cushion vehicle (ACV) is a form of transportation that travels just above land or water on a cushion of air. It is also known as a hovercraft or ground-effect machine.