How high up is outside the atmosphere?

The answer is the air just keeps getting thinner and thinner as you go up, and we are not exactly sure where it completely stops. Satellites that are orbiting about 100 miles up are outside nearly all the atmosphere, but there are still a few air molecules even at that altitude, and when the satellite bumps into them it is slowed down a little bit. And when it is slowed – even a very small amount – it drops to a slightly lower orbit, where there are still more molecules of air to bump into and where the pull of gravity is more powerful. And so it goes until it is slowed down so much that the pull of gravity wins the tug-of-war, and the satellite plunges down through the air, usually burning up on the way because its high velocity creates such intense heat from friction with Earth’s atmosphere. Even at extremely high altitudes – say, 2000 miles or more where there are no air molecules – there are other things to bump into. For example, there is cosmic dust (debris ejected from comets), as well as solar wind (electrified particles of gas which stream outward from the Sun), and even radiation pressure from the rays of the Sun. The effects of these things are very small, of course, but over long periods of time they will generally cause any orbit to decay.

The result of this is that satellites at low altitudes, say about 100 miles, stay up for only a few weeks. Those that are as high as 1000 miles can stay up for many years, and as we know the moon has been up there for millions of years.

Satellite orbits come in different sizes and shapes, depending on what people want to use the satellite for. If you just launch a satellite into orbit to see if you can do it (that was the main idea with the first few satellites launched), it will go into an elliptical orbit. The point in that ellipse where the satellite comes closest to the Earth is called the perigee, and the point where it is farthest away is called the apogee.

Sometimes people want a satellite to be in an elliptical orbit. One of the first little Environmental Research Satellites (ERS) vehicles was sent up to measure particles in the Van Allen radiation belt, and for this purpose the TRW wanted it to pass through many different layers at different distances from the Earth. So they put it in a highly elliptical orbit, with a perigee of 5357 miles and an apogee of 69,316 miles, or about 13 times as far from the Earth as the perigee.

If you should want to put your satellite into a circular orbit, the easiest way to do it is first to put it into an elliptical orbit with the apogee at the same altitude you want for your circular orbit. Then when the satellite arrives at apogee, you point it along that circular path and fire a small rocket engine, often called an apogee motor. That adds enough speed so that the satellite will now stay in the circular orbit instead of falling back down to its old perigee as it goes around the Earth. Very often, the apogee motor is carried in the satellite itself and is not a part of the launch vehicle.

For satellites that are “looking” at the Earth with cameras or radio beacons – such as weather or communication satellites – we prefer circular orbits, so that they will stay at the same altitude all the time.

red diffraction

“Without atmosphere a painting is nothing.” (Rembrandt). Illustration: Megan Jorgensen

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