What keeps satellites in orbit?

The International Space Station

Above is a picture of the International Space Station (ISS). The first parts of the ISS were launched in 1998, and crews began occupying it in the year 2000. It is a collaboration of 5 space agencies representing 15 countries doing science experiments and technology demonstrations. (Here is a more detailed history.)

This is a real photograph of the ISS, not an artist's conception. Who took the picture? The ISS has been upgraded and repaired by many space missions, including the U.S. Space Shuttle. This photo was taken in May of 2010 by the crew of the Space Shuttle Atlantis just after it undocked from the Station at the conclusion of a servicing mission.

Here's a quick quiz. The ISS orbits the Earth about once every 90 minutes. It is at an altitude of about 250 miles. What keeps such a thing in its orbit? Why doesn't it fall to the ground?

(a) It stays up by catching a magnetic field

(b) It's in outer space, and there is no gravity in outer space

(c) It continuously fires a rocket to keep it up

(d) Once at orbital velocity, its own momentum keeps it in orbit

I'll give the correct answer below. To prepare the way, let's do a thought experiment with a humbler projectile, a baseball.

A baseball throwing contest

Parabolic trajectories of a ball thrown horizontally from the top of a cliff

I stand at the edge of a cliff with Cy Young, one of the greatest baseball pitchers of all time. I'm curious to know how much farther he can throw than I can. So I challenge him to a contest. Each of us throws a baseball horizontally. The picture above shows the trajectories. Mine is #1 and Cy's is #2. His ball hits the ground about three times as far from the cliff as mine. If we could get rid of the friction of the air, each ball would maintain its horizontal speed while having a vertical acceleration toward the ground because of gravity. These vertical and horizontal motions combine to make the parabolic curves shown in the picture. Cy can give his ball a much faster horizontal speed, so I lose the contest pretty badly.

Trajectories of a ball thrown horizontally from the top of a cliff, including one thrown fast enough to be in a circular orbit

Suddenly, out of the blue, Superman lands nearby and boasts that he can do even better. He picks up a baseball and throws it at over 20 times the speed of sound, nearly 17,700 mph. The ball travels so far in such a short time that I can no longer draw an accurate picture with the ground being flat. I have to include the curvature of the Earth. Above is what Superman's trajectory, labelled #3, might look like compared to Cy's and mine. If he throws the ball at just the right speed (again neglecting air resistance) it gets deflected by gravity along a curved path that matches the curvature of the earth. The baseball is in a state of free fall, but by the time it falls by one foot, the Earth's surface has also curved by one foot. The ball stays the same distance above the ground. Now there’s a technical name for this trajectory. It’s called an orbit.

Falling around the earth

The best answer to the quiz question is (d), but it's not a perfect answer.

Earth's gravitational influence extends into outer space. At an altitude of 250 miles, it's only about 12% weaker than at sea level. Like Superman's baseball, a satellite is actually on a free fall trajectory that matches the Earth's curvature. ISS passengers experience "weightlessness" because they are also in free fall.

In reality, Superman's ball won't stay in orbit, the friction from the air would both slow it down and burn it up. If it survives, it will fall to the ground. In rocket science parlance, its orbit decays, which means that it loses height because of frictional drag. This is why satellites are launched into space. If we can fly the satellite in a vacuum, we get rid of this friction and the satellite could stay up indefinitely.

Satellite orbits do decay, however, because the vacuum isn't perfect. At the altitude of the ISS, the atmospheric pressure might typically be only one trillionth that at sea level. That sounds like nothing, but because the satellite moves at over 17,000 mph, there is enough friction to cause a slow decay. The ISS, for example, does require an occasional firing of a booster rocket.