## Once a ship is in orbit, do we have to do anything to keep it there?

Yes. Satellite orbits will degrade (slow and fall slightly) over time. As a ship travels for a long time, it goes through space, which is almost but not completely empty. The collective force from millions of tiny collisions with floating matter in space will decrease an orbiting object's speed. This force is very small since the floating matter is usually nothing more than dust and occasional clouds of gas. Overall, the effect of all these tiny forces hitting the satellite will act like drag or resistance on a plane flying in our atmosphere.

Drag is the slowing or resistance force caused by air. Try swinging your arm around your body with your palm facing the direction your hand is moving. Do you feel the wind on your palm? That's drag. As you might guess, the faster your hand moves, the harder the air molecules hit your hand, and the more drag force will be exerted on your hand. If your hand is not moving at all compared to the air, then there isn't any drag force acting on it. We say that the drag force on an object like your hand is "directly proportional" to its speed. That just means that as the speed goes up the force goes up, and as the speed goes down, the force goes down.

Now, back to satellites in space. If space were a perfect vacuum, meaning there was absolutely nothing in it, we probably wouldn't have to worry about any of this, and stuff would stay in orbit as long as we liked because of inertia. But space isn't a perfect vacuum. Even though that dust and dirt and gas that hits the satellite is very thin and spread out, its effect is like an extremely thin atmosphere. Even though any orbiting object is moving at thousands of miles per hour (speeds which would cause an object to break apart and burn up if it was in the atmosphere being hit by bazillions of air molecules) too few particles are hitting it to cause a significant drag force like the one you felt on your hand. However, over long periods of time, the effect of the particles colliding with the orbiting object are significant and slow the ship. For instance, NASA scientists estimate that the space shuttle, about the size of a passenger plane, can stay in orbit for about a month before this force causes it to slow enough that it falls out of its orbit.

Sometimes NASA scientists dip a satellite into the atmosphere of a planet on purpose so that drag will slow it. This is called aerobraking.

What is aerobraking and retroburning?
What could cause an orbit to fail?