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Life in Free Fall

Ken Tapping, February 6, 2008

In the sky this week...

> Mars still dominates the southern sky during the night.

> Saturn rises about 7 p.m. and Jupiter and Venus rise, fairly close together, around 6 a.m.

> The Moon will be New on February 6.


How many times have we heard people say or write that astronauts float around while orbiting the Earth because in space there is no gravity pulling at them? Of course that is absolute rubbish. There is gravity everywhere in the universe. Imagine the gravitational forces holding the Moon in its orbit around the Earth, or our Earth in its orbit around the Sun. For that matter, consider the forces holding our galaxy together. OK then, how can astronauts orbiting the Earth in the shuttle or space station float around as though they have no weight?

Imagine we could build a tower 1,000 kilometres high. You go to the top and then jump off. What would happen? The answer is that the same thing would happen as would take place if you leaped off the Eiffel Tower. It would just take you a bit longer to get to the ground. Now imagine this tower is strong enough for you to take a nice big cannon to the top. You load it with gunpowder and a cannonball, point the gun horizontally, and fire. The cannonball will start off moving horizontally, and then because of the Earth's gravity, start to curve downwards until it eventually hits the ground. Load the gun again. This time put in a lot more gunpowder. Now fire. The cannonball will fly a lot further before it curves down to hit the ground. As we load up with more and more powder, the cannonball goes further and further before it hits the ground.

However, remember that the Earth is not flat; it is a sphere, so we could load enough powder in the gun to give the cannonball a trajectory that never hits the ground. As the cannonball's path curves, the Earth curves away beneath it, so that it never hits the ground. Your cannonball is falling around the Earth. It is in orbit.

Since we cannot make a gun that powerful, and we do not have a tower 1,000 kilometres high, we use rockets instead. The rocket does two things: it lifts the spacecraft up above the atmosphere, and then gives it enough horizontal speed for the curvature of its path to take it around the Earth. The reason the astronauts can float around is that they and their spacecraft, or space station, are all falling together. In fact, the best way to describe an orbiting spacecraft or weightless astronauts is that they are in "free fall". Similarly, the Moon is falling around the Earth, and the Earth is falling around the Sun, and so on. Getting from one planet to another consists of putting yourself in the right trajectory and then falling all the way there.

One of the items in the news at the moment is that an American spy satellite is about to re-enter our atmosphere and burn up. That satellite is orbiting the Earth at a low altitude, where there is still some atmosphere. As the satellite moves through the air at several kilometres a second, pushing the air out of the way exerts a small drag. This eats away at the satellite's kinetic energy. Gradually, the satellite drifts lower and lower, into denser and denser atmosphere, until it burns up and falls to Earth. Falling satellite debris is obviously a problem, but all the junk orbiting higher up, so that it can remain orbiting the Earth for centuries, is a bigger one. 

TappingKen Tapping is an astronomer at the National Research Council Herzberg Institute of Astrophysics (NRC-HIA), and is based at the Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9 Tel (250) 493-2277, Fax (250) 493-7767,