Table of Contents

A Guide to Microgravity for All Ages

Weightlessness

In orbit, objects seem to float as if there were no gravity. Actually, at an orbital altitude of 200 km, the height of the space shuttle's orbit, gravity is only 10% less than at the earth's surface. Objects seem to float with respect to one another because they are all falling at the same rate. If you drop a pencil, it falls. You observe the effect of gravity. If you drop two pencils, they fall at the same rate and their position with respect to one another does not change.

In the absence of air drag, all objects fall at the same rate. You can demonstrate this by dropping two objects of different weight and size, such as a basketball and a squash ball. Another demonstration can be done using a sheet of paper and the basketball. The plain sheet and the basketball fall at different rates due to air drag. If you tightly crumple up the sheet, the drag on the sheet is reduced and they will fall at closer speeds. In a vacuum, where there is no drag, even a feather and a hammer would fall at the same rate. Another useful demonstration is placing a penny on top of a magazine and dropping them. The penny will stay on top of the magazine, since they fall at the same rate.

Imagine that you are inside an elevator and the cable is cut. You fall at the same rate as the elevator so you don't move with respect to the elevator. Similarly if you release a pencil in front of you, it also falls at the same rate and does not move with respect to you. It appears to float in front of you because it is falling at the same rate as you are. If you were standing on a chair inside the elevator and stepped off it, you would stay at the same height as the chair. Because you are falling at the same rate as the chair and the elevator, you appear to float just like an astronaut.

This is how the space shuttle orbits-it's like a falling elevator car. Imagine driving down the highway and looking out the window at another car. The other car seems to be stationary to you, because both cars are moving at the same speed. In fact, were it not for the scenery going by, you wouldn't even be able to tell that you were moving.