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Weightless in orbit

Gravity and orbits · level 4, lesson 4 of 6

A question to start

At the space station, gravity is still 89% as strong as on the ground. Yet the astronauts float, and their drinks form wobbly balls in the air. If gravity is nearly as strong, why do they float?

The idea

Imagine standing on a bathroom scale in a lift. If the cable snapped (do not try this), you, the scale and the lift would all fall together, at the same speed. Your feet would no longer press on the scale. It would read zero. You would float inside the lift, just like an astronaut.

Floating does not mean gravity has gone. It means everything around you is falling together with you, so nothing presses on anything else. This is called free fall.

The space station is in free fall all the time: it is Newton's cannonball, falling round the Earth and always missing. The astronauts fall with it, so they float inside it. We call it "weightless", but a better word is free-falling.

You can feel a few seconds of it on Earth. At the top of a trampoline bounce, or the moment a roller coaster tips over a hill, your stomach "floats". Special aircraft fly in big arches to give scientists and astronauts about 20 seconds of free fall at a time.

On the way to Mars, the ship will be falling round the Sun, so the crew will float for the whole trip of six months or more. To stay healthy they must exercise hard every day. A ship could make its own gravity by spinning: in a spinning wheel, the floor pushes you round the circle, and that push feels like weight. A wheel 100 m across would need to turn about once every 23 seconds to give the 0.38 g of Mars, and about once every 14 seconds for a full 1 g.

Worked examples

1. A dropped cup in the lift. In the falling lift, you let go of a cup. It stays in front of your hand. It is falling, but so are you, at the same rate.

2. Water in orbit. Water poured on the space station does not fall into the cup; it floats as a wobbling ball, held together by its own surface.

Try it

  1. Why does a bathroom scale read zero in a freely falling lift?
  2. Is there gravity at the space station? How strong?
  3. Why is "free-falling" a better word than "weightless"?
  4. How could a ship to Mars give its crew a feeling of weight?
Answers
  1. You and the scale fall together, so your feet do not press on it.
  2. Yes, about 89% as strong as on the ground.
  3. Gravity is still pulling; the astronauts float because everything is falling together.
  4. By spinning part of the ship, so the floor pushes them round in a circle.

Common mistake

"Astronauts float because they are above the gravity." They are not above it; they are falling in it. If the station stopped moving sideways, it would fall straight down.

Where it leads