Everyone has seen video and pictures of the astronauts in space. We watch on them each mission, as well as astronauts on the International Space Station, gently floating in space. If you asked anyone why the astronauts are weightless in space, they would probably all say ‘because there isn’t any gravity’ – but that, would be wrong.
Here is a situation and a question for you: We know that the earth has a gravitational pull on the moon, and that the moon is 384,400 km away. So if the earth has a gravitational pull on the moon, why wouldn’t it also affect the International Space Station, which is only 330 km?
The answer to this is just a bit more complicated, but the basis is that there is some gravity in space, so the weightless condition of the astronauts doesn’t have anything to do with gravity.
In reality, the astronauts are actually falling and so is the space station. We also have to remember that there are tons and tons of space debris as well as satellites that are out in space around the planet. They are all falling as well.
Right now, you are probably saying ‘no way’. If the space station and the astronauts were falling, they would have already hit the earth. They haven’t hit the earth due to the fact that they are falling and going very fast. It’s also speed that is keeping them from falling to earth. The combination of falling with the speed gives the astronauts and the International Space Station, along with the satellites and everything else that is floating out there, the sense of weightlessness.
There is a specific speed that objects around the earth have to travel so that they don’t crash and burn into the earth’s atmosphere. They have to travel at least 28,160 km/hour. In addition, the earth is round and that means that as they ‘fall’ it is curving away underneath them. The speed, combined with the curvature of the earth means that as they move faster towards the earth it is actually curving away from them. Since the astronauts and the space station are going at the same speed, they have a feeling of weightlessness.
There are situations here on earth that you can feel the same thing that the astronauts are experiencing. If you have ever been on a roller coaster you know that as it heads up and over to the top you feel as though you are going to fly out of your seat. It is the same weightless feeling and is due to the combination of speed and the curvature of the roller coaster as it is coming down. And yet, you are, indeed falling from the top of the roller coaster.
The same principles of physics that apply to the roller coaster, apply to the astronauts out in space: speed, combined with falling toward something that is curving away can seem like weightlessness. If there are two objects, such as the space station and an astronaut, and they are both going at the same speed, it will appear as if neither is moving and yet both are weightless.