Falling and missing
Here is the strangest true thing in this whole course: astronauts on the space station are falling. All the time. They just keep missing the ground.
Imagine standing on a very tall mountain and throwing a ball. It curves down and lands. Throw it harder, and it lands farther away. Now imagine throwing it so hard that as it falls, the Earth curves away underneath it just as fast as it drops. The ball never lands. It goes all the way around and comes back to you. That is an orbit.
Isaac Newton drew this picture with a cannon on a mountain nearly 350 years ago, long before anyone could try it. He was right.
Why astronauts float
Because they are falling! When you fall, you feel weightless — think of the tummy-drop moment on a rollercoaster. On the space station, that moment lasts for months. Gravity is still there — it is almost as strong as on the ground — but everything is falling together, so nothing presses on anything.
Fast means low, slow means high
- The space station is about 400 km up and goes around the Earth every 90 minutes — 16 times a day. The astronauts see 16 sunrises every day!
- Weather and TV satellites sit much higher, at 36,000 km. Up there an orbit takes exactly one day, so the satellite stays over the same spot on Earth. We call that geostationary.
- The Moon is 384,000 km away and takes about 27 days to go around.
The higher you are, the slower you can go and still miss the ground.
Try it
Pick a height and see how fast a satellite has to go, and how many laps of the Earth it does each day. Try 400 (the space station) and 36,000 (a TV satellite).