Making The Sky Stay Put

I think I knew this somewhere in the storage closet of my mind but never thought about it before. Did you realize that the International Space Station is constantly falling out of orbit and needs constant boosts to stay up there? Yup. And these can only be done by supply ships that visit every so often; the station doesn't have it's own engines to keep it circling the Earth.

Here's a chart from the very useful Heavens Above site showing how far it's fallen in recent days and how much they've had to push it back up.