At some point this week, the Chinese space station Tiangong-1 is going to crash down to Earth. When and where? We can't know for sure. And for that, we have physics to blame. Tiangong-1 is in orbit around the Earth at an altitude of about 138 miles. At first approximation, there is only one force acting on the space station—the gravitational pull from the Earth. This gravitational force pulls the space station towards the center of the Earth so that it moves in a mostly circular orbit.

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