When and where will the Chinese space station land? We can’t know for sure—but we can examine some variables.
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. It’s just like taking a ball on a string and swinging it around your head. The string pulls the ball so that it moves in a circle.
But wait! There’s actually more than just the gravitational force. We like to pretend that outer space is an empty vacuum—but there are plenty of gases up there. And in the case of objects like Tiangong-1 in low Earth orbit, there are also still some atmospheric effects from the Earth’s air. Oh sure, the air is super thin up there, but even a little bit of air resistance can make a difference over a long time. In fact, this is why the International Space Station gets an occasional reboost so that it doesn’t slow down from the interaction with the air.
This tiny air resistance force pushes on the space station in the opposite direction of its motion. It’s just like sticking your hand out the window of a moving car—but in space. This backwards pushing force decreases the speed of the space station so that it has a lower orbit. Oh, and what happens when the orbit gets lower? Yup, the object moves further into the atmosphere where there is even more air and greater air resistance. This can quickly lead to a runaway effect that heats up the object—so much that most of it will burn up during re-entry.
Now, if the basic physics of re-entry is understood, then why don’t when know when and where the Tiangong-1 will land? The short answer is that the motion of the space station is very sensitive to initial conditions. Let me explain this by making a model of something similar to the Tiangong-1 in low Earth orbit.
