Returning a spacecraft back to Earth can be a dire situation. How do we make sure they won’t smash into cities?

SeekerPublished: July 10, 2018Updated: July 11, 2018
Published: July 10, 2018Updated: July 11, 2018

The old adage of “what goes up must come down” applies to things in space, too. Granted, not all of them. A spacecraft launched with enough velocity on the right trajectory won’t come back to Earth, but things in Earth orbit will. For example, China’s prototype space station, Tiangong-1, is expected to fall back to Earth late in March or early April of 2018. The unusual thing is, it’s going to do it out of control. Which raises the question of what happens when spacecraft die and have to home.

We have to do something with this old equipment, because most of the time they’re going to come down no matter what we do. Satellites and space stations orbit because their speed, about 17,500 miles per hour, balances against the pull of gravity.  The higher the orbit, the more stable it is, too. That’s because low-Earth orbit isn’t a perfect vacuum. There’s no hard line where our atmosphere ends. Instead, it gradually peters out meaning there’s a thin veil of molecules that extends to where some satellites do orbit.

Those molecules create drag, slowing the satellites until they can’t keep up the fight against the pull of gravity and they reenter the atmosphere. This also happens in the region where the International Space Station orbits — about 200 miles above the planet. So, to keep satellites in orbit we need to boost them continually. Small bursts of speed kick satellites and stations up just enough to offset the drag of the upper atmosphere. Since most satellites and space stations are designed with a limited lifetime, mission planners have figure out how to bring them home before they’re even launched.

When they’re ready to die, retro rockets fire against the direction of travel slow the payload deliberately and start a reentry into the atmosphere, where most small satellites just burn up. But, there is another scenario. If a satellite or space station is dead or nearly out of fuel, things can get a little more complicated. Without power, engineers can’t control the deorbit burn to bring it down in predictable place. This is exactly where engineers are with Tiangong-1.   This video, "Returning a spacecraft back to Earth can be a dire situation. How do we make sure they won’t smash into cities?", first appeared on

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