A Brief History of Things Falling Uncontrollably From Space

If you've seen stories about the Chinese space station Tiangong-1 uncontrollably falling to Earth sometime this weekend, you may worry the sky is falling. But the sky has been falling all along, you just didn't realize it.

Humans have been putting satellites into orbit for six decades now, and that means that there are more than 18,000 objects up there right now. But what goes up must come down, and every year, hundreds of satellites do just that.

"We've had space junk falling out of the sky on a regular basis," Jonathan McDowell, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Massachusetts who tracks launching and reentering satellites on Twitter, told Newsweek. "Most are really small and burn up completely."

Read more: Here's Why Chinese Satellite Tiangong-1 is Falling Uncontrollably, and Why Earth's Atmosphere Can't Stop it

Tiangong-1 is larger than most reentering satellites and there's a good chance the atmosphere won't be able to destroy it entirely. But it's hardly the largest satellite that's ever fallen to Earth uncontrolled—there have been 49 heavier examples, McDowell has calculated.

The largest of those was the space shuttle Columbia, which fell to pieces as it carried seven astronauts back to Earth in 2003 after a piece of foam broke off a fuel tank and punctured a wing. McDowell says that this is the rare example of an uncontrolled reentry that could have been dangerous. Even still, as in other larger reentries, the debris was spread across a path a couple hundred miles long—so even over Texas and Louisiana, no one was injured.

Another particularly large example was a space station just like Tiangong-1, this time owned by the U.S.—Skylab, which re-entered in 1979. Skylab was one of the last large satellites built before people began to prioritize controlled reentry, and although the U.S. realized they could use a space shuttle to control the station's reentry, those vehicles weren't ready in time and the station ended up falling on its own.

"No one would let a Skylab reenter on its own now days except by accident," McDowell said. "Tiangong-1 is a tenth of a Skylab, it's really no big deal in comparison."

U.S. space station Skylab fell to Earth in 1979. NASA Johnson Space Center

There are plenty of smaller but still relatively large examples over our decades of space exploration, McDowell added—things like stages of the Saturn V rockets that powered missions to the Moon and Russian space stations that have faced technical problems. "Things of this size reenter every two to three years and usually don't make the headlines," McDowell said.

Just in January, a Russian rocket segment fell in Peru, and no one farther afield heard about the incident. "You get a bunch of nice YouTube videos of people going, 'Holy shit, what's that?'," McDowell said. "No mass casualties or anything like that."

The space community has known for two years that Tiangong-1 would be joining this list sooner or later. That's because the station stopped functioning in March 2016, so scientists could no longer carefully control its reentry, steering it over a swath of open ocean as they normally would with larger satellites.

"We've been monitoring Tiangong-1 because we knew it had stopped maintaining its orbit," McDowell said. "We knew then that it was a goner, it was just a matter of time."