What Happens When a Space Station Falls Out of the Sky

Sometime this weekend, an abandoned Chinese space station the size of a school bus will plummet back to Earth and mostly disintegrate in the atmosphere. Whatever chunks survive the intense heat of the journey will probably land in the ocean or a remote part of land, away from populated areas.

It’ll be quick, and chances are nobody will witness the reentry from the ground. So what exactly will happen to Tiangong-1 as it comes hurtling down?

“Have you ever seen Gravity?” Ted Muelhaupt says. “The depiction of the reentry in that movie is actually pretty accurate. It was one of the better things about that movie.”

Muelhaupt is an associate principal director at the Aerospace Corporation, a nonprofit research group that works on space issues. He and his colleagues have spent weeks tracking Tiangong-1’s orbit as the space station steadily loses altitude, and they’ve discussed working in shifts over the weekend to catch the big event. Tiangong-1 is expected to reenter sometime between the night of March 31 and late evening on April 1, according to the European Space Agency’s Space Debris Office, which has been tracking the station.

China launched Tiangong-1 into space in September of 2011 as a prototype for a future permanent space station in low-Earth orbit. The station was twice visited by Chinese astronauts, or taikonauts, including the country’s first female taikonaut, Liu Yang.

Tiangong-1 station was never intended to remain in orbit forever. China would eventually stop firing the station’s engines to keep it in a stable orbit, and it would lose altitude until it neared the top of Earth’s atmosphere. China would then carry out what’s known as a controlled reentry. Using quick engine bursts, engineers would guide Tiangong-1 toward a trajectory that would see it fall safely over the ocean. Spacefaring nations often carry out such controlled reentries, for satellites and stations alike.

But in March of 2016, Tiangong-1 mysteriously stopped working. China could no longer command the station to do anything.

In one of the final, dramatic scenes in the 2013 film Gravity (warning: spoilers ahead), Sandra Bullock’s character is stranded alone in a Soyuz capsule, the International Space Station destroyed behind her. After a brief, oxygen-deprived hallucination featuring George Clooney’s character, Bullock maneuvers the Soyuz toward a fictional version of a Tiangong space station and gets in just as the station starts to reenter Earth’s atmosphere.

Bullock’s descent is quite similar to what will happen to Tiangong-1, Muelhaupt says, minus the Steven Price score. Powerful wind streams will buffet the station, shaking it violently. Solar arrays, antennae, and any other protruding hardware will be among the first to snap off.

Before that, it’s possible that Tiangong-1 may hit the top of the atmosphere at such an angle that it could bounce back into space.

“If you put something at a right angle in the air it’s moving through, you can generate some lift. So when an object’s going to naturally decay and reenter and come down into the denser atmosphere, it might generate lift and then come back out again,” Muelhaupt says. “You have to hit the air for reentry at just the right angle.”

Once it makes it into the atmosphere, Tiangong-1 will travel at hypersonic speeds, faster than the speed of sound. “As the thing goes deeper into the reentry, the air piles up in front of it faster than it can get away, and you get this shock wave,” Muelhaupt says. “The molecules in the air literally start coming apart.”

A layer of hot plasma will envelop the space station, and metal will begin to melt from the extreme temperatures. “During that period, where you get this intense heating, if you don’t design a vehicle to survive that heating, it’s going to come apart,” Muelhaupt says. Based on photos China has shared publicly of Tiangong-1, the station does’t appear to have any kind of heat shielding, Muelhaupt says.

Tiangong-1 will break apart into pieces, all of them flying together on the way down, like a flock of mismatched birds. Lighter material will slow down faster and thus experience less heat, which means some of it may survive the fall. Heavier material will experience the most severe heat, but some objects may be so dense that they escape melting before they reach Earth’s surface, too.

Then, minutes after it begins its final descent, Tiangong-1 will disappear from radar trackers around the world.

If you happen to witness Tiangong-1’s reentry, “you’d probably see a bright, burning trail go across the sky, with multiple bright objects all in a tight cluster” for about a minute, Muelhaupt says.

It’s difficult to predict which parts of Tiangong-1 will survive the ride down, particularly because the station’s composition is not publicly known. There’s nothing to worry about, though. The chances of getting struck by falling space debris are extremely low, lower than the chances of getting hit by lightning.

Wherever bits of Tiangong-1 land, they will still be considered the property of China, according to international space law. Any resulting damage will also be China’s responsibility. If debris lands in a remote part of a foreign country, China may choose to leave it there, as other nations have done and continue to do.

“I’m not aware of any aggressive recovery efforts,” Muelhaupt says. “If something survives an uncontrolled reentry, it’s usually in pretty bad shape. Unless you’re trying to do something specific with the object you recover, it’s much more a curiosity than anything else.”

There have been some outliers, like the reentry of a Soviet satellite in 1977 that scattered radioactive debris over northern Canada. In that case, the debris couldn’t be abandoned. The Canadians carried out the cleanup and billed the Soviets for the work.

Today, Earth is littered with bits of spacecraft, the burnt remains of once-gleaming pieces of remarkable human engineering. Nasa officials have said one to two objects from reentries are found somewhere in the world each year. Some are in museums, like the remains of Skylab, the first American space station, which in 1979 made a controlled reentry over the southern Indian Ocean but deposited some pieces along the coast of western Australia. (Skylab weighed 10 times as much as Tiangong-1, and no one got hurt on the ground.) Australia fined the United States for littering as a joke, then placed the leftovers on display. Others are strewn across grassy fields in rural areas, like bits of China’s family of Long March rockets. This weekend, if they don’t sink in the sea, scorched parts of Tiangong-1 may join them.