In a few days, China’s prototype space station Tiangong-1 will come crashing through Earth’s atmosphere as a dazzling fireball—and no one knows where the debris will hit.
“But we do know where it isn’t going to come down,” says William Ailor, principal engineer for the Center for Orbital and Reentry Debris Studies (CORDS) at The Aerospace Corporation.
Speeding around our planet at about four miles per second, the uncrewed spacecraft is out of control in a decaying orbit, tumbling through the uppermost reaches of the atmosphere. Ground-based observers have been able to track its orbital trajectory with some precision, and experts know that its orbit (and reentry path) is confined to a zone around the Earth between the latitudes of 43 degrees North and 43 degrees South. Anywhere north and south of that zone, there is a zero chance of the spacecraft reentering.
Within those zone, however, is where most of the Earth’s population resides.
“The best case—and most likely case—is that it will come down over water some place, and never be seen again,” says Ailor. “The worst would be if some debris came down in a [populated] area.”
Tiangong-1, which means “Heavenly Palace-1,” is the first space station China launched into space in 2011. It was inserted into an orbit approximately 217 miles (350 kilometers) high, and was used as a testbed to demonstrate rendezvous and docking technologies ahead of China’s plans to launch a permanent crewed outpost in the 2020s. In November 2011, a robotic spacecraft called Shenzhou-8 made history as the first Chinese space vehicle to successfully dock in orbit, followed by two crewed missions (Shenzhou-9 and 10) in June 2012 and June 2013. (A second small station, Tiangong-2, was launched in 2016.)
Bigger than a school bus with a mass of 8,500 kilograms (9.4 tons), Tiangong-1 is by no means the biggest humanmade object to come hurtling through the atmosphere. In 1979, NASA’s 77,111-kilogram (85-ton) Skylab space station reentered over Australia, scattering debris near the town of Esperance. Other, larger spacecraft have reentered, such as the 129,700-kilogram (143-ton) Russian Mir space station—though its reentry over the Pacific Ocean was controlled to minimize any risk of debris hitting populated regions.
On March 21, 2016, an official Chinese statement said that communications with Tiangong-1 had ceased. Although the Chinese government has yet to confirm whether the reentry will be uncontrolled, international observers believe it has been orbiting uncontrolled since at least June 2016, so it’s doubtful China has any influence over where it will reenter.
There is uncertainty with any spacecraft reentry, due to the changing density of the upper atmosphere. Planetary atmospheres are the ultimate satellite-killer, where even the thinnest wisps of gas exert aerodynamic friction, or drag, on a vehicle’s direction of motion, causing it to slow and therefore lose altitude.
What makes this effect even more unpredictable is that Earth’s atmosphere is very sensitive to solar activity, adds Ailor. “That atmospheric density varies due to solar storms, other solar events, like sunspots, all kinds of things affect atmospheric density,” he says. Indeed, the fact that our sun is currently experiencing low activity in its solar cycle means the atmospheric gases have been less dense at higher altitudes, allowing Tiangong-1 to stay aloft longer than originally predicted.
Now, however, Tiangong-1’s time has come; spacecraft trackers know that it’s going to reenter the Earth’s atmosphere some time between March 30 and April 2, but will not be able to predict exactly where it will reenter until its final moments.
What are the odds?
Although the odds are vanishingly small of a chunk of space station hitting you—The Aerospace Corporation estimates the odds of that happening are less than 1 in a trillion—should you find a piece of Tiangong-1 in your back yard, you shouldn’t approach it, as it could contain toxic chemicals. And you wouldn’t be able to keep it anyway.
According to space law expert James Dunstan, the founder of Mobius Legal Group and a Senior Adjunct Fellow at TechFreedom, space treaties will prevent you from collecting souvenirs. “It is still technically property of China,” he says. “Ownership and jurisdiction over a space object retains in the launching country. So just by putting it into space and returning it to Earth, it’s still the property of the Chinese.”
If you do become the unluckiest person on the planet to suffer injury or loss because of falling space junk from Tiangong-1, those same space treaties mean you would have some cause for complaint.
“The Chinese would be liable,” says Dunstan. “Now, the caveat in that is that individuals would not be free to go and sue the Chinese government.” Instead, he points out, you’d need to lodge a complaint with your government, who might then seek damages on your behalf.
But we’re getting ahead of ourselves; the good news is that Earth is 70 percent ocean, so the most likely outcome will be Tiangong-1 having a watery grave.