The 9.4-ton spacecraft is expected to fall from the sky Sunday morning, break up, and sprinkle debris over Earth's surface.
Objects as large as Tiangong-1 can tumble and "skip" off the atmosphere, experts say.
That and other factors make advanced predictions of a reentry date, time, and location nearly impossible.
But pieces of Tiangong-1 are extremely unlikely to hit people.
The first space station China ever launched is about to return to Earth as a mess of ultra-hot, supersonic space junk.
China launched Tiangong-1, or "Heavenly Palace," in 2011. After six successful missions to Tiangong-1 — three of which were crewed — China abandoned the spacecraft in June 2013.
Since then, the two-room, 9.4-ton vessel has orbited the planet without any Chinese astronauts aboard. But in May, China told the United Nations that it'd lost contact with Tiangong-1 in March 2016 after it "fully fulfilled its historic mission."
Tiangong-1 may reenter Earth's atmosphere at 3:15 a.m. EDT on Sunday, April 1, give or take 20 hours, according to the latest prediction by the Aerospace Corporation, a nonprofit spaceflight-research company.
When that happens, chunks of the space station are likely to rain down over our planet's surface. Some of the gear left inside the vessel may even reach the ground intact.
No one knows when or where Tiangong-1's debris will land, but the good news is that the garbage will most likely fall into the ocean. You're about 1 million times as likely to win the Powerball jackpot as you are to get hit by any piece of Tiangong-1.
It seems as if the brightest minds on Earth should be able to pinpoint when and where giant spacecraft will reenter Earth's atmosphere, but it's not so simple. Here's why.
Skipping off the atmosphere
To circle Earth from about 250 miles up, a spacecraft must reach a blistering speed of 17,500 mph, meaning it orbits the planet once every 90 minutes.
Even that high up, however, the outer fringes of Earth's atmosphere drag on spacecraft like Tiangong-1. If a vessel isn't sped up every so often to correct its orbit, it will eventually slow down and fall from the sky.
"You often hear space starts at 100 kilometers — that's based on where aerodynamic forces start having an effect to where you can actually control your [craft] with wings," Jesse Gossner, an orbital-mechanics engineer who teaches at the US Air Force's Advanced Space Operations School, told Business Insider.
"Above 100 kilometers" — or 62 miles — "it's a lot, lot, lot thinner than down here, and you certainly wouldn't be able to survive," Gossner said. "But it's thick enough to slow you down."
That's what has been happening to Tiangong-1 — and the reason it will soon fall. But even now, about 80 hours from the space station's expected crash, the timing estimate has a 20-hour window of uncertainty on either end. It could come down as early as Saturday morning or as late as Sunday night.
"The 21st Space Wing, using its global Space Surveillance Network of ground- and space-based radar and optical sensors, is tracking Tiangong-1," Diana McKissock, a flight lead with the US Air Force's 18th Space Control Squadron, told Business Insider in an email.
Gossner said the uncertainty was due to the nature of Earth's atmosphere and how high-speed objects behave in it.
"You'd be surprised just how inaccurate and random it is because of the atmosphere," Gossner said. "Have you ever skipped a stone on a lake?" he added. "It bounces a few times, then eventually goes into the water."
A hypothetical out-of-control spacecraft like Tiangong-1, Gossner said, may behave like that stone.
"This thing can bounce off the atmosphere because it's going so fast," he said. "If it hits on its smooth side, sort of like a rock skipping on a lake, it'll bounce. But if it hits on a pointy end, or on one end of a cylinder, in the direction of the velocity, it could dig in."
Any large spacecraft that dips below an altitude of about 125 miles has just a few days left in orbit, Gossner said — and that's roughly where Tiangong-1 is drifting. Once it's about 80 miles up, it'll be within one orbit (about 90 minutes) of crashing.
"Even if you knew exactly where it hits in the atmosphere, the debris and stuff can spread out to a pretty big area," Gossner said.
"It's really just a guessing game," he said, adding: "There's just no way to tell where it's going to land."
How Tiangong-1 will die — and which pieces may survive
When space stations come down, "a funny thing" happens that helps doom the spacecraft, Gossner said.
"You start going really, really fast," he said. "Then you get slowed down really, really fast."
That's because the spacecraft is losing its forward speed, allowing gravity to accelerate the space station toward Earth. The air is still too thin to slow it down much, so it plummets faster and faster.
As the spacecraft falls into thicker air, the drag begins to rip off solar panels, antennas, and other loosely attached pieces. Superheated plasma heats the vessel to thousands of degrees, melting and disintegrating it.
Only a few types of materials can withstand such punishment.
"Titanium is a good one," Gossner said.
But there is a chance that some gear and hardware left aboard could survive intact all the way to the ground, according to Bill Ailor, an aerospace engineer who specializes in atmospheric reentry. That durability is thanks to Tiangong-1's onion-like layers of protective material.
"The thing about a space station is that it's typically got things on the inside," Ailor, who works for the Aerospace Corporation, previously told Business Insider. "So basically, the heating will just strip these various layers off.
"If you've got enough layers, a lot of the energy is gone before a particular object falls out, it doesn't get hot, and it lands on the ground."
For example, he said, after NASA's Columbia space shuttle broke up over the US in 2003, investigators recovered a working flight computer — an artifact that ultimately helped explain how the deadly incident happened.
The most likely place Tiangong-1 will fall
Tiangong-1 is likely to crash over the ocean, as water covers about 71% of Earth's surface. In fact, space agencies try to de-orbit large spacecraft over the Pacific Ocean "graveyard" since it's such a huge and innocuous target.
"So much of it lands in the ocean — that's our saving grace," Gossner said.
But some pieces of the Chinese space station may strike land, as the crash will leave a long, thin footprint of debris.
"The whole footprint length for something like this could be 1,000 miles or so," Ailor said, with heavier pieces at the front and lighter ones toward the back.
If anyone is lucky enough to witness Tiangong-1's atmospheric breakup from an airplane, it may look similar to the destruction of the European Space Agency's 14-ton Automated Transfer Vehicle, which used to resupply the International Space Station. Once astronauts and cosmonauts unloaded the vehicle's supplies, it was filled with garbage and sent careening back to Earth.
Ailor says pieces of China's space station are "really unlikely" to hit any people on Earth though.
"It's not impossible, but since the beginning of the Space Age ... a woman who was brushed on the shoulder in Oklahoma is the only one we're aware of who's been touched by a piece of space debris," he said.
Should a hunk of titanium, a computer, or another piece smash through a roof or windshield, however, international space law covers compensation for victims.
"It's China's responsibility if someone gets hurt or property gets damaged by this," a NASA representative previously told Business Insider.
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