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.