Science & Technology

No fooling: Skywatchers zero in on fiery fall of China’s Tiangong-1 space lab

After months of tracking China’s uncontrollable Tiangong-1 space lab, satellite watchers have narrowed down the time frame for its final, fiery plunge through the atmosphere — and it’s this weekend.

Here are a few of the current predictions:

The latest predictions are about half a day later than they were earlier in the week, largely because an expected solar storm didn’t sweep past Earth. “This means that the density of the upper atmosphere, through which Tiangong-1 is moving, did not increase as predicted (which would have dragged the spacecraft down sooner),” ESA explained.

Where will it fall? Nobody knows, precisely. But experts have mapped out a large zone where it won’t fall — and it won’t fall anywhere close to Seattle. The reason experts can be so sure is that when Tiangong-1 was launched in 2011, it was put in a specific orbital inclination that strays only so far from the equator: 42.8 degrees latitude, north and south.

Seattle is beyond that territory, along with the rest of the northern tier of the U.S., plus Alaska, Canada, northern Europe and Russia. You’d have to drive almost as far south as Grants Pass, Ore., to reach the potential re-entry zone. (Update for April 1: The final orbital tracks show that Tiangong-1 won’t fall anywhere near North America.)

Put the emphasis on “potential”: As with other cases of uncontrolled re-entry, the chances are great that Tiangong-1’s space junk will blaze through the skies over one of the world’s oceans, just because that accounts for most of the zone’s area.

But not all of the area. The possibility that debris from the space lab will fall on a populated area has been stoking headlines for days. If that’s what you’re worried about, don’t overstress: The chances of being injured by space debris are astronomically small — less than 1 in a trillion, by some estimates. And as previously noted, the chances are zero outside the potential re-entry zone.

Only one person has ever reported being hit by orbital debris: That’s Lottie Williams of Tulsa, Okla., who said in 1997 that she was struck a glancing blow by a 6-inch metal fragment later traced to a Delta rocket stage. Even then, she wasn’t injured.

The coordinates for Tiangong-1’s demise are uncertain because the abandoned lab’s Chinese controllers lost contact with it in 2016, three years after it was last occupied. That means the lab’s descent is subject to the vagaries of atmospheric drag rather than a precisely planned series of commanded thruster firings. That’s the preferred way to bring a spacecraft down.

Tiangong-2, a successor space lab that China launched in 2016, is still going strong.

In terms of tonnage, Tiangong-1 isn’t even close to the largest piece of hardware to fall from the sky. That honor goes to Russia’s 120-ton Mir space station, which made a controlled descent in 2001. The biggest object to make a less than fully controlled fall was NASA’s 74-ton Skylab station, which littered debris over Australia in 1979.

Tiangong-1, which weighed 8.5 tons when it was launched with a topped-off tank of fuel, is more on a par with the European Space Agency’s 12-ton ATV or Japan’s 10-ton HTV space transports.

The most likely effect of Tiangong-1’s re-entry will be to put on a shooting-star show, as was the case with ATV and HTV re-entries. The only question is whether anyone will be in the vicinity to see it. By Monday, we’ll know.

This report was originally published at 1:25 p.m. PT March 28, and last updated at 2:54 p.m. PT April 1.

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