A spacecraft the size of a small house is about to re-enter Earth’s atmosphere. We don’t know when, exactly, and we certainly don’t know where. Pieces of the spacecraft might survive the fiery re-entry and make it to the ground — or maybe they won’t. We’ll be able to see it happen if it re-enters over a populated area, probably.

As you can tell, there are a lot of unknowns for something that’s going to happen in a matter of days. Forecasters have a pretty good idea of what the weather is going to be like on April 1 – the operators of this totally man-made and human-controlled spacecraft have only a rough idea when it’s going to fall out of the sky.

The operators of the Chinese space lab, Tiangong-1, lost contact with it back in 2016. They didn’t come right out and say that, though. At first, China’s space agency said it was being disabled. Then amateur skywatchers took notice and found that the spacecraft was actually out of control. China admitted as much later, saying they expected Tiangong-1 to make an uncontrolled re-entry in late 2017. Now it looks like the spacecraft will blast into Earth’s atmosphere on March 31 or April 1.

It’s not really supposed to go like this.

“We have protocols for every launch and every spacecraft to make sure we can properly bring it down,” J.D. Harrington, a spokesman for NASA, told The Washington Post. “There are times when those plans don’t come to fruition, and that’s something all space agencies have to deal with.”

[A Chinese space station will crash into Earth by April 1]

NASA isn’t tracking this spacecraft. (The military is, however, at the Joint Space Operations Center at Vandenberg Air Force Base.)

But NASA knows a few things about space debris and how to prevent satellites from falling out of the sky, uncontrolled. The protocol usually consists of having enough fuel on board to make a controlled re-entry, Harrington said. If operators can steer the spacecraft, they can pinpoint exactly where and when it will enter Earth’s atmosphere.

In some cases, NASA will use fuel intended for re-entry to keep the science mission going. That was the case with the TRMM satellite, which was decommissioned and fell back to Earth in 2015. NASA and the broader research community decided it was worth the risk to use the extra fuel to extend the mission — monitoring tropical rainfall, which provided invaluable contributions to things like hurricane and monsoon research.

The difference between TRMM and Tiangong-1, though, is size. TRMM was a relatively small research satellite, weighing around 5,700 pounds when it re-entered the atmosphere. It burned up into small debris that didn’t pose much of a threat to anything on the ground. Tiangong-1 is nearly 19,000 pounds and the size of a small house.

Neither NASA nor the European Space Agency know how the spacecraft is constructed or what it’s made of. Large titanium or steel components could withstand the heat of re-entry.

[Amateur rocket man launches himself off Earth to prove it’s flat]

The space agencies have performed controlled re-entries on spacecraft of this size before. Based on those, “it can be surmised that Tiangong-1 will break up during its atmospheric re-entry and that some parts will survive the process and reach the surface of Earth,” the European Space Agency says.

There are no laws that govern the movement of objects in space, says Holger Krag, the head of the Space Debris Office at the European Space Agency. The only international law that applies to space objects is the Liability Convention, which was reached by the U.N. General Assembly in 1971. It says that when something falls out of space and lands on the ground, the country where that object originated is absolutely liable for any damage it causes.

“There’s no similar thing for property in space, though,” Krag told The Post. Contact is “much more likely to occur there than an injury on the ground. This isn’t covered by any international space law.”

(By the way, you can see all of the stuff in space in real time, at the website stuffin.space.)

[NASA working on a plan to avert a chance asteroid strike in 2135]

So far, an international space crisis has been avoided because of the good relationship between all of the space nations, including the United States, Russia, Europe and China, to name a few. Plus, it helps that 71 percent of Earth is covered in water.

“That’s 71 percent odds that it’s going to go in the drink,” Harrington said. “We’ve been fortunate.”

Tiangong-1 is far from the biggest thing to fall out of space uncontrolled. The 39-ton second stage of the Saturn V rocket used to launch Skylab made an uncontrolled re-entry in 1975, two years after it was finished guiding Skylab into place.

Skylab itself, weighing in at 74 tons, made an infamous semi-controlled reentry in 1979. The spacecraft didn’t burn up as fast as NASA thought it would, so instead of landing in the ocean southeast of Cape Town, South Africa, its pieces scattered across southwest Australia.

One town even fined NASA $400 for littering. As far as we know, that fine was never paid.

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