The best guess of expert astronomers and space junk-trackers is that China's decommissioned, out-of-control space station, Tiangong-1, will reenter Earth's atmosphere anywhere between late Friday night and Sunday.

In the words of the European Space Station: "This is highly variable."

What is clear, is that it's very hard to track pieces of space junk like Tiangong.

"There are many factors acting on an object as it decays and reenters the atmosphere," Maj. Cody Chiles, spokesperson for the Joint Force Space Component Command, told UPI. "These factors include how it tumbles and breaks up, variations in the gravitational field of a landmass or ocean, solar radiation pressure and atmospheric drag."

"These factors complicate our ability to predict what happens after reentry occurs; however, reentry predictions do become more accurate as the event approaches," Chiles said.

The U.S. Strategic Command maintains a catalogue of most of the known satellites in the orbit around Earth.

"By tracking and listing these objects and making that information available, we enable spaceflight safety and increase transparency in the space domain," Chiles said.

Tiangong is currently orbiting Earth at an altitude of 200 miles. Each day, the space station's orbit drops roughly 2.5 miles.

Because Tiangong-1's orbit remains highly variable, researchers aren't sure where the space station will reenter Earth's atmosphere. The space station swings around Earth twice every three hours. Its orbit sees the craft travel from 43 degrees North latitude to 43 degrees South, which means Tiangong-1 could break up almost anywhere but Antarctica.

"The fall location is hard to predict till the last minute," Vishnu Reddy, an assistant professor at the University of Arizona's Lunar and Planetary Laboratory, told UPI.

Launched in 2011, Tiangong served as a base on which China's space station tested a variety of spaceflight technologies.

It's likely that most of Tiangong-1 will burn up in the atmosphere, but there's a chance fragments as large as 220 pounds survive the trip and strike the Earth. If pieces of the space station do make their way to Earth's surface, the odds are they will land harmless into the ocean -- oceans cover 70 percent of Earth's surface.

Tiangong-1 isn't the only large satellite to come crashing out of space. When NASA de-orbited Skylab in 1979, fragments landed in the middle of Australia. Space is full of junk, and many space agencies are concerned about the dangers these hunks of orbiting metal pose other functioning satellites.

"I think we will have our first environmental superfund site in Earth orbit in the next decade given the amount of debris that is there right now and the projected increase of objects in the coming years due to cheap access to space," Reddy said. "The reentry of Tiangong-1 brings some awareness to this long ignored problem of space debris. In addition, it helps us get information how we can deorbit large debris and our short comings in their predicted behavior due to drag and solar radiation."

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