A Space Station Is About to Fall From the Sky—But Where Will It Hit?

The Tiangong-1 space station is expected to break apart in Earth’s atmosphere sometime between March 30 and April 2.

A Chinese space station is on a collision course with Earth, and the latest predictions say it could come crashing down almost anywhere on the planet sometime between March 30 and April 2.

Named Tiangong-1, which translates to “Heavenly Palace,” the craft was placed in orbit in September 2011. The station was designed to be a testbed for robotic technologies, and it has seen multiple vehicle rendezvous, dockings, and taikonaut visits during its operational lifetime. This activity lays the groundwork for a more permanent space station the Chinese plan to launch in the near future.

In May 2017, Chinese officials released their first report to the United Nations saying that Tiangong-1 had ceased operating on March 16, 2016. Although it has maintained its structural integrity, the craft is all out of fuel and can no longer be controlled by teams on the ground.

Now circling Earth at an average altitude of 320 kilometres, the station is experiencing significant drag as it brushes against the planet’s denser outer atmosphere, and it’s losing altitude at a rate of about 4 kilometres a day. Eventually, Tiangong-1 will reach an altitude of about 70 kilometres and make a fiery reentry.

If pieces of Tiangong survive the descent, it’s hard to know where on the planet they will fall. The space station loops around Earth twice every three hours, and its orbit takes it between 43 degrees North latitude and 43 degrees South, which means every continent except Antarctica is in the potential fall zone.

“All we can say is that it will come down somewhere between that latitude limit, and that answer won't change until it actually has come down,” says Jonathan McDowell, an astronomer at the Harvard-Smithsonian Center for Astrophysics.

The precise re-entry date for Tiangong-1 also remains unknown at this point, because the density of the upper atmosphere changes depending on solar activity,

“When it gets to about 24 hours out, we may be able to predict re-entry time to about three hours or so,” McDowell says.

A crewed space capsule docks with the Tiangong-1 space station in an illustration released by Chinese state media in 2013.

Tracking Trouble

Our ability to predict how falling satellites will behave is still very much in its infancy, in part because we lack the ability to do real-time global monitoring of atmospheric dynamics, says Moriba Jah, an astrodynamicist at the University of Texas at Austin.

“We don’t understand all of the dynamics and mechanics of the atmosphere with sufficient accuracy and precession to predict well into the future,” he says. “This translates into such a huge uncertainty in the orbital track or trajectory of the threatening object, that it could mean the difference between entering over the Pacific Ocean or over the United States.”

When Tiangong-1 does re-enter the atmosphere, the space station is expected to break apart, and there is the possibility that fragments as hefty as 100 kilometres will come crashing to Earth. Something similar happened in the 1979, when NASA’s Skylab station de-orbited and did not burn up as quickly as expected. (Parts of the station rained down over Australia, and while no one was hurt, one shire did fine NASA for littering.)

In their report to the UN, Chinese officials downplay the risk of fragments surviving the trip, and Jah and McDowell agree that the odds are in our favour.

“Most of the planet is covered by ocean, and so in all likelihood, if there are any survivable pieces, the highest probability is that they will land somewhere in the ocean,” Jah says.

“Even if some do impact land, most of the world’s population is centered around specifically coastal areas, which will further reduce the probability that any given survivable piece will hit a densely populated area.”

Cluttered Space

Still, the uncertainty surrounding Tiangong-1 should be a lesson for space agencies and satellite manufacturers to have back-up plans if we lose control of large assets in space, says Jah.

The International Space Station, which is the size of an American football field, is on a similar orbital path as Tiangong-1, and Jah says that NASA is extremely concerned about how to properly de-orbit it at the end of its life.

“Perhaps this [Chinese space station de-orbiting] will be a motivation for government agencies worldwide to provide some funding to academics and research instructions to really get a grasp on the science of reentries, because this problem will not go away but will repeat itself,” he says. “Risks will continue to go up as well, especially with the growing number of orbiting objects.”

With more than 50,000 pieces of space junk now being tracked in orbit around Earth, the real concern for is in-orbit collisions, McDowell adds.

“If we don't start cleaning up our act, space could become unusable because of all the shrapnel flying around up there,” he says.

Eyes on the Skies

For now, telescopes around the world are keeping close watch on Tiangong-1 as it spirals down on its death dive. Sky-watchers can join in the vigil, including via a live webcast from the Virtual Telescope Project, which will stream on March 28 starting at 11:00pm AEDT.

The station is also visible to the naked eye, and it’s easy to tell the difference between it and a passing airplane: Like many satellites and the ISS, Tiangong-1 looks like an unblinking white light gliding swiftly across the sky.

People living in mid-latitude areas in both the Northern and Southern Hemisphere have the best chance of seeing the station re-enter, depending on the date it happens.

North American observers in particular will have clear views of the star-like station in their early morning skies this week, while viewers above 60 degrees latitude won’t see the station rise above their local horizon. You can find the specific times to look from your vantage point by entering your location into various satellite tracking websites, such as Heavens Above.

Clear skies!

Michael Greshko contributed reporting.

Andrew Fazekas, the Night Sky Guy, is the author of Star Trek: The Official Guide to Our Universe. Follow him on Twitter, Facebook, and his website.

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Lead Image: A Long March 2F rocket ferried the Tiangong-1 space station into orbit in September 2011. PHOTOGRAPH BY LINTAO ZHANG, GETTY

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