Here's Why Chinese Satellite Tiangong-1 Is Falling Uncontrollably, and Why Earth's Atmosphere Can't Stop it

Let's clear something up right away: Although the Chinese space station Tiangong-1 is indeed falling to Earth completely out of control, there is no need to panic. The chances of it harming anyone as it lands are astronomically tiny.

That's because humans are so scattered across Earth's surface that there's plenty of room for a satellite to crash with no ill effects. But the only reason there's any chance of parts of Tiangong-1 reaching the surface is because of its large size. Hundreds of satellites fall out of orbit each year and are completely destroyed before they hit the ground, thanks to the thick blanket of atmosphere enveloping Earth.

"In outer space, it's a vacuum—except that it's not," Jonathan McDowell, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Massachusetts who tracks launching and re-entering satellites on Twitter, told Newsweek. "The Earth's atmosphere dribbles out, very slowly, very thinly." It's a very, very good vacuum, but not quite a perfect one.

That means that even at the highest levels of orbits around the planet, where the atmosphere is thinnest, any satellite will bump into particles of atmosphere now and again. Because a satellite is orbiting at a speed of about 17,000 miles per hour, hitting even an occasional tiny particle can slow it down just a smidge—the space equivalent of the drag a swimmer faces in water.

Read more: Tiangong-1: How Long Was Chinese Space Station in Orbit Before It Started Falling To Earth?

Here's where things get a little mind-bending. If a satellite slows down just a smidge, it falls just a smidge as well, orbiting closer to Earth. But that makes it orbit faster—picture the tip of a shorter pendulum compared to a longer one, then imagine completing the circle. "The net thing is that you spiral in," McDowell said. "Each orbit is a little smaller than the last orbit."

(Satellites meant to stay in orbit for a long time carry miniature rockets they use to nudge themselves back up to a steady orbit, and Tiangong-1 received more than a dozen of these little nudges between its 2011 launch and 2016.)

The lower the orbit gets, the thicker the atmosphere around the satellite and the stronger the drag, which means the spiraling effect speeds up. Eventually, the collisions with all the particles in the atmosphere causes the satellite to burn up and fall out of orbit down to Earth.

The same basic principle holds with any atmosphere, which is why NASA disposed of its Cassini spacecraft by steering it into the atmosphere around Saturn when its mission was over, and why Juno will meet the same fate at Jupiter. It also holds true of Earth's atmosphere with natural intruders, like meteorites.

The whole process is made a little bit less predictable because the amount of drag a satellite faces depends on how much of its surface is facing into the atmosphere—consider a parachute as compared to a missile.

Tiangong-1 launched in 2011. Lintao Zhang/Getty Images

The materials a satellite is made of also affect re-entry. Many satellites contain lots of aluminum, which tends to burn up pretty thoroughly at the end of this spiraling process. But denser parts of a spacecraft don't always burn up all the way. "You get lumps of metal falling through the sky and hitting the surface of the Earth," McDowell says. Most hit unpopulated and undeveloped parts of Earth's surface.

These days, scientists steer most dying satellites with onboard rockets, controlling the re-entry area to a known broad swath of uninhabited surface, like the southern Pacific Ocean. This is exactly what was supposed to happen to Tiangong-1, which like most larger modern satellites was equipped with thrusters to give scientists control over where it re-entered.

But the Chinese were worried the space station's successor, Tiangong-2, wouldn't be able to launch smoothly, and they wanted to be sure they had something up there to keep their space program growing, no matter what. During that extra time in space, Tiangong-1's control mechanisms broke—which is why the atmosphere is in control of things now. "They made a bet that they shouldn't have made," McDowell said.

"It's gonna come down just fine," he continued. "But it's not best practice."