A space junk disaster is a real possibility — here's how the US government helps prevent a chain of collisions that'd threaten human access to space

China's school-bus-size Tiangong-1 modular space station is expected to fall to Earth in a fiery blaze on or around Easter Sunday.

The US government is tracking the orbit of Tiangong-1 and about 23,000 other human-made objects larger than a softball. These satellites and chunks of debris zip around the planet at more than 17,500 mph — roughly 10 times the speed of a bullet.

However, there are millions of smaller pieces of space junk orbiting Earth, too.

"There's lots of smaller stuff we can see but can't put an orbit, a track on it," Jesse Gossner, an orbital-mechanics engineer who teaches at the US Air Force's Advanced Space Operations School, told Business Insider.

As companies and government agencies launch more spacecraft, concerns are growing about the likelihood of a "Kessler syndrome" event: a cascading series of orbital collisions that may curtail human access to space for hundreds of years.

Here's who is keeping tracking of space junk, how satellite collisions are avoided, and what is being done to prevent disaster on the final frontier.

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Thousands of launches since the dawn of the Space Race have led to a growing field of space debris. Most space junk is found in two zones: low-Earth orbit, which is about 250 miles up, and geostationary orbit, about 22,300 miles up.

In addition to 23,000 objects the size of a softball or larger — like rocket stages, satellites, and even old spacesuits — there are more than 650,000 objects that are softball-to-fingernail-size.

Another 170 million bits of debris as small as a pencil tip may also exist — including things like explosive bolts and paint flecks.

Source: ESA

Countless pieces of tiny debris were added to orbit in 2007, when China intentionally smashed one of its old satellites with a "kill vehicle." Then in 2009, an old Russian satellite and US satellite collided, adding even more dangerous junk.