As humanity launches more stuff into space, the odds of spacecraft bumping into each other will go up.
SpaceX and the European Space Agency (ESA) got the most recent and talked-about taste of the problem on Monday. On that day, there was a 1-in-1,000 chance of collision between one of SpaceX's new Starlink internet satellites and the ESA's wind-monitoring Aeolus spacecraft.
The ESA decided to fire a thruster on Aeolus and avoid risking a hit, but there will inevitably and always be more close calls in the future â and sometimes deliberate incidents, such as India's satellite shoot-down in May â that generate countless tiny pieces of space junk.
The US government tracks about 23,000 human-made objects floating in space that are larger than a softball. These satellites and chunks of debris zip around the planet at more than 17,500 mph â roughly 10 times the speed of a bullet. Until April 1, the list of space junk even included China's school-bus-size Tiangong-1 space station, which burned up in Earth's atmosphere.
However, there are millions of smaller pieces of space junkâ sometimes called micrometeoroids â 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 in 2018.
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.
This story has been updated. It was originally published on March 27, 2018.