Not to alarm you, but a SpaceX Falcon 9 second-stage rocket booster is on track to crash into the moon. The giant tin can has been careening around the Earth and moon since it deployed a space weather-monitoring spacecraft for the National Oceanic and Atmospheric Administration in 2015. Now its wandering will end when it smashes into the far side of the moon on March 4, according to projections by Bill Gray, who writes software for tracking near-Earth objects.

As far as anyone can tell, it will be the first piece of space junk to smash into the moon, but it’s not expected to do any harm. On the other hand, it’s a symptom of a larger problem. Many derelict boosters have been abandoned over the past few decades in orbits around the Earth and sun, where their looming hulks pose risks of impacts with active spacecraft, including those that provide communications, broadband, GPS, and other services we depend on. Recently, news coverage has focused on the hazards of tiny shrapnel, like the bits from a derelict satellite that Russia blew up in November, which subsequently nearly sideswiped the International Space Station. But things can always get worse.

“Every year, there’s a couple of school bus-sized things that come within 100 meters of colliding. And if they did, it would be an order of magnitude worse than the worst anti-satellite test,” says Brian Weeden, director of program planning at the Secure World Foundation, a nonpartisan think tank based in Broomfield, Colorado.

The oldest giant chunk of space garbage dates back to 1959: It’s a Vanguard rocket body, left over after deploying a US Navy weather satellite, one of the early launches of the Space Age rivalry between the US and USSR and one of the first tracked orbiting objects. Then, as the space race picked up, the discarded rocket bodies accumulated. And necessarily so: They are part of the multistage rockets needed to propel a spacecraft into orbit or beyond the Earth’s atmosphere. When the upper-stage rocket booster deploys that craft, it’s often left in orbit or on a similar trajectory. Some even have residual fuel left in them, which also makes them an explosion hazard. Now, there could be as many as thousands of spent boosters floating haphazardly in orbit, Weeden says.

And those are just the ones in low Earth orbit. Similar challenges apply further out in space, even if there’s more room to work with. “Any time we put something into interplanetary orbit, or deep space sensors or something like that, there’s likely a booster associated with it. Space is so vast that people never think about it,” says Bruce McClintock, head of the Space Enterprise Initiative of the Rand Corporation, a nonprofit research organization based in Santa Monica, California.

The first booster that ended up orbiting the sun arrived there in 1959, but that was an accident. A Soviet lunar mission was supposed to throw a spacecraft into the moon, but the upper-stage rocket misfired and it narrowly missed its target, McClintock says. When SpaceX launched a red Tesla roadster four years ago, with a spacesuit-clad mannequin in the driver’s seat, it ended up in a solar orbit, too.

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