The moon is a wonderland of craters—thousands of them, carved by asteroids hitting the surface over billions of years. Space rocks are still at it, and every year the bombardment scoops out dozens of craters big enough for moon-orbiting spacecraft to notice. Today, because of human beings and their little space things, Earth’s celestial companion got one more dent.
This morning, a piece of space junk smacked right into the far side of the moon. The debris, a piece of a rocket about the size of a school bus, had been floating in space for seven years. The hardware came barreling head-on at the lunar surface at 5,800 miles an hour, into an existing, enormous crater. Astronomers can’t see the aftermath yet but, based on the size of this thing, they believe that the impact has scooped out a new crater as wide as 20 meters, or 65 feet. “It would almost completely cover a basketball court,” Bill Gray, an amateur astronomer who tracks space objects, told me.
According to Gray, the space junk turned out to be a discarded rocket booster from a Chinese mission that sent a spacecraft toward the moon in 2014. The hardware, no longer useful after a successful launch, was just discarded in space. When you think about it that way, we’re kind of throwing trash at the moon.
Humankind has actually been chucking stuff at the moon for decades, and not just astronauts. But we’ve usually done it for specific reasons, such as national glory or scientific curiosity. The first spacecraft to ever reach the lunar surface, for example, smashed into it on purpose in 1959 because the Soviet Union was eager to beat the United States to it, even if it meant destroying a probe. But this latest incident appears to be the first of its kind: entirely unplanned.
Gray, who is well known in the space community for monitoring objects in orbit—he provided the tracking software that helped pinpoint a space rock that used to orbit Earth like a second moon—first noticed the debris in 2015. Back then, he believed that it was an abandoned rocket booster from a recent SpaceX launch, which had deployed a satellite mission for the National Oceanic and Atmospheric Administration; the orbital data and the timing seemed to line up. Gray and others have kept an eye on it over the years, noting whenever the object popped up in telescope observations of asteroids. There’s that rocket chunk again, they’d say, and update the numbers for its orbital path.
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This past January, Gray noticed that the rocket debris was tracing a path directly toward the moon. The collision sparked a bunch of news stories because, well, we’re just throwing trash at the moon now, and this trash appeared to have originated with SpaceX and its divisive CEO, Elon Musk. But when someone who worked on that NOAA satellite reached out to Gray with doubts, the astronomer did a deeper analysis, running the object’s trajectory backward in time, and realized that he’d been wrong.
Astronomers will now attempt to find the new crater using observations from two missions, NASA’s Lunar Reconnaissance Orbiter and India’s Chandrayaan-2, which both loop around the moon. “We haven’t seen any big things like rocket upper stages hit the moon since the Apollo program,” Robert Wagner, a research specialist at Arizona State University who works on the NASA orbiter’s camera team, told me. And those were planned events; NASA dropped the rocket parts to see whether the seismometers that the astronauts had left behind would pick up any vibrations.
The Lunar Reconnaissance Orbiter has captured the lunar surface in great detail, including the Apollo landing sites and the things the astronauts left behind more than 50 years ago. Experts will soon go through before-and-after photos of the spot where the rocket hit on the moon’s far side. Wagner expects they’ll see a big splash of material on the landscape, and then, when they zoom in, the shape of the crater itself. It probably won’t be bowl-shaped, like the moon’s most prominent craters, but kind of lumpy around the edges. “It’s always exciting when you see something new on the moon’s surface,” Julie Stopar, a scientist at the Lunar and Planetary Institute who works on the orbiter’s imaging team, told me. Scientists might even learn something interesting about craters by examining the way this one formed, she said.
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The team doesn’t expect to see any signs of the debris. The hardware was likely vaporized upon impact today, Gray said. The dust has joined the remains of fast-moving missions past, early efforts like the Soviet mission in the early 1950s, but recent collisions too: The NASA mission that hurled itself at the moon’s south pole in 2009, in an attempt to see whether the impact would dislodge any grains of pure water ice. A pair of probes that NASA deliberately sent down in 2012, after they were decommissioned. A couple of moon landers from 2019 that were supposed to do what their name suggests, but ended up slamming into the surface instead. Gray suspects that other human-made objects have crashed into the moon, but they were too small to detect or simply slipped past our observation instruments.
People will probably lob more stuff toward the moon in the coming years, as government space agencies and private companies mount new missions to the lunar surface. Launch providers could avoid such moon litter by ensuring that their discarded hardware falls back into Earth’s atmosphere after launch, or wafts somewhere else in space. The rocket booster that helped propel the James Webb Space Telescope last year, for example, was designed to end up in a peaceful orbit around the sun; Gray said the booster will swing past Earth sometime in 2047, but at a safe distance. (Proper disposal of rocket parts is generally a good move, considering the panic that set in when a different Chinese booster came down last year in an “uncontrolled reentry,” keeping experts guessing about where it would end up.)
The moon’s newest crater is a permanent addition. The moon lacks any of the natural processes that long ago erased evidence of the asteroids that once rained down upon Earth in its early history. On the moon, there’s no weather to wear away the sharp edges, or tectonic plates to shift the land around, or volcanoes to fill in the basins—just a wispy, airless atmosphere that preserves the surface and its indentations, whether they’re basins shaped by ancient asteroids, boot prints of long-departed astronauts, or craters chipped into existence by a piece of rubbish from next door.
