A rogue rocket part collided with the moon

It's the first piece of space junk found on a lunar collision course—and astronomers worry the problem will only get worse. 

In the first known event of its kind, a new crater has been carved into the lunar surface by a piece of space junk. Around 7:25 a.m. ET on March 4, a discarded rocket stage that’s been drifting through deep space since at least 2015 collided with the moon.

The rogue rocket was completely obliterated, punching a crater in the lunar surface and launching a plume of sharp, corrosive dust into orbit that may linger for hours. Humans couldn’t see the action, though—the rocket smashed into the moon’s far side in 350-mile-wide Hertzsprung crater, and lunar orbiters did not see the rocket come down.

Astronomers believe the derelict rocket stage came from China’s Chang’e 5-T1 mission that launched in 2014, but experts are not completely certain. Regardless of where it came from, the rocket’s impact did not damage anything other than the lunar surface.

“It’s not a big deal at the current level of occupation of the moon, which is currently population: robots, a couple of dozen; humans, zero. And maybe alien mutants grown from the human poop left there,” says astronomer Jonathan McDowell of the Harvard-Smithsonian Institute for Astrophysics. “There’s not much for it to hit.”

That likely won’t always be the case. Lunar exploration is ramping up, with humans once again setting their sights on establishing moon bases either on the surface or in orbit. Private companies are joining the handful of nations that have already sent hardware to the moon. And even if this particular collision isn’t a big deal, the next one could be.

“We need to make sure that we go to space responsibly,” says Michelle Hanlon, a space lawyer at the University of Mississippi and the founder of the nonprofit For All Moonkind, which seeks to preserve lunar heritage sites. “That means making sure we do the right things—we figure out where our rockets are going and make sure we don’t just start throwing stuff on the moon for the heck of it.”

So what is this thing that hit the moon?

The object is designated WE0913A and was tracked intermittently since it was first spotted in 2015 during surveys designed to look for potentially hazardous asteroids. In January, astronomer Bill Gray ran a computer program that plotted its trajectory. A blinking line popped up on his screen, highlighted in red: “IMPACT,” written in all caps. Gray’s program calculated that WE0913A would smash into the moon on March 4, 2022.

Based on its dimensions—roughly 10 feet across and 41 feet tall—scientists are confident that the object was an upper rocket stage, a propulsive segment that separates from the lower part of a rocket to carry a payload farther into space. It was bright and tumbled in orbit about once every 185 seconds, traits that are common for the discarded upper stages of rockets.

It had “a very characteristic light curve of a rocket body tumbling—that was a giveaway,” says Vishnu Reddy of the University of Arizona, whose group’s observations would ultimately help characterize the object.

OK, but which rocket did it come from?

Scientists aren’t 100 percent sure.

Early observations suggested the object could have been the upper stage of a SpaceX Falcon 9 rocket that had delivered the National Oceanic and Atmospheric Administration’s DSCOVR satellite to a point in deep space in February 2015. But it soon became clear that the upper stage from DSCOVR’s rocket had ended up orbiting the sun and was nowhere near the moon.

So astronomers went hunting for other rockets that had launched onto moon-crossing orbits around 2015, reasoning that anything as bright as WE0913A would have been picked up by asteroid surveys shortly after it had been launched.

They landed on a solid candidate from China’s Chang’e 5-T1 mission, an early test of a lunar sample-return program. In late 2014, a Chinese Long March 3C rocket launched a small spacecraft to loop around the moon once and return to Earth. Part of the rocket was left in a weakly bound orbit around Earth that occasionally encountered the moon—a path that McDowell says is consistent with the object that impacted the lunar far side.

Reddy’s group found another piece of the puzzle in February, when he and his students observed WE0913A as it passed overhead and gathered data about how its painted coating reflected light.

The team compared those observations to other upper stages from SpaceX and Chinese rockets in Earth orbit and found that the paint on WE0913A was a better match with the Chinese rocket parts.

“SpaceX uses a different paint than the Chinese, and the moon impactor is very similar to the Long March booster we have in Earth orbit,” Reddy says.

So, case closed?

Not exactly. Wang Wenbin, a spokesperson for China’s Ministry of Foreign Affairs, said that the Chang’e 5 rocket had already deorbited and burned up. But it appears China’s response may have been the result of a miscommunication, referring instead to the subsequent Chang’e 5 mission and not Chang’e 5-T1.

The U.S. Space Command, which tracks objects in orbit, initially said that the Chang’e 5-T1 rocket had completely deorbited, but later revised its statement. “While U.S. Space Command can confirm the CHANG’E 5-T1 rocket body never de-orbited, we cannot confirm the country of origin of the rocket body that may impact the moon,” Space Command said in an emailed statement.

McDowell says all the signs point to Chang’e 5-T1, and he’s reasonably sure that’s where the errant rocket came from—but he’s still not 100 percent confident.

Will we ever know for sure where the rocket came from?

Only if better orbital tracking data is found that allows astronomers to more reliably rewind the object’s path through space.

For the last eight or so years, WE0913A had been looping around Earth on what McDowell calls “a chaotic deep-Earth orbit.” Its path carried it from about 15,000 miles above the planet’s surface to more than twice the distance of the moon, where it occasionally got tossed around and tugged at by lunar gravity. Its path was also influenced by solar radiation, and all these factors make it tough to rewind its journey through space.

The rocket itself was completely destroyed when it hit the moon. It hurtled toward our nearest neighbor at roughly 5,000 miles an hour, and unlike Earth, the moon doesn’t have an atmosphere to slow it down as it approaches.

When the rocket stage hit the moon, it gouged out a crater between 60 and 100 feet wide. In the coming months, NASA’s Lunar Reconnaissance Orbiter will attempt to snap an image of the crash site, but the exact impact location is still uncertain, so identifying the new crater might take some time.

Has this type of thing happened before?

“Almost certainly yes,” McDowell says. He estimates that, over the course of the space age, 50 or so objects have been launched into orbits that could have led to a collision with the moon. But because tracking data are sparse, he says, we don’t know their ultimate fates.

We do know that some fraction of them have likely smashed into the moon and escaped our notice, another fraction were nudged into orbits around the sun, and a third fraction are still in their evolving orbits or have burned up in Earth’s atmosphere.

Humans have also deliberately crashed spacecraft into the moon multiple times—sometimes for science, sometimes at the end of lunar missions.

“This crater now will become part of China’s archaeological record on the moon, and we have to consider it in relation to the other Chinese sites,” says Alice Gorman, a space archaeologist at Flinders University who studies the physical record of human-made objects in space.

“What we’re creating here is a sort of a modern Anthropocene bombardment phase, potentially. Kind of a geological era created by human activities that will leave its marks on the surface of the moon. It’s already started.”

Should someone be keeping better track of these things? 

A number of organizations, including the U.S. military, use radar to track objects in Earth orbit, from the satellites that hover just above the atmosphere to those more than 22,000 miles up.

But almost no one tracks space debris once it leaves Earth orbit. Satellites are small, and bright objects like the moon and sun make them difficult to find once they are too far away. And when these things turn up in asteroid surveys, it’s usually to the disappointment of astronomers who were hoping for newly discovered worlds.

A number of experts say that needs to change. About a dozen missions are currently scheduled to launch to the moon this year, some of which could leave another accidental impactor in space.

“At some point in the future, an event like this isn’t just going to be a curious thing to observe from the outside. It’s going to be something that people in lunar orbit or on the surface of the moon are worried about,” Gorman says.

Gorman and others say we could use better regulations for proper disposal of space junk, such as requiring rocket stages to perform end-of-life maneuvers that send them into solar orbits rather than leaving them tumbling between the moon and Earth.

“This rocket body hitting the moon—thank goodness it’s going to the far side. What if it were heading to one of the Apollo sites, or the Chang’e 4 rover?” asks Hanlon. “We’re not thinking properly about the moon.”

Editor’s Note: This story was originally published on March 3. It was updated on March 4 to reflect the impact on the moon. 

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