A discarded part of a rocket crashed into the Moon’s far side just after noon, scientists say. They were expecting the impact to occur at 12:25 GMT on Friday.
The rocket segment, which weighs three tons, had been tracked for years, but its origin wasn’t clear.
For a time, astronomers thought it belonged to Elon Musk’s SpaceX firm, then they said it was Chinese, which China denies.
“Space junk” rocket crashes into Moon
The effects of such an impact on the Moon should be pretty minor, experts say.
The rocket stage would have dug out a small crater, creating a plume of dust, although scientists hope to get confirmation of this in the coming days or weeks.
The rocket part was first sighted from Earth in March 2015, with a NASA-funded space survey in Arizona spotting it. It was quickly forgotten about when the object wasn’t shown to be an asteroid, however.
The rocket part is known as “space junk,” or hardware discarded from missions or satellites.
Some pieces are closer to Earth, but others, like this booster, are thousands of kilometers away in high orbit.
The European Space Agency estimates there are now 36,500 pieces of space junk larger than 10 cm (4 inches).
No single space program or university tracks deep space junk, at least formally. Monitoring space is expensive and the risks to humans overall are low, so volunteer astronomers take the reins.
Is the rocket China or SpaceX’s?
Astronomer and data scientist Bill Gray identified the rocket that crashed on the moon as a SpaceX booster.
There is no visible insignia on the rocket, and astronomers normally piece together their identities by tracking their route backward through space. They match their orbit to dates and locations of rocket launches and trajectories.
China’s space missions, however, do not publicize their routes.
“For a Chinese mission, we know the launch date because they are televised,” Gray, in a BBC News interview, explains. “So I take a guess that it’s going to get to the Moon – usually in four or five days. Then I compute an approximate orbit.”
Weeks after the SpaceX identification, Gray received an email from another observer, who revealed that his identification was impossible.
China denies rocket is theirs
After running the numbers again, he conducted it was a rocket from China’s lunar mission Chang’e 5-T1, launched in October 2014.
China denied this, however, saying it had already re-entered the Earth’s atmosphere and burned up.
Gray is sticking to his prediction, however. He believes China is mixing up the tracking of the two rocket parts, saying, “I’m 99.9% sure it’s the Chang-e 5-T1.”
Wherever it came from, no one will know now, most likely. And unfortunately no one witnessed the booster’s last moments; evidence as to what happened won’t come until two satellites that orbit the Moon pass over the impact site and photograph the crater.
Since the booster certainly broke into thousands of pieces at impact, scientists believe no one will ever be able to collect any physical evidence of its origins.