The Moon is about to be hit by three tons of space debris, a blow that will create a crater large enough to fit several tractor-trailers.
The remains of a rocket will collide with the far side of the Moon on Friday at 9,300 kilometers (5,800 miles) per hour, far from the view of telescopes. It could take weeks or even months for the impact to be confirmed with satellite images.
Experts believe that junk has been floating around in space since China launched the rocket nearly a decade ago. Although the Chinese authorities have expressed doubts that it is theirs.
No matter who launched it, scientists estimate the object will punch a hole 10 to 20 meters (33 to 66 feet) in diameter and kick up lunar dust that could drift hundreds of kilometers (miles) across the moon's barren surface.
SpaceX was initially blamed for what will become lunar debris, after the Bill Gray asteroid tracker pinpointed the collision course in January. A month later he corrected his observation and pointed out that the "mysterious" object was not the upper stage of a SpaceX Falcon rocket from the 2015 launch of a NASA climate observatory.
Gray said it was likely the third stage of a Chinese rocket that sent a sample capsule to the moon and back in 2014. But Chinese officials say that stage returned to Earth's atmosphere and burned up.
The Moon already has countless craters, reaching 2,500 kilometers (1,600 miles) in diameter. The Moon, which has little or no atmosphere, is defenseless against the constant barrage of meteors and asteroids and sometimes spacecraft, including some deliberately crashed for scientific purposes. Without weather, there is no erosion, so impact craters are permanent.
China has a lunar probe on the far side of the moon, but it will be too far away to detect Friday's impact, just north of the equator. NASA's Lunar Reconnaissance Orbiter will also be out of range. India's Chandrayaan-2 orbital probe is unlikely to pass through the area at that time.
The object is about 12 meters long (40 feet) and 3 meters (10 feet) in diameter.
Tracking down debris from space missions is difficult, explained Jonathan McDowell of the Harvard Center for Astrophysics and the Smithsonian. The Moon's gravity can alter the course of an object as it passes by, adding to the uncertainty. And there are no easily accessible databases, except for those that experts like him and Gray have put together on the spur of the moment.
"We're in an era where a lot of countries and private companies are taking things into deep space, so it's time to start tracking them," McDowell said. "Right now no one does it, just a few hobbyists in their spare time."