Mystery rocket that smashed into the moon left 2 craters, NASA says

NASA has photographed the crash site of the mysterious rocket that smashed into the far side of the moon in March, and the unidentified spacecraft left behind a weird double crater that has scientists puzzled.

Images of the crash site were taken by NASA's Lunar Reconnaissance Orbiter (LRO) on May 25 and released on June 24. The photos show that the wayward debris (the origins of which are still contested) somehow punched out two overlapping craters when it smashed into the far side of the moon traveling at roughly 5,770 mph (9,290 km/h). 

The unexpected dual craters add an extra layer of strangeness to a mystery that has confounded space watchers since January, when Bill Gray, a U.S. astronomer and developer of software that tracks near-Earth objects, predicted that the orbiting piece of space junk would hit the moon's far side in a matter of months, Live Science previously reported. When Gray first spotted the debris, he suggested that it was the second stage of a Falcon X rocket launched by Elon Musk's SpaceX in 2015. But later observations and analysis of orbital data hinted that the object was the spent upper stage of China's Chang'e 5-T1 rocket, a spacecraft (named after the Chinese moon goddess) which launched in 2014. Chinese officials, however, disagreed, claiming that this rocket's upper stage burned up in Earth's atmosphere years ago. 

Related: 5 strange, cool things we've recently learned about the moon

To date, at least 47 NASA rocket bodies have crashed into the moon, according to Arizona State University, but "the double crater was unexpected," NASA wrote in a statement. "No other rocket body impacts on the Moon created double craters."

Although scientists were unable to directly observe the moment of impact, experts predicted that the discarded rocket stage struck the lunar surface at Hertzsprung crater on the moon's far side, on March 4 at 7:25 a.m. EST (12:25 GMT). Observations from the LRO show the two indentations on the lunar surface — the eastern crater measures 59 feet (18 meters) wide, while the western crater measures 52.5 feet (16 m) across. If NASA's LRO had been positioned to capture images of the impact, it would have likely documented a plume of lunar dust erupt hundreds of miles high.

Scientists are still hypothesizing about what could have created the two craters. One possibility is that the craters were formed by a piece of debris  that had two large masses at each end — although this scenario would be unusual, NASA representatives said.

"Typically a spent rocket has mass concentrated at the motor end; the rest of the rocket stage mainly consists of an empty fuel tank," according to the statement. 

Is it really Chang'e 5-T1's booster?

As the rocket booster is likely to have totally disintegrated upon impact, it’s uncertain if investigating the craters will provide any big clues to its controversial origin. But some astronomers think they have most of the mystery figured out already. Gray wrote on his blog soon after the images were released that the object is "quite conclusively identified as the Chang'e 5-T1 booster."

"I'm fairly convinced that there's no way it can be anything else," Gray told Live Science. "At this point, we rarely get anything quite this certain."

Gray made his first prediction that the controversial debris would collide with the moon after it was spotted tumbling through space in March 2015. The object (assigned the temporary name WE0913A) was first glimpsed by the Catalina Sky Survey, an array of telescopes near Tucson, Arizona that scans our cosmic neighborhood for dangerous asteroids that could smash into Earth. However, WE0913A wasn't orbiting the sun, like an asteroid would, but was orbiting Earth instead. Gray suspected that the object was human-made.

After initially misidentifying the mystery trash as a SpaceX Falcon 9 rocket, Gray went back to the data to find that another spacecraft was a near match for the trajectory of the moon-bound debris: the upper stage of China's Chang'e 5-T1 mission, which launched in October 2014 as part of a preliminary mission to send a test capsule to the moon and back.

Chinese foreign ministry officials denied that the space junk is theirs, insisting that the Chang'e 5 rocket already burned up on its return trip to Earth in 2014. But U.S. experts contested this claim, suggesting that Chinese officials could be mixing up the 2014 rocket with a similarly designated rocket from a 2020 mission, and that the former is what hit the moon. On March 1, the U.S. Department of Defense's Space Command, which tracks low-Earth orbit space junk, released a statement saying that China's 2014 rocket never deorbited.

Gray believes his orbital data, which is a near-perfect match to the Chinese rocket's initial trajectory, is conclusive.

"It's on the orbit that an awful lot of lunar missions take; its inclination means that, in the past, it was headed out over China; it was going east in the way Chinese lunar missions do; and its estimated launch time falls within 20 minutes of the Chang'e 5-T1 rocket," Gray said.

An amateur radio satellite (or "cubesat") was attached to the Chang'e 5-T1 for the first 19 days of its flight, and the trajectory data sent back from that satellite matches the rocket debris' current trajectory perfectly, according to Gray. Others have also identified important clues that support Gray's conclusion; NASA Jet Propulsion Laboratory's Center for Near Earth Object Studies confirmed Gray's analysis of the orbital data, and a University of Arizona team identified the rocket as part of the Chang'e 5-T1 mission by analyzing the light spectrum reflected by paint on the crashed debris.

Although this is the first piece of space junk to unintentionally collide with the moon, it isn't the first time a human-made satellite has crashed there. In 2009, NASA's Lunar Crater Observation and Sensing Satellite was deliberately fired into the moon's south pole at 5,600 mph (9,000 km/h), unleashing a plume that enabled scientists to detect the chemical signatures of water ice. NASA also disposed of the Apollo programme Saturn V rockets by hurling them at the moon.

Gray said the confusion surrounding the object's identity highlights a real need for space-faring agencies and private companies everywhere to develop better procedures for tracking the rockets they send into deep space (which would also keep such objects from being mistaken for Earth-threatening asteroids).

"From my selfish standpoint, it would help us to track asteroids better," Gray said. "The care that's given to low-Earth orbit satellites has not been applied to those in high-Earth orbits because people figured it really doesn't matter. My hope is that, with the U.S. now considering a return to the moon and other countries sending stuff there too, that attitude might change."

Originally published on Live Science.