In November, we reported how an impact on the Moon from a Chinese Long March rocket booster created an unusual double crater. For a single booster to create a double crater, some researchers thought there must have been an additional – perhaps secret – payload on the forward end of the booster, opposite from the rocket engines. But that may not necessarily be the case.

Other researchers feel the extra mass wasn’t anything secretive, but possibly an inert structure such as a payload adapter added to the rocket to support the primary mission payload.

Chang’e 5-T1 was an experimental robotic spacecraft, launched on October 23, 2014, by the China National Space Administration (CNSA) to test out the return capsule design planned for use on the future Chang’e 5 mission, China’s first-ever sample-return effort. Chang’e 5 landed on the Moon in November 2020 and successfully gathered lunar samples from the Moon’s Ocean of Storms region, with the container landing back on Earth on December 16, 2020.

Before engaging in the first sample return effort for the country (and first in over forty years), China wanted to test out procedures and their sample return capsule. That was one of the 5 T-1 mission’s goals.

“The rocket was carrying a ‘Service Module’ satellite with a sample return capsule attached,” said Phillip Stooke, professor emeritus at the University of Western Ontario, in an email to Universe Today.  “It would need a fairly substantial support structure (called a payload adaptor) to support the mass against the vibration and acceleration of launch.”

Stooke explained how the Service Module flew around the Moon and back to Earth, where it released the capsule to test its ability to survive atmospheric re-entry. Then the Service Module headed back out to the Earth-Moon L2 point, staying there for a few months before entering a low lunar orbit, possibly to perform a gravity mapping mission. The Service Module is still in lunar orbit.   

“The combination Service Module and capsule had a mass of 2,500 kg – 2.5 tons,” Stooke said, “so it can’t just sit on top of the rocket’s fuel tanks.  I can’t guess at the mass [of the payload adapter] but it would be quite significant.”

Payload adaptors for medium-sized payloads can weigh anywhere from 135 Kg (300 lbs.) to 225 kg (500 lbs) or more.

Chang’e 5-T1 did also have additional payloads, but they were small (and known to be onboard) and couldn’t account for the mass large enough to create a second crater. The two payloads were a small radiation exposure experiment for bacteria and plants, as well as the first commercial payload to the Moon called the 4M mission (Manfred Memorial Moon Mission) for the German space technology company OHB System, in honor of the company’s founder, Manfred Fuchs, who died in 2014. That payload weighed only 14 kilograms but contained two scientific instruments: a radio beacon to test a new approach for locating spacecraft and a radiation dosimeter (provided by the Spanish company iC-Málaga) to continuously measure radiation levels throughout the satellite’s circumlunar journey. The 4M mission was mounted in the equipment bay of the booster.

“There would be no reason to suspect the rocket had anything else attached to it other than 4M and the usual flight electronics,” said citizen scientist Scott Tilley, who monitors the orbits of artificial satellites of the Earth and the Moon. “There would also be some extra mass to support the payload adapter and related structure for supporting the payload stack, which was likely at the limit of the rocket’s capabilities.  Consider this is the first mission they launched toward the Moon with stacked payload.  It would have likely been more complex to mount and secure it than the other payloads, which were more self-contained.”

The ongoing debate on the extra mass and what it might be would not have ensued if not for two things: the unusual double crater created by this booster’s impact and the denial by Chinese foreign ministry officials that the space junk and the impact is from their rocket. They insist that the Chang’e 5T-1 rocket already burned up on its return trip to Earth in 2014. However, on March 1, 2022, 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 de-orbited.

Additionally, Chinese officials have never commented on the nature of the double crater.

The crater was imaged by NASA’s Lunar Reconnaissance Orbiter (LRO).

“Typically, a spent rocket has mass concentrated at the motor end; the rest of the rocket stage mainly consists of an empty fuel tank,” wrote Mark Robinson, principal investigator with LRO Camera (LROC), back in June of 2022 when the images were released.

A team of researcher from the University of Arizona discovered the errant booster (it was initially thought to be an asteroid), tracking its movements to determine it came from the Chang’e 5-T1 mission. They also conducted spectroscopic analysis of the object from ground-based telescope observations during several Earth flybys, which showed conclusively that the object was the Long March 3C rocket body from the Chang’e 5-T1 mission. They were able to predict approximately where and when the booster would impact the Moon, which was why the LRO team could search for and easily find the impact crater in their data.  

Everyone was surprised the impact created a double crater. No other rocket body impacts on the Moon created double craters, as seen in these images of craters from four Saturn rocket boosters from Apollos 13, 14, 15, 17.

The researchers from the University of Arizona said there had to be additional, undisclosed mass at the front end of the rocket body.

“The results from the Bayesian analysis imply that there may have been additional mass on the front of the rocket body,” wrote Tanner Campell, Vishnu Reddy and several others in their paper “Physical Characterization of Moon Impactor WE0913A.”  “Comparing the pre- and post-impact images of the location shows two distinct craters side by side that were made by the Chang’e 5-T1 R/B. The double crater supports the hypothesis that there was additional mass at the front end of the rocket body, opposite the engines, in excess of the published mass of the secondary permanently affixed payload.”

Asked about the payload adaptor as the possible culprit for the excess mass, team member Vishnu Reddy didn’t want to venture a guess without more data.  

“It is hard to speculate on the support structure because we are not aware of anything like that on usual boosters sent to the Moon,” he said.

Tilley told Universe Today that among both amateur and professional satellite and rocket trackers, it is known that China’s space agency has “struggled” in the past with their aim for having these type boosters to re-enter the Earth’s atmosphere or get ejected from the Earth-Moon system to properly dispose of the object.

“The Chinese expected the rocket to re-enter the Earth’s atmosphere,” Tilley explained, noting details in a paper by LuxSpace, the company that operated the 4M mission. “That didn’t happen so it seems that part of their mission failed, which is likely why the Chinese denied it was their rocket later on.”

Subsequently, however, more recent missions, such as the booster for the Chang’e 5 sample return mission successfully re-entered Earth’s atmosphere and was properly disposed of.   

Another question about the impact is understanding the dynamics of why a single booster, even if it had substantial weight at each end, would create a double crater.

“Regarding the double crater,” Vishnu explained, “I think the booster impacted at a near vertical angle, so the engines created the first crater and the secondary mass toppled over and created the second crater.” Vishnu added, however, it is also possible that if the booster was tumbling and happened to be horizontal when it hit, it could create the two craters.

But like much of this unusual space drama story, questions still remain.

“That is why we leave the actual mechanism for a future paper when we have better data to model,” Vishnu said.