Tracking an errant space rocket to a mysterious crater on the moon

In March 2022, an inoperative segment of a lunar probe plummeted toward the moon’s crust and made an impact adjacent to the Hertzsprung Crater, a vast crater situated on the moon’s side that remains perpetually obscured from our planet’s view.

Interestingly, in contrast to other lunar debris, this particular instance resulted in not a single depression, but two adjacent ones, instigating debates about the nature of the object that settled on the lunar soil, as voiced by Tanner Campbell, a graduate researcher at the University of Arizona’s Aerospace and Mechanical Engineering Department, within the Engineering Faculty, and the principal investigator of the study.

In a research article presented in the Planetary Science Journal, a collective of scholars at UArizona offers decisive evidence indicating that the debris was in fact a propellant booster originating from a Chinese lunar probe, which had meandered through space for numerous years. The inquiry also uncovered that the discarded stage possibly transported a concealed, supplementary payload.

The lunar environment has frequently been the deposit site for advanced debris—the remnants of space crafts that have concluded their voyages and impacted the lunar terrain. Exemplary of such refuse are several rocket propellants left over from NASA’s Apollo explorations, which could be encountered by future moonwalkers traversing the barren, silent, and breathless lunar expanse.

Seven years beforehand, the Catalina Sky Survey operated by UArizona, recognized as a paramount program in the detection and analysis of asteroids that pose a threat to Earth, apprehended sight of a swiftly moving body between our planet and the moon. It received the designation WE0913A, but its identity remained a mystery.

Tracing its celestial trajectory, the initial belief was that WE0913A could be an astray SpaceX Falcon 9 rocket propellant from a 2015 launch, on a collision course with the lunar surface. Pertinent observations utilizing the Raptor Telescope, constructed by the student collective, and spectral analysis subsequently detected distinct light reflections emanating from the object’s exterior.

It was deduced that WE0913A’s reflective properties and its manners of celestial travel pointed to a plausible identity as a propellant hailing from the Chang’e 5-T1 mission, a spacecraft deployed in 2014 within the ambit of the Chinese space agency’s lunar expedition project. Despite assertions by the Chinese space agency that the propellant had disintegrated in Earth’s atmosphere on re-entry, confirmation by the U.S. Space Command indicated the projectile’s third stage never penetrated back through Earth’s atmospheric boundaries.

The Chang’e 5-T1 mission, planned as a rehearsal for a mission to retrieve lunar soil samples back to our globe, featured an automated spacecraft mounted atop a Long March 3C rocket. This uppermost portion of the rocket, later pinpointed as object WE0913A by the Catalina Sky Survey, supplied the necessary impetus to dispatch the payloads into a lunar obit. Post task fulfillment, it dispelled the orbiting component and the sample return capsule, leaving the spent booster to drift in space, which is a regular protocol for these components after they’ve exhausted their purpose.

While the propellant dimensions are too minute to discern even by potent telescopes, the gathered observations revealed a distinctive pattern of light intensity fluctuations due to its rotation.

“As the object revolves, we witness shifts in the intensity of light it reflects, as the surface area facing us oscillates,” said Vishnu Reddy, a space sciences professor at UArizona’s Lunar and Planetary Laboratory and director of the Space4 Center. Reddy, a co-author of the paper and one of Campbell’s mentors, added, “You perceive heightened illumination when the object’s wider section aligns with your line of sight. As it spins, the light emitted from that side diminishes.”

Assessing the light curve information against computational emulation of countless speculative objects adrift in space, it was inferred that WE0913A’s characterization was unaligned with that of a conventional booster.

“An artifact enduring space’s environment as long as this one has, is susceptible to terrestrial and lunar gravitational dynamics as well as solar radiation,” commented Campbell. “Thus, a slight wobble could be anticipated, especially considering the shape of the rocket: a hollow structure with a weighty engine on one end. Contrary to expectations, its rotation was quite constant, akin to tumbling end-to-end in a remarkably organized manner.”

In essence, the booster’s constitution must have incorporated a counterbalancing mass to the duo of motors, each individually weighing 1,200 pounds unfueled.

“We acknowledge that an instrumentation panel was affixed to the booster’s apex, though it’s considerably lightweight at just over 60 pounds,” added Campbell. “Through a torque balance evaluation, it became apparent that such a mass would shift the gravitational center a mere inches—it was nowhere near sufficient to justify the observed stable spin. It’s this realization that prompts us to surmise the existence of an additional mass attached to the forepart.”

Fresh insights emerged from the impactitself: When the rocket booster smashed into the lunar surface, it spawned dual craters roughly 100 feet apart, rather than a solitary one. This occurrence is quite peculiar, in Campbell’s view, who noted that craters resulting from Apollo rockets are typically circular, if the impact was vertical, or elongated, if the trajectory was more horizontal.

“This marks the inaugural instance we’ve witnessed a paired crater formation,” he elaborated. “Given that Chang’e 5 T1 descended almost perpendicularly, to produce two craters of similar dimensions, you’d require two mass components of comparable size separated from one another.”

The research underscores a pressing requirement: Monitoring obsolete space apparatus post-operation is crucial for sustaining cosmic voyage. Thus, investigative initiatives focused on what’s termed space situational awareness are pivotal, with UArizona’s Space4 Center playing an integral part in this domain, as stated by Roberto Furfaro, the Center’s vice director and Campbell’s guidance collaborator.

Furfaro, who is a co-author of the paper and functions as a professor within the Department of Systems and Industrial Engineering, contributes along with additional UArizona contributors such as Adam Battle, a postgraduate scholar at their Lunar and Planetary Lab, and Neil Pearson, who manages Reddy’s laboratory.

“There’s an intensive drive at the levels of both governance and enterprise to journey to the moon,” mentioned Furfaro, “and with the incremental introduction of entities upon the lunar landscape, it grows increasingly critical not just to monitor these entities, but also to decipher their intended effects upon arrival.”

Concerning the Chang’e 5 T1 rocket‘s secondary cargo, its essence could likely stay elusive, suggested Campbell.

“Clearly, we’re in the dark about its potential nature—might be some redundant support mechanism, extra instruments, or an altogether different entity,” he remarked. “The likelihood is we may never determine its true nature.”