An unusual double crater on the far side of the moon explained

On March 4, 2022, a rocket smashed into the far side of the Moon — an event not in itself unusual, as a number of space vehicles from different nations have crashed into our celestial companion. But what's strange about the recent crash, as NASA's Lunar Reconnaissance Orbiter later discovered, was that it resulted in two craters instead of one.

No other , such as NASA's Apollo missions, have left similar double craters on the Moon. What was different about this one? As no nation stepped forth to take credit, the mystery of its origin and nature persisted. A from the University of Arizona, published in , may now put the mystery to rest.

The researchers, led by Tanner Campbell, a doctoral student in the Department of Aerospace and Mechanical Engineering, claim they found "definitive proof" that a booster made the impact from the , which likely carried another unknown payload. This extra payload explains the existence of the second crater, according to the scientists.

Typically, once a rocket's fuel is spent, its mass is concentrated at one end, as the other end of the rocket contains the empty fuel tank. The details The object was first spotted tumbling between the Earth and the Moon in 2015 and dubbed WE0913A by the University of Arizona's team, which tracks objects that may pose a danger to Earth.

The impact occurred near the Hertzsprung Crater, a huge site (570 kilometers or 350 miles in diameter) on the side of the Moon you can't directly see from Earth. The craters left by the rocket crash were also considerable in size — the eastern crater extends about 18 meters (~ 59 feet) in diameter.

In contrast, about 100 feet away, the western crater stretches 16 meters in diameter (about ~52 feet). Based on the object's path toward the Moon, the team's first hypothesis was that it was a booster from SpaceX's Falcon 9 rocket launched in 2015. But by studying the object's light reflection signature and its path through space, the scientists concluded that it was much more likely a booster from the Chang'e 5 T1.

Part of the Chinese Space Agency's lunar program, it was launched in 2014. For its part, China's Space Agency denied it had anything to do with the object, stating their rocket's booster burned up on its return when it entered Earth's atmosphere. The U.S. Space Command, however, does not believe that ever happened and that the rocket's third stage never re entered the atmosphere.

Evidence for the booster The Chang'e 5 T1 was an experimental, robotic spacecraft that traveled atop a Long March 3C rocket. The mission was intended to bring back a sample of lunar soil to Earth. The booster that slammed into the Moon is thought to be the rocket's third stage. Its role was to provide the thrust necessary to propel payloads into lunar orbit.

According to a from the University of Arizona, once it accomplished its mission, the booster was supposed to jettison the orbiting module, return the capsule, and just float off into space. That's the usual process. What happened to the booster next would have been a mystery, especially as it's too small to be observed by even high powered telescopes.

However, the researchers were able to spot it using spectroscopic analysis, identifying a light curve that brightened and dimmed in a way that's characteristic of the booster's rotation. "As the object is spinning, we see variations in the right light it reflects as the visible surface area is changing," shared Vishnu Reddy, one of the paper's co authors and a University of Arizona professor of planetary science at the as well as the director of .

"When the broad side of the rocket is pointed at you, you get more light, and as it turns, you get less light from that side," he added. The cause of the double crater But why did the booster crash cause a double crater? Answering this question required some detective work, especially as 47 NASA rocket bodies have crashed into the Moon previously without causing a similar outcome.

, four craters that resulted from the impact of the Apollo SIV B stages were a bit unusual in outline and much larger (over 35 meters or 114 feet) than either of the double craters. Even so, those were single craters. To understand this part of the mystery, scientists compared computer simulations of thousands of potential objects that could be found traversing space to the light curve data.

They discovered that WE0913A wasn't behaving in an expected manner. "Something that's been in space as long as this is subjected to forces from the Earth's and the moon's gravity and the light from the sun," Campbell explained in the . "So you would expect it to wobble a little bit, particularly when considering that the rocket body is a big empty shell with a heavy engine on one side.

But this was just tumbling end over end, in a very stable way." What was the reason for this behavior? The researchers believe there must have been a counterweight to the two engines, which weigh 1,200 pounds (544 kilograms) each when they are empty of fuel. Campbell said they knew the booster had an on its top end, weighing about 60 pounds (27 kilograms).

A torque balance analysis showed that wasn't enough, however, to explain the rotation of the booster. This led the team to think something else was mounted to the front of the booster. There was also crucial evidence visible at the impact site. Seeing two craters from a rocket crash is unusual.

"This is the first time we see a double crater," Campbell pointed out. "We know that in the case of Chang'e 5 T1, its impact was almost straight down, and to get those two craters of about the same size, you need two roughly equal masses that are apart from each other." Campbell isn't sure we'll ever find the answer to the question of what exactly this extra payload was, speculating it was possibly an additional support structure or some instrumentation.

What's important, say the scientists, is to focus on the growing need for a plan on how to manage that fulfill their missions and are no longer needed. They need to be tracked and disposed of in more sustainable ways so they do not present a danger to other space missions. Check out the full study "Physical Characterization of Moon Impactor WE0913A" in.