After months of scrutinizing photographs of the lunar surface, scientists have finally found the crash site of a forgotten rocket stage that hit the far side of the moon in March.
They still don’t know for sure which rocket the wayward debris came from. And they are puzzled as to why the impact left two craters and not just one.
“It’s cool, because it’s an unexpected result,” said Mark Robinson, professor of geological sciences at Arizona State University, who is the principal investigator for the camera aboard NASA’s Lunar Reconnaissance Orbiter. who has been photographing the moon since 2009. “It’s still much more fun than if the prediction of the crater, its depth and its diameter, had been exactly right.
The intrigue of the rocket crash began in January when Bill Gray, developer of Project Pluto, a suite of astronomical software used to calculate the orbits of asteroids and comets, tracked what looked like the abandoned upper stage of a rocket. He realized he was on a collision course with the far side of the moon.
The crash was certain, around 7:25 a.m. Eastern time on March 4. But the exact orbit of the object was not known, so there was some uncertainty as to the time and place of impact.
Mr Gray said the rocket part was the second stage of a SpaceX Falcon 9 that launched the Deep Space Climate Observatory, or DSCOVR, for the National Oceanic and Atmospheric Administration in February 2015.
He was wrong.
A NASA engineer pointed out that DSCOVR’s launch trajectory was incompatible with the orbit of the object Mr. Gray was following. After some further research, Mr Gray concluded that the most likely candidate was a Long March 3C rocket launched from China a few months earlier on October 23, 2014.
University of Arizona students reported that an analysis of the light reflected from the object revealed that the mixture of wavelengths matched similar Chinese rockets rather than a Falcon 9.
But a Chinese official denied it was part of a Chinese rocket, saying the rocket stage of that mission, which launched the Chang’e-5 T1 spacecraft, had re-entered Earth’s atmosphere and had burned.
Whichever rocket it was a part of, the object continued to follow the spiral path dictated by gravity. At the scheduled time, it slammed into the far side of the moon in the 350-mile-wide Hertzsprung crater, out of sight of anyone on Earth.
The Lunar Reconnaissance Orbiter was unable to observe the impact, but the hope was that a freshly dug crater would appear in a photograph the spacecraft later took.
Mr. Gray’s software made a prediction of the impact site. Experts from NASA’s Jet Propulsion Laboratory calculated a location a few miles to the east, while members of the Massachusetts Institute of Technology’s Lincoln Laboratory expected the crash to be tens of miles away. the west.
That meant researchers had to search a strip about 50 miles long for a crater a few tens of feet wide, comparing the lunar landscape before and after the crash to identify recent disturbances.
Dr Robinson said he feared ‘it will take us a year of imaging to fill in the box’.
While the Lunar Reconnaissance Orbiter has photographed the vast majority of the moon multiple times over the past 13 years, there are places it has missed. Some of the gaps turned out to be close to the intended crash site.
Dr. Robinson remembers thinking about Murphy’s Law and joking, “I know exactly where it’s going to hit.”
Because the accident had been predicted a month in advance, the mission team was able to fill in most of the gaps.
Then the search began.
Usually a computer program does the comparison, but it works best if the before and after photos are taken at the same time of day. For this search, many images were taken at different times and the difference in shadows confused the algorithm.
With all the false positives, “we just sat back and had several people manually sift through the millions of pixels,” Dr. Robinson said.
Alexander Sonke, a senior with Arizona State’s Department of Geological Sciences, contributed to the effort. He estimated that he spent around 50 hours over several weeks performing this tedious task.
Mr. Sonke graduated in May. He got married. He went on honeymoon. A week and a half ago, it was his first day back at work – he is about to start his graduate studies with Dr Robinson as his adviser – and he resumed the search for the impact site.
He found it.
Mr Sonke said he saw “a group of pixels that were very different in brightness” as the before and after images flickered back and forth.
“I was pretty confident when I saw it that it was a new geological feature,” Mr Sonke said. “I definitely jumped out of my seat a bit, felt like that was definitely it, and then tried to contain my excitement.”
The eastern crater, about 20 meters in diameter, is superimposed on the slightly smaller western one, which most likely formed a few thousandths of a second before the eastern one, Dr Robinson said.
This is not the first time that a part of a spacecraft has hit the Moon. For example, pieces of the Saturn 5 rockets that took astronauts to the Moon in the 1970s also dug craters. But none of these impacts created a double crater.
Why this one did it could point to his mysterious identity. China’s October 2014 mission carried the Chang’e-5 T1 spacecraft, a precursor to another mission, Chang’e-5, which landed on the Moon and brought rock samples back to Earth.
The precursor T1 spacecraft did not include a lander, but Dr. Robinson assumes it had a heavy mass atop the stage to simulate the presence of one. If so, the rocket engines at the bottom and the lander simulator at the top could have created both craters.
“This is pure speculation on my part,” Dr. Robinson said.
The other parts of the rocket stage would have been made of thin and light aluminum, and were unlikely to make a large dent in the lunar surface.
The actual impact site is between the sites predicted by Mr. Gray and NASA’s Jet Propulsion Laboratory, close to NASA’s. “It was within the margins of error that we had calculated,” Mr Gray said.
Fortunately, the Lunar Reconnaissance Orbiter team had filled in the gaps – called gores, in cartographer parlance – in the images. “As Murphy would have said, this thing impacted what was one of the gores,” Dr. Robinson said. “If I hadn’t been alerted, we wouldn’t have had a picture before.”
Scientists may possibly have found the crash site. The dirt thrown up by a dug crater is usually brighter and darkens over time. This is how scientists identified the craters caused by Saturn’s 5 stages.
But they would still be looking for a small bright spot in the moon’s haystack.