After months of closely examining photos of the lunar surface, scientists have finally found the crash site of a forgotten rocket stage that slammed into the far side of the moon in March.
They’re still not sure which missile the wayward debris came from. And they are baffled as to why the impact excavated two craters and not just one.
“It’s cool because it’s an unexpected outcome,” said Mark Robinson, a professor of geological sciences at Arizona State University who serves as the principal investigator for the camera aboard NASA’s Lunar Reconnaissance Orbiter, which has been photographing the moon since 2009. . always much nicer than if the prediction of the crater, its depth and diameter, had been just right.”
The rocket crash intrigue began in January when Bill Gray, developer of Project Pluto, a suite of astronomical software used in calculating the orbits of asteroids and comets, tracked what appeared to be the discarded top stage of a rocket. He realized it was on a collision course with the far side of the moon.
The crash was certain, at approximately 7:25 AM Dutch time on March 4. But the exact trajectory of the object was unknown, so there was some uncertainty about the time and place of the impact.
Gray said the rocket portion was the second stage of a SpaceX Falcon 9 that launched in February 2015 from the Deep Space Climate Observatory, or DSCOVR, for the National Oceanic and Atmospheric Administration.
He was wrong.
A NASA engineer pointed out that DSCOVR’s launch trajectory was incompatible with the orbit of the object Mr. Gray followed. After some more digging, Mr. Gray said the most likely candidate was a Long March 3C rocket launched from China a few months earlier, on October 23, 2014.
Students at the University of Arizona reported that an analysis of the light reflected from the object showed that the mix of wavelengths matched comparable Chinese rockets rather than a Falcon 9.
But a Chinese official denied it was part of a Chinese rocket, saying that the mission’s rocket stage, which launched the Chang’e-5 T1 spacecraft, had re-entered Earth’s atmosphere and burned up.
Regardless of which rocket it was part of, the object continued to follow the spiral path determined by gravity. At the predicted 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 not in a position to observe the impact, but the hope was that a newly sculpted crater would appear in a photo the spacecraft took later.
The software of mr. Gray made one prediction of the impact site. Experts at 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 occur dozens of miles to the west.
That meant the researchers had to search a strip about 50 miles (80 km) long for a crater several tens of meters wide, comparing the lunar landscape before and after the crash to identify recent disturbances.
dr. Robinson said he was concerned that “it would take us a year of imaging to fill the box.”
Although the Lunar Reconnaissance Orbiter has photographed the vast majority of the moon several times over the past 13 years, there are spots it has missed. It turned out that some holes were near the expected crash site.
dr. Robinson recalled thinking about Murphy’s Law and joking, “I know exactly where it will hit.”
Because the crash was predicted a month in advance, the mission team was able to fill 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 of the images were taken at different times and the difference in shadows confused the algorithm.
With all the false positives, “we just sat down and had several people manually go through the millions of pixels,” said Dr. Robinson.
Alexander Sonke, a senior in the Arizona state geological sciences department, contributed to the effort. He estimated that over several weeks he had spent about 50 hours performing this tedious task.
Mr Sonke graduated in May. He is married. He went on a honeymoon. A week and a half ago it was his first day back at work – he’s about to begin his graduate school studies with Dr. Robinson as his advisor – and he resumed the search for the site of the impact.
He found it.
Mr. Sonke said he saw “a group of pixels that looked significantly different in brightness” as the before-and-after images flickered back and forth.
“When I saw it, I was pretty sure this was a new geological feature,” said Mr. sonke. “I definitely jumped out of my seat a little bit, felt like this was it for sure, then tried to contain my excitement a little bit.”
The eastern crater, about 20 meters in diameter, is superimposed on the slightly smaller western crater, which most likely formed a few thousandths of a second earlier than the eastern one, said Dr. Robinson.
It 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 have also carved craters. But none of those impacts created a double crater.
The reason this one did that could point to his mysterious identity. The October 2014 Chinese mission carried the Chang’e-5 T1 spacecraft, a precursor to another mission, Chang’e-5, which landed on the moon and returned rock samples to Earth.
The predecessor T1 spacecraft had no lander, but Dr. Robinson suspects it had a heavy mass at the top of the stage to simulate the presence of a lander. If so, rocket engines at the bottom and the lander simulator at the top could have created the two craters.
“That’s pure speculation on my part,” said Dr. Robinson.
The other parts of the rocket stage would have been thin, light aluminum and probably won’t make many dents on the lunar surface.
The actual impact site was between the places predicted by Mr. Gray and the NASA Jet Propulsion Laboratory, close to NASA. “It was within the margin of error we calculated,” said Mr Gray.
It was also fortunate that the Lunar Reconnaissance Orbiter team filled in the gaps – called gores, in mapmakers’ language – in the images. “As Murphy would have it, that thing affected what was one of the bleeds,” said Dr. Robinson. “If I hadn’t been warned, we wouldn’t have had an example.”
The scientists would eventually have found the crash site. Dirt thrown from a grooved crater is usually brighter and darkens over time. That’s how scientists identified the craters caused by Saturn 5 stages.
But they would still be looking for a small ray of light in the moon’s haystack.