After months of examining photos of the lunar surface, scientists have finally found the crash site of a forgotten rocket stage that hit the other side of the moon in March.
They still do not know with certainty from which rocket the wayward debris came. And they are confused about why the impact dug up 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 chief investigator for the camera aboard NASA’s Lunar Reconnaissance Orbiter, which the moon has 2009 photographs. “It’s always much nicer than when the prediction of the crater, its depth and diameter, was exactly right.”
The rocket crash plot began in January when Bill Gray, developer of Project Pluto, discovered a series of astronomical software used to calculate the orbits of asteroids and comets that looked like the discarded upper stage of a rocket. I realized it was on a collision course with the other side of the moon.
The accident was certain, around 7:25 a.m. Eastern time on March 4th. But the exact orbit of the object was not known, so there was some uncertainty about the time and place of the impact.
Mr. Gray said the rocket part was the second phase of a SpaceX Falcon 9 that the Deep Space Climate Observatory, or DSCOVR, launched for the National Oceanic and Atmospheric Administration in February 2015.
He was wrong.
A NASA engineer pointed out that the launch orbit of DSCOVR was incompatible with the orbit of the object that Mr. Gray traced. After a little more digging, 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.
Students at the University of Arizona reported that an analysis of the light reflected by the object found that the mixture of wavelengths matched similar Chinese rockets rather than a Falcon 9.
But a Chinese official denied that it was part of a Chinese rocket, saying that the rocket stage of that mission, which launched the Chang’e-5 T1 spacecraft, re-entered and burned the Earth’s atmosphere.
Regardless of which rocket it was part of, the object continued to follow the spiral path determined by gravity. At the predicted time, it crashed against the other side of the moon inside the 350-mile-wide Hertzsprung crater, out of sight of anyone on Earth.
The Lunar Reconnaissance Orbiter was not in a position to see the impact, but the hope was that a freshly carved crater would appear in a photo the spacecraft later took.
Mr. Gray’s software 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 take place tens of miles to the west.
This meant that the researchers had to search a strip about 50 miles long for a crater a few tens of feet wide, and compared the lunar landscape before and after the crash to identify recent disturbances.
Dr. Robinson said he was concerned that “it’s going to take us a year of imaging to fill the box.”
While the Lunar Reconnaissance Orbiter has photographed the vast majority of the moon several times over the past 13 years, there are places that have missed it. It turned out that some of the gaps were close to the expected crash site.
Dr. Robinson remembers thinking of Murphy’s Law and joking, “I know exactly where it’s going to hit.”
Because the accident was predicted a month ahead of time, 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 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 went through the millions of pixels by hand with several people,” said dr. Robinson said.
Alexander Sonke, a senior in Arizona State’s Department of Geological Sciences, contributed to the effort. I estimated that I spent about 50 hours over a few weeks performing the tedious task.
Mr Sonke graduated in May. He got married. He went on his honeymoon. A week and a half ago, his first day was back at work – he was about to start his postgraduate school studies with Dr. Robinson as his adviser – and he summed up the search for the impact site.
I found it.
Mr. Sonke said he saw “a group of pixels that differ significantly in brightness” as the before-and-after images flickered back and forth.
“I was pretty confident when I saw it was a new geological feature,” he said. Sonke said. “I definitely jumped out of my chair a bit, had a feeling it was definitely it, and then tried to subdue my excitement.”
The eastern crater, about 20 meters in diameter, is on top of the slightly smaller western one, which most likely formed a few thousandths of a second ahead of the eastern one, dr. Robinson said.
This is not the first time a spacecraft has hit the moon. Pieces of the Saturn 5 rockets that astronauts took to the moon in the 1970s, for example, carved craters. But none of those impacts created a double crater.
The reason why this one did may indicate 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 brought rock samples back to Earth.
The forerunner T1 spacecraft did not include a lander, but dr. Robinson assumes that it had a heavy mass at the top of the stage to simulate the presence of one. If so, then rocket motors at the bottom and the lander simulator at the top could have created the two craters.
“This is pure speculation on my part,” said Dr. Robinson said.
The other parts of the rocket stage would have been thin, light aluminum, which probably would not have made much of a dive on the lunar surface.
The actual impact site lies between the sites identified by Mr. Gray was predicted and the NASA jet propulsion laboratory, near the NASA one. “It was within the margins of error that we calculated,” he said. Gray said.
It was also fortunate that the Lunar Reconnaissance Orbiter team filled the gaps – called gores, in the language of cardmakers – in the images. “As Murphy wanted it to be, that thing had an impact on what was one of the fears,” said Dr. Robinson said. “If I had not been warned, we would not have had an example.”
The scientists could have finally found the crash site. Dirt thrown from a notched crater is usually clearer and darkens over time. This is how scientists have identified the craters caused by Saturn’s 5 stages.
But they would still be looking for one small bright spot in the hay mountain of the moon.