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A rocket stage set to hit the moon March 4 might not be from SpaceX after all.
The astronomer credited with discovering the forthcoming impact, Bill Gray, announced Saturday (Feb. 12) that he made an error in identifying the rocket as an old SpaceX Falcon 9 rocket stage that helped launch the Deep Space Climate Observatory satellite in 2015.
Rather, Gray suggests the stage could part of a Long March 3C rocket that launched China's moonbound Chang'e 5-T1 mission in October 2014. This spacecraft was a predecessor to Chang'e 5, the 2020 mission that did a robotic lunar sample return.)
Gray manages the Project Pluto software used to track near-Earth objects and placed a correction notice on his website Saturday (Feb. 12) after receiving a note from an engineer at NASA's Jet Propulsion Laboratory, Jon Giorgini.
"He [Giorgini] wrote to Gray on Saturday morning explaining that the DSCOVR spacecraft's trajectory did not go particularly close to the moon, and that it would therefore be a little strange if the second stage strayed close enough to strike it," wrote Eric Berger on Ars Technica, who first reported the errant booster three weeks ago, on Saturday.
The rocket stage, regardless of its origin story, is still expected to slam into the far side of the moon on March 4 at 7:25 a.m. EDT (1225 GMT), and it won't be visible from Earth. Nevertheless, Gray explained on his website why he believed he made an error in identification.
"Prompted by Jon's e-mail, I dug into my e-mail archives to remind myself why I had originally identified the object as the DSCOVR stage in the first place, seven years ago. I did that digging in full confidence it would prove that the object was, in fact, the DSCOVR second stage," Gray wrote in his update.
He was using data from the Catalina Sky Survey that usually tracks near-Earth objects to assess threats to Earth, Gray wrote. Catalina found an object about a month after DSCOVR's launch, designated WE0913A and at first believed to be a natural object.
"Shortly thereafter, an astronomer in Brazil noted on a newsgroup that the object was orbiting the earth, not the sun, suggesting it might be a human-made object," Gray said. After some conversation with the astronomer, Gray and other researchers found that WE0913A went past the moon two days after DSCOVR's launch.
"I and others came to accept the identification with the second stage [of Falcon 9] as correct. The object had about the brightness we would expect, and had showed up at the expected time and moving in a reasonable orbit," Gray continued, but noted the evidence was "circumstantial" rather than fully conclusive.
"In hindsight," Gray continued, "I should have noticed some odd things about WE0913A's orbit. Assuming no maneuvers, it would have been in a somewhat odd orbit around the earth before the lunar flyby. At its highest point, it would be near the moon's orbit; at its lowest (perigee), about a third of that distance. I'd have expected the perigee to be near the earth's surface. The perigee seemed quite high."
At first Gray thought these variations might be due to leaking of leftover fuel, which is highly common in old rocket stages. That said, such a change in DISCOVR's trajectory would have required an unusual amount of fuel, though still possible.
"I didn't have a trajectory for DSCOVR at the time, and the lunar flyby seemed quite plausible [as] spacecraft often use a lunar flyby to adjust their orbits," Gray said. But after receiving the e-mail, he searched the records for an object launching not too long before March 2015, in a "high orbit going past the moon" that few spacecraft achieve.
That led him to Chang'e 5 T1; the evidence is still not fully conclusive and based on running its projected orbit through time, but added confidence comes from Jonathan McDowell, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics who commonly tracks space objects and space junk.
"McDowell has sent orbital elements for an amateur radio cubesat that got a 'ride share' with the booster, and it's a very close match," Gray wrote, adding. "In a sense, this remains 'circumstantial' evidence. But I would regard it as fairly convincing evidence."