As soon as he saw the data, Paul Chodas knew something was strange about the near-Earth object that had been designated 2020 SO.
It should have been just another of the tens of thousands of space rocks that astronomers have spotted breezing through our neighborhood in space. This solar system rubble is mostly harmless, but scientists identify and track all they can in case an object appears to be on a collision course with Earth. As head of NASA's Center for Near-Earth Object Studies at the Jet Propulsion Laboratory in California, Chodas evaluates observations of these objects every day.
And to Chodas, 2020 SO didn't look like an asteroid. Instead, it looked like something much rarer: an abandoned rocket body that had once ferried a spacecraft to the moon.
"Even on the night it was announced, I said, 'I think this is such-and-such rocket stage. That's my guess, all circumstantial evidence points to that,'" Chodas told Space.com in late September.
Now, nearly a month of ongoing observations have confirmed that 2020 SO is moving more like a spent rocket stage than a space rock, buffeted by simply the sunlight hitting it.
"Our latest orbit calculations for this object clearly show that it is being affected by non-gravitational forces, presumably solar radiation pressure," he wrote in an email update, calling those calculations "very strong evidence" that the object is not a space rock.
"An asteroid is not easily pushed around," Chodas told Space.com during his original interview. "But an empty can, like a rocket stage, would be pushed around."
But he didn't need the ongoing observations to suspect that the erstwhile space rock was actually human debris, the spent Centaur upper stage of the rocket that, in 1966, sent a NASA mission called Surveyor 2 to the moon.
Chodas is one of the scientists who have had their eyes peeled for just such an object for more than a decade now. "We've kind of mused about whether [or] when this would this happen," he said. "I've over the years looked at asteroid orbits to see if any of them was in an orbit around the sun that was likely to have been associated with a launch."
And 2020 SO is the best match to date. Two characteristics in particular stand out about the object's journey: its slow speed and how closely its trajectory around the sun aligns with Earth's own orbit. Asteroids don't tend to behave that way — they travel much more quickly and without regard to Earth's own path around the sun.
Instead, Chodas said, 2020 SO's orbit shouts that it's a rocket body from a mission to the moon. "It was clearly not from a launch to Mars or Venus or any of the others because they would be in orbits that would carry them out toward those planets," Chodas said. "It has all of the hallmarks of a lunar mission," he said of the object's orbit.
In particular, 2020 SO looks like an object that was trying to land a spacecraft on the moon — gently, and therefore not particularly fast. "The spacecraft had to slow down, you want to approach the moon fairly slowly," Chodas said. "So the rocket body missed the moon and went into orbit around the sun, just barely." (Meanwhile, the Surveyor 2 spacecraft itself botched the soft landing and crashed into the lunar surface.)
That's what caused the slow, near-Earth orbit that so stuck out to Chodas. "That's why I was suspicious in the first place that this could be a rocket body, and from a lunar mission," he said.
Chodas was thenable to rewind the orbit, so to speak, to determine when 2020 SO might have left the Earth-moon system. The answer? Late 1966. But the '60s, of course, were the height of the moon-bound space race, and between the U.S. and the Soviet Union, 10 missions launched to the moon that year.
But between the later launch date, the details of the trajectory needed to land softly on the moon rather than simply orbit it, and the relative size of the potential rocket bodies at play, Chodas was soon confident that 2020 SO is actually the Centaur upper stage used to launch NASA's Surveyor 2 mission on Sept. 20, 1966.
That identification isn't confirmed, he noted in his update email, although it's "looking increasingly likely," he wrote. He and his colleagues are still analyzing the forces that would have acted on the object over the past half century.
But the initial data is compelling circumstantial evidence, he said.
"This orbit is known so accurately that I can be very sure of the energy with which the rocket left the moon back in 1966, and the direction and geometry," Chodas said during the original interview. "Everything fits the Surveyor 2 mission — the velocity, the nearness to the moon, the date — everything matches to that launch, and it doesn't match to the other launches."
Email Meghan Bartels at email@example.com or follow her on Twitter @meghanbartels. Follow us on Twitter @Spacedotcom and on Facebook.
Get the Space.com Newsletter
Breaking space news, the latest updates on rocket launches, skywatching events and more!
Meghan is a senior writer at Space.com and has more than five years' experience as a science journalist based in New York City. She joined Space.com in July 2018, with previous writing published in outlets including Newsweek and Audubon. Meghan earned an MA in science journalism from New York University and a BA in classics from Georgetown University, and in her free time she enjoys reading and visiting museums. Follow her on Twitter at @meghanbartels.