So 'asteroid' 2020 SO was actually 1960s space junk. It may be the first of many to come.

A diagram showing the path of 2020 SO, which is likely a Centaur upper stage from a lunar mission launched during the 1960s.
A diagram showing the path of 2020 SO, which is likely a Centaur upper stage from a lunar mission launched during the 1960s. (Image credit: NASA/JPL-Caltech)

The detective story of 2020 SO, an erstwhile asteroid now formally identified as a 54-year-old piece of space junk, sounds like a wild yarn today but may become the first installment in a long series of such puzzles.

The object dubbed 2020 SO was spotted in September by an asteroid survey, but there was always something a little fishy about the space rock. One NASA expert theorized simply from its orbit that it was likely an upper-stage rocket body from the 1966 launch of a lunar mission called Surveyor 2. By the end of November, additional observations of the object had cemented its status as decades-old space junk wandering back for a surprise visit of its planet of origin. The hunk of metal made its closest approach to Earth on Dec. 1 and is expected to hang around for about four months before being kicked away to orbit the sun again.

The story of the object's loss and rediscovery forms a stark contrast with large debris in Earth orbit, which experts monitor for their potential to collide with active satellites. "We're so obsessively tracking everything we can in Earth orbit because we've created this problem with it," Alice Gorman, an archaeologist at Flinders University in Australia focused on spaceflight heritage, told

Related: A vintage NASA moon rocket body is officially back in Earth orbit … for now

But not so for objects orbiting the sun. Here, records may indicate what's been lost out in the expanse, but no one would ever waste time trying to monitor them. 

"It's kind of like they're orphaned, they're all by themselves in the dark and nobody is looking out for them," Gorman said. "They're in the darkness and then suddenly they appear in view and we're interested in them again for a short time."

Vishnu Reddy, who studies natural and artificial objects in Earth's neighborhood at the University of Arizona, was interested in the object because he and his colleagues wanted to find a way to observationally confirm what precisely it was, rather than relying on 2020 SO's strange orbit. But the object was still quite far away, and therefore difficult to see in detail.

"I looked around to find out what's the biggest piece of telescope we can throw at it, and it turned out to be the Large Binocular Telescope," Reddy said, referring to an observatory in Arizona with twin telescopes looking into the skies like a pair of eyes.

Despite its size, that instrument could offer only a basic, four-color spectrum for the object, which Reddy and his colleagues could compare with the two most common flavors of asteroids, carbon-based and silicate-based. No match — but that's not a conclusive sign that the object wasn't just a particularly weird space rock.

And so the hunt continued.

Working from the hypothesis that 2020 SO was indeed that specific upper stage from the 1966 launch, the researchers tracked down photos of the rocket before flight, then the manufacturer who supplied the white paint they saw in those images. Paint samples, the researchers applied the same four-color view — but the mystery object didn't match that, either.

So the team reached out to a NASA historian, who said that the rocket body would have been covered in foam that fell away after launch, revealing plain stainless steel below. Same deal: find the right stainless steel, get a piece, take the four-color spectrum, check for a match. This time, tada.

Meanwhile, 2020 SO had been coming ever closer, and by mid-November, Reddy realized the team could get a proper, much more detailed spectrum by borrowing NASA's Infrared Telescope Facility (IRTF) on Hawaii. A first try was messy, but a second observing run at the end of the month offered cleaner data. Again, the spectrum matched the stainless steel samples, but with something else mixed in.

Back to the historian the team went for one last clue; he noted that the upper stage would have had Mylar covering its electronics bay. And the spectral overlay matched the organic signature of this plastic, the researchers realized — solving the mystery.

But throughout this process, the researchers had also been looking for an even truer comparison: tracking down one of the countless Centaur upper stages that litter Earth orbit to observe in the same way as its supposed long-lost cousin.

"We don't have that much stuff out there beyond the Earth-moon system, and it doesn't come back that often so we don't get the term to really have a squiz at it," Gorman said. "We have all these other Centaurs that are in orbit as well, so we have direct comparison."

But the Centaur Reddy and his colleagues chose to compare to 2020 SO posed its own challenges: Things orbiting Earth move very, very quickly, and IRTF has a tiny field of view, so researchers were unlikely to be able to time observations correctly and obtain a spectrum. A plan to use backyard telescopes to home in on the object failed as buildings and a recalcitrant chimney blocked the scientists's views.

They tried anyway. 

"We went to the IRTF praying, you know, give it a shot — I had like a 1% chance this would work," Reddy said. "Sure enough it came in and the telescope operator pulled a magic miracle and he managed to grab it" — all while, in true 2020 fashion, on a video call spanning time zones to connect the collaborators.

And there it was: a perfect match. "We got a couple sets of data," Reddy said. "Boom: Same plastic organic bands, same spectral shape, it's like a slam dunk."

Comparisons aside, just by popping up on scientists' screens, 2020 SO has offered a host of new information. Even spotting it as early as scientists did, way back in September and earlier in archived images that scientists checked after the discovery, is crucial, giving scientists a sense of how strong their detection skills are.

"Frankly the fact that we can see this at a large distance is encouraging," Paul Chodas, who leads NASA's Center for Near Earth Object Studies and who first hypothesized that 2020 SO was the Surveyor 2 Centaur stage, told earlier this fall. 

Now that the object is decisively identified, scientists can reference known statistics about its size and other characteristics and evaluate their observations to better understand real near-Earth asteroids.

In particular, the case of 2020 SO is the first time scientists have positively re-identified a lost rocket stage. The next most confident case was in September 2002, when scientists spotted what may have been the third stage of the Saturn V rocket used during the Apollo 12 mission. But two decades ago, that identification was tentative — and NASA had spotted only 2,000 near-Earth objects.

Today, that number is more than 10 times larger and is only going to keep growing. Lost space hardware wandering the sun like 2020 SO will come too, whether scientists try or not.

"We'll get rocket bodies as a side benefit of the asteroid search missions," Chodas said. "I certainly expect that we'll see more examples of old hardware found in the midst of the hundreds and hundreds and in fact thousands of asteroids that we will find from the new generation of asteroid search capabilities."

And as scientists spot more and more lost objects, these rocket parts may begin to tell more of the stories of their absence.

"I think this makes it interesting when an object like this Centaur suddenly pops back up, having having been away for a while, like a long-lost cousin, just pops out of nowhere," Gorman said. "Suddenly, there's an opportunity to ask it, 'Well, what did you see while you were on that journey, in your particular little orbit?'"

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Meghan Bartels
Senior Writer

Meghan is a senior writer at and has more than five years' experience as a science journalist based in New York City. She joined 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.