Astronomers Confirm Earth’s Newest Mini-Moon Is Actually a Long-Lost Rocket

The piece of space debris, called 2020 SO, is the upper stage rocket booster from a failed 1966 mission to the moon

Photograph of a 1966 rocket launch
Surveyor 2 launched on a Centaur/Atlas rocket in September of 1966. The upper stage, called Centaur, was lost in space until it returned to Earth's orbit this November. Courtesy of NASA / 1966

In September 1966, NASA launched its second Surveyor spacecraft to study the surface of the moon. Unfortunately, it crash-landed on the lunar surface, but that’s not the end of Surveyor 2’s story. The upper stage of its Centaur rocket has spent decades floating through around the solar system.

This week, astronomers confirmed that the upper stage of the Centaur rocket has returned to Earth for a brief visit, per a statement by NASA. Researchers suspected that a strange-looking object was a piece of human-made space debris when they first spotted it in September, Nora McGreevy reported for Smithsonian at the time. But some serious detective work was required to confirm its identity.

Scientists around the world have been studying the booster since September, when the then-unidentified object received the name 2020 SO. By observing how light reflects off of 2020 SO and comparing those observations to space debris of a similar age and material, scientists were able to confirm the object's identity, Katherine Kornei reports for the New York Times.

The 1966 Centaur's upper stage re-entered Earth’s orbit in November and astronomers got the best look at the rocket booster on December 1, when it made its closest pass to Earth.

“I managed to get a tracked image of the object, but also a trail [upper left in the photo] and the latter shows a dotted pattern, basically a bright dot, followed by a fainter one and so on,” says astronomer Gianluca Masi of the Virtual Telescope Project 2.0 to Gizmodos George Dvorsky via email. “This suggests the object was rotating, with a period of about 10 seconds.”

The tumbling rocket booster caught astronomers’ attention because its orbit is very similar to Earth’s, but it was moving much slower than most asteroids. Paul Chodas, the manager of NASA’s Center for Near Earth Object Studies, studied 2020 SO’s orbit and ran his simulation in reverse to find out where the object came from. The path traced back to Earth around September 1966, when Surveyor 2 launched, per the New York Times.

Surveyor 2 was carried to space by a Centaur upper stage rocket, supported by an Atlas first stage. When a stage runs out of fuel, it disconnects to reduce weight, and either falls back to Earth or remains in orbit depending on how fast and far from Earth it is when it disconnects. And sometimes, the forces in space can bump an orbiting object just right so that it escapes Earth’s orbit and begins orbiting the Sun. That's why 2020 SO’s orbit around the Sun takes just a few weeks longer than Earth’s.

“There are so many factors in the space environment, like gravitational factors and other things that affect movement, that it can sometimes be quite unpredictable,” says Flinders University space archaeologist Alice Gorman to ScienceAlert in September. “You have to keep tracking these things, or you can just sort of lose sight of them really easily. And if they do something a little bit unpredictable, and you look the wrong way, then you don’t know where it’s gone. It is quite astonishing, the number of things that have gone missing.”

Astronomers measured the visible light reflecting off of 2020 SO, which matched the light reflecting from a different, known Centaur booster orbiting Earth. But to clinch the identity confirmation, the researchers needed infrared measurements. So when 2020 SO passed close to Earth on Tuesday, astronomers measured its infrared spectrum and compared that to the known Centaur booster.

“There’s very little ambiguity in the infrared,” says University of Arizona planetary scientist Vishnu Reddy to the New York Times. It was a perfect match, “the ultimate apples to apples comparison.”

The booster is only staying until March, but it will be back to visit again in 2036.