This Apollo-Era Rocket Stage, Lost For Half a Century, Turned Up in a Telescope Search

Found in a hunt for asteroids, an old pal checks in on its way around the sun.

Centaur upper stage
A Centaur upper stage like this one, photographed in 1964, was recently identified by a telescope that normally looks for asteroids. (NASA)

Astronomers have confirmed that a small object temporarily captured by Earth’s orbit is the Centaur upper-stage rocket booster that helped lift NASA’s ill-fated Surveyor 2 spacecraft toward the moon in 1966.

The object, designated 2020 SO, was initially detected by the Panoramic Survey Telescope And Rapid Response System, which monitors near-Earth objects such as asteroids that might pose a threat to Earth. Upon closer examination, scientists at the Center for Near-Earth Object Studies (CNEOS) realized that this was no ordinary asteroid. Typically, the orbit of an asteroid is more elongated and tilted relative to Earth’s orbit. However, before 2020 SO was captured by this planet’s gravity, it was orbiting around the sun in a near circle and in an orbital plane that almost matched Earth’s. Adding to the mystery, the trajectory of 2020 SO was changing slightly in response to getting pushed by the solar wind, suggesting it was likely hollow.

Suspecting that they had discovered an old rocket booster, CNEOS director Paul Chodas calculated the object’s orbit backward in time and found that 2020 SO’s approach in late 1966 would have been close enough that it might have originated from Earth—coinciding with the launch of the Surveyor 2 spacecraft aboard an Atlas-Centaur rocket. A thruster malfunction had caused the spacecraft to crash into the moon on September 23, while the spent Centaur upper-stage rocket sailed into space.

Final confirmation of the identity of 2020 SO came from a team led by Vishnu Reddy, an associate professor and planetary scientist at the Lunar and Planetary Laboratory at the University of Arizona. “My job description is pretty simple—if something is going to hit the Earth, I tell what it is made of…before it hits us of course!”says Reddy, who performed follow-up observations using NASA’s Infrared Telescope Facility. He and his team compared the spectrum data from 2020 SO with that of 301 stainless steel, the material used to construct Centaur rocket boosters in the 1960s.

While the match was good, it wasn’t perfect, so they investigated further. “A colleague in the Air Force alerted us to look at other similar rocket bodies in Earth orbit,” says Reddy. “My grad student was able to initially get visible wavelength spectra of two of the Centaur rocket bodies from the 1970s using our small 24-inch telescope at the university.” The spectra matched with 2020 SO, confirming its identity as a fellow Centaur booster.

2020 SO made its closest approach to Earth on December 1, 2020, and will remain within Earth’s sphere of gravitational dominance until it escapes back into a new orbit around the sun in March 2021.

But even after we say farewell to the Centaur rocket booster, this might only be the beginning of encounters with historic space artifacts. Says Reddy: “The number of objects we put in space each year is only going up, so we are bound to run into objects like 2020 SO in the future as humanity spreads its wings across the inner solar system.” Who says you can’t go home again?