That’s no Asteroid, it’s a Rocket Booster

Back in September, astronomers using the Panoramic Survey Telescope and Rapid Response System 1 (Pan-STARRS1) noticed an object in a distant orbit around Earth. Initially, the object (designed 2020 SO) was thought that be a near-Earth Asteroid (NEA). But based on the curious nature of it’s and the way solar radiation appeared to be pushing it off course, NASA scientists theorized that 2020 SO might be a spent rocket booster.

This was the tentative conclusion reached by staffers at the NASA Center for Near-Earth Object Studies (CNEOS) at NASA JPL. Specifically, they theorized that the object was the spent upper stage booster of the Centaur rocket that launched the Surveyor 2 spacecraft towards the Moon in 1966. This theory has since been confirmed thanks to new information provided by CNEOS and the NASA Infrared Telescope Facility (IRTF).

The first hint that 2020 SO might be an artificial object arose from its curious orbit, which is nearly circular, as distant from the Sun as Earth, and in the same orbital plane. The next came when follow-up observations over the course of three months (made by Pan-STARRS and observatories around the world) revealed how solar radiation was altering the object’s trajectory significantly.

Last, further analysis of 2020 SO’s orbit revealed that the object had made several close flybys with Earth over the past few decades. One of these approaches occurred in 1966, where it was close enough to Earth to suggest it may have originated here. From all this, Paul Chodas (CNEOS director) concluded 2020 SO was a spent upper stage of the ill-fated Surveyor 2 mission.

The purpose of this orbiter was to scout potential landing sites for the crewed Apollo missions that would follow three years later. Unfortunately, after separating from its upper stage booster, one of the spacecraft’s three thrusters failed to ignite and the spacecraft lost control. On Sept. 23rd, 1966, it crashed into the moon just southeast of the Copernicus crater. The spent Centaur upper-stage rocket continued to sail past the Moon.

From there, it disappeared into an unknown orbit about the Sun. As Chodas said, it’s likely that it never escaped the gravity of the Earth-Moon system:

“One of the possible paths for 2020 SO brought the object very close to Earth and the Moon in late September 1966. It was like a eureka moment when a quick check of launch dates for lunar missions showed a match with the Surveyor 2 mission.”

Building on this knowledge, a team led by planetary scientist Vishnu Reddy performed follow-up spectroscopic observations using the IRTF – located at the summit of Mauna Kea, Hawaii. Reddy is an associate professor with the Lunar and Planetary Laboratory (LPL), which is part of the University of Arizona’s School of Science. Additional observations conducted with the Large Binocular Telescope (LBT) indicated that it was not an asteroid.

These observations allowed Reddy and his team to analyze 2020 SO’s composition and compare its spectral data to the material the Centaur rocket boosters were made of during the 1960s (301 stainless steel). Initially, the results were not a perfect match since the spent booster had spent 54 years in space being exposed to the harsh conditions of space weather. As Reddy explained in a recent NASA news release:

“We knew that if we wanted to compare apples to apples, we’d need to try to get spectral data from another Centaur rocket booster that had been in Earth orbit for many years to then see if it better matched 2020 SO’s spectrum. Because of the extreme speed at which Earth-orbiting Centaur boosters travel across the sky, we knew it would be extremely difficult to lock on with the IRTF long enough to get a solid and reliable data set.”

Persisting, Reddy and his team observed another Centaur D rocket booster on the morning of Dec. 1st, this one from a 1971 launch of a communication satellite in Geostationary Transfer Orbit (GTO). After obtaining spectra from it, Reddy and his team were able to compare it against 2020 SO and found that the two objects were a match. This definitively proves that 2020 SO is a piece of Apollo-era rocket booster.

“This conclusion was the result of a tremendous team effort,” said Reddy. “We were finally able to solve this mystery because of the great work of Pan-STARRS, Paul Chodas and the team at CNEOS, LBT, IRTF, and the observations around the world.”

The orbital path that 20202 SO has since brought it back to Earth for two more close approaches. On Nov. 8th, 2020, it drifted into Earth’s “Hill Sphere”, a region of gravitational dominance that extends about 1.5 million km (930,000 miles) from Earth. Its closest approach happened on Dec. 1st, and it will remain in Earth’s Hill Sphere until it escapes into a new orbit around the Sun by March 2021.

Besides the nostalgic appeal of being visited by an Apollo Era object over fifty years later, the detection of 2020 SO is significant for many reasons. For one, it demonstrated the effectiveness of the instruments involved, which were able to determine the object’s composition. Next, there was the effective detective work by all those involved that helped trace the origin of the object (based on its orbit) back to Earth.

Third, it highlights the importance of being able to differentiate between natural and artificial objects, which is an indispensable requirement for Earth defense and monitoring potentially-hazardous NEAs. This is becoming all-the-more important because of the growing population of artificial objects that find themselves in Low Earth Orbit (LEO) or in orbit around the Sun.

Astronomers will continue to observe this relic from the early Space Age until it breaks free of Earth’s gravity and assumes a new solar orbit. The visit seems fitting considering the current effort to send astronauts back to the Moon for the first time since the Apollo Era. If you’re the sentimental type, you could choose to believe it’s the cosmos’ way of wishing us luck on that journey!

Further Reading: NASA