As early as September, astronomers using the Panoramic Surveying Telescope and Rapid Response System 1 (Pan-STARRS1) noticed an object in a distant orbit around the Earth. Originally, the object (designed 2020 SO) was considered a near-earth asteroid (NEA). Due to the strange nature and the way solar radiation seemed to keep them off course, NASA scientists theorized that SO could be a spent rocket booster in 2020.
This was the preliminary conclusion of the staff at NASA's Center for Near-Earth Object Studies (CNEOS) at NASA JPL. In particular, they theorized that the object was the spent booster of the Centaur upper stage rocket that fired the Surveyor 2 spacecraft toward the moon in 1966. This theory has since been confirmed thanks to new information from CNEOS and the NASA Infrared Telescope Facility (IRTF).
The first indication that 2020 SO could be an artificial object came from its strange orbit, which is almost circular, as far from the sun as the earth and in the same orbit plane. The next came when follow-up observations over a period of three months (conducted by Pan-STARRS and observatories around the world) showed how solar radiation changed the object's trajectory significantly.
Animation of the orbit of 2020 SO on November 8, 2020. The movement was accelerated millions of times faster than in real time. Photo credit: NASA / JPL-Caltech
Most recently, further analysis of 2020 SE orbit revealed that the object had made several close flybys with Earth in the past few decades. One of these approaches took place in 1966 when it was close enough to Earth to suggest that it may have originated here. From all of this, Paul Chodas (CNEOS director) concluded that 2020 SO was a depleted high school of the ill-fated Surveyor 2 mission.
The purpose of this orbiter was to identify potential landing sites for the manned Apollo missions that would follow three years later. Unfortunately, after disconnecting from its upper stage booster, one of the spacecraft's three engines failed to ignite and the spacecraft lost control. On September 23, 1966, it crashed into the moon southeast of Copernicus crater. The spent Centaur rocket of the upper stage continued to sail past the moon.
From there it disappeared into an unknown orbit around the sun. As Chodas said, it is 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 the Earth and the Moon in late September 1966. It was like a Eureka moment when a quick review of the 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 conducted spectroscopic follow-up observations with the IRTF at the summit of Mauna Kea, Hawaii. Reddy is an Associate Professor at the Lunar and Planetary Laboratory (LPL), which is part of the University of Arizona School of Science. Additional observations with the Large Binocular Telescope (LBT) showed that it was not an asteroid.
NASA infrared telescope facility on Mauna Kea, a volcanic mountain on the island of Hawaii. Credits: University of Hawaii Institute of Astronomy / Michael Connelley
These observations allowed Reddy and his team to analyze the composition of the 2020 SO and compare its spectral data to the material from which the Centaur rocket amplifiers were made in the 1960s (301 stainless steel). Initially, the results did not agree perfectly as the spent booster had spent 54 years in space exposed to the harsh conditions of space weather. As Reddy recently stated in a NASA press release:
"We knew that if we were to compare apples to apples, we would have to try to get spectral data from another Centaur rocket booster that had been in Earth orbit for many years and then see if it was better on the 2020 spectrum SO fits. " Because of the extreme speed at which earth-orbiting Centaur boosters move across the sky, we knew it would be extremely difficult to connect to the IRTF long enough to get a solid and reliable data set. "
Reddy and his team observed another Centaur D rocket booster on the morning of December 1 that came from a 1971 launch of a communications satellite in Geostationary Transfer Orbit (GTO). After Reddy and his team received spectra, they were able to compare them to 2020 SO and found that the two objects matched. This definitely proves that 2020 SO is a piece of rocket amplifiers from the Apollo era.
"That conclusion was the result of tremendous team effort," said Reddy. "Thanks to the great work of Pan-STARRS, Paul Chodas and the team from CNEOS, LBT, IRTF and the observations around the world, we were finally able to solve this puzzle."
The Centaur high school booster on an Atlas V rocket. Photo credit: NASA
The orbital path that 20202 SO has since brought back to Earth for two more approximations. On November 8, 2020, it drifted into the Earth's "hilltop", a region of gravitational dominance that extends about 1.5 million km from Earth. The closest approximation was on December 1 and will remain in the globe of the earth until it escapes into a new orbit around the sun by March 2021.
In addition to the nostalgic appeal of being visited by an Apollo-era object over fifty years later, the 2020 SO detection is important for many reasons. On the one hand, the effectiveness of the instruments involved was demonstrated, with which the composition of the object could be determined. Next there was everyone involved in effective detective work, which 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 man-made objects, which is an essential requirement for defending the earth and monitoring potentially dangerous NEAs. This becomes all the more important as more and more artificial objects are living in Low Earth Orbit (LEO) or in orbit around the sun.
Astronomers will watch this relic from early space age until it breaks free from Earth's gravity and takes on a new solar orbit. The visit seems appropriate given current efforts to send astronauts back to the moon for the first time since the Apollo era. If you are the sentimental type, you can believe that this is the way in the cosmos to wish us luck on this journey!
Further reading: NASA