What’s 2020 SO? ‘Mini-moon’ to bid farewell to Earth on February 1 as it moves into new orbit around the Sun

Temporarily captured by our planet since November 2020, this artificial mini-moon or space debris from the Surveyor 2 lunar mission will soon leave our neighborhood


                            What’s 2020 SO? ‘Mini-moon’ to bid farewell to Earth on February 1 as it moves into new orbit around the Sun
The Virtual Telescope Project in Rome will host an online farewell to 2020 SO on February 1 (Getty Images)

An object, discovered in September last year by astronomers searching for near-Earth asteroids, sparked interest among scientists due to its size and unusual orbit. Dubbed as the Earth’s “mini-moon,” researchers confirmed that near-Earth object (NEO) 2020 SO is, in fact, a 1960’s-era Centaur rocket booster of the Surveyor 2 space mission. Temporarily captured by our planet since November 2020, this artificial mini-moon will soon leave our neighborhood, escaping into a new orbit around the Sun in March 2021.

What is a mini-moon?

A smaller object gets temporarily captured in our planet’s orbit now and then. They remain for a short period before being flung out back into space. Such objects are referred to as mini-moons. 

2020 SO will make a close approach to Earth on February 1-2 before drifting off into space. 2020 SO made its closest approach to Earth on December 1, 2020, and will remain within Earth’s sphere of gravitational dominance - a region in space called the “Hill sphere” that extends roughly 930,000 miles (1.5 million kilometers) from our planet - until it escapes back into a new orbit.

When Earth captured the rocket booster (NASA/JPL-Caltech)

“2020 SO is about to make one more close approach to Earth on February 2, 2021. It’ll pass farther away this time, but still within 0.58 lunar-distances (140,000 miles, or 220,000 km). Afterward, in March 2021, Earth’s gravity will relinquish its hold on the object. It’ll no longer be a mini-moon for Earth. Instead, it’ll be orbiting the Sun,” explains Earthsky.org.

The Virtual Telescope Project in Rome will host an online farewell to the object on the night of February 1. The live feed is scheduled for the night of February 1, 2021, starting at 22.00 UTC, which is February 1 at 4 pm Central, 5 pm Eastern, 2 pm Pacific in North America.

“After its extremely close fly-by last December 2020, SO is safely coming very close again, this time to say farewell. We will say it goodbye, live: join us from the comfort of your home!” writes Italian astronomer Gianluca Masi. 

This photograph shows a model of the Surveyor lander (NASA/JPL-Caltech)

How was the object found?

2020 SO was discovered by the Pan-STARRS survey on September 17, 2020. Using data collected at NASA’s Infrared Telescope Facility (IRTF) and orbit analysis from the Center for Near-Earth Object Studies (CNEOS) at NASA's Jet Propulsion Laboratory, scientists were able to determine what the object is. 

Analysis of 2020 SO’s orbit revealed the object had come close to Earth a few times over the decades, with one approach in 1966 bringing it close enough to suggest it may have originated from Earth. Comparing this data with the history of previous NASA missions, CNEOS director Paul Chodas concluded 2020 SO could be the Centaur upper stage rocket booster from NASA’s ill-fated 1966 Surveyor 2 mission to the Moon. 

NASA’s Surveyor 2 was supposed to make a soft landing on the Moon, but a thruster failed to ignite, putting the spacecraft into a spin. The lander crashed on the Moon.

NASA's Infrared Telescope Facility was used to study the object (University of Hawaii Institute for Astronomy / Michael Connelley)

Equipped with this knowledge, a team led by Vishnu Reddy, an associate professor and planetary scientist at the Lunar and Planetary Laboratory at the University of Arizona, performed follow-up spectroscopy observations of 2020 SO using the IRTF. Due to the extreme faintness of this object following CNEOS prediction, it was a challenging object to characterize.

On December 1, however, the team observed another Centaur D rocket booster from the 1971 launch of a communication satellite that was in Geostationary Transfer Orbit, long enough to get a good spectrum. With this new data, investigators were able to compare it against 2020 SO and found the spectra to be consistent with each other, thus definitively concluding 2020 SO to also be a Centaur rocket booster.

According to scientists, as NASA-funded telescopes survey the skies for asteroids that could pose an impact threat to Earth, the ability to distinguish between natural and artificial objects is valuable as countries continue to explore and more artificial objects find themselves in orbit about the Sun. Astronomers will continue to observe this particular relic from the early Space Age until it is gone.