On September 20, 1966, NASA launched the Surveyor 2 lander to the moon on the back of an Atlas-Centaur rocket. It would be the second soft-landing attempt to study the properties of the lunar soil in advance of the manned Apollo missions. Unfortunately, during a mid-course correction, one of the thrusters failed to fire, causing the spacecraft to tumble and lose contact with mission control.

 This 1964 photograph shows a Centaur upper-stage rocket, similar to the one that helped launch Surveyor 2 in 1966, before it was joined with the Atlas booster.  (NASA)
This 1964 photograph shows a Centaur upper-stage rocket, similar to the one that helped launch Surveyor 2 in 1966, before it was joined with the Atlas booster. (NASA)

Surveyor 2 continued to the moon and ultimately crashed near Copernicus crater, while the upper Centaur stage that helped launch the mission blew past the moon and entered into an orbit around the sun, out of sight and out of mind. End of story, right? Fast-forward to Sept. 17, 2020 when the 71-inch (1.8-meter) Pan-STARRS1 telescope atop Mt. Haleakala on Maui discovered a new, near-Earth asteroid with the temporary name of 2020 SO. Almost immediately, astronomers suspected that the object was artificial.

A familiar face

For one it was moving slowly relative to the planet, around 1,800 mph (2,900 km/hour). Asteroids typical clock in at tens of thousands of miles an hour. Also, 2020 SO's orbit was nearly circular (like Earth's) and revolved around the sun in the same plane. Astronomers next examined sunlight reflected from the object with NASA's infrared telescope at the Mauna Kea Observatory in Hawaii and made an amazing discovery. Instead of rock they detected a signature resembling stainless steel, the metal used to make that era's Centaur boosters.

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Further analysis of its orbit revealed that the object had cozied up to Earth several times in the past including a close approach in 1966. Radar images also showed that 2020 SO had an elongated shape about 33 feet (10 meters) long by 10 feet (3 meters) wide, an excellent match to the upper Centaur stage.

This solar system diagram shows the positions of the inner planets and 2020 SO on Jan. 21. Notice how similar the defunct rocket's orbit is to the Earth's. (JPL HORIZONS)
This solar system diagram shows the positions of the inner planets and 2020 SO on Jan. 21. Notice how similar the defunct rocket's orbit is to the Earth's. (JPL HORIZONS)

Putting it all together, astronomers determined that the once forgotten rocket had been nudged close enough to Earth by the pressure of sunlight to be captured by the planet's gravity and become a temporary mini-moon. You and I can't feel it, but light exerts pressure especially on long, hollow objects like empty rocket stages floating around in frictionless outer space.

For the same reason, 2020 SO's sojourn as a moon will be brief. During a very close pass of just 31,000 miles (50,000 km) on Dec. 1, 2020 it briefly became bright enough to track in larger amateur telescopes. Its next rendezvous occurs on Feb. 2 at 139,500 miles (224,500 km). After that — ta ta.

A Hill Sphere is the region around a planetary body where its own gravity dominates in attracting satellites and moons compared to to the sun or other celestial bodies. (University of Arizona)
A Hill Sphere is the region around a planetary body where its own gravity dominates in attracting satellites and moons compared to to the sun or other celestial bodies. (University of Arizona)

On March 7 the booster will leave the Hill Sphere — a region that extends roughly 930,000 miles (1.5 million km) from Earth, where its gravity dominates — and resume orbiting the sun, free of the planet's grasp.

Come say goodbye

Before it goes, Italian astrophysicist and astronomy popularizer Gianluca Masi invites you to join him in wishing the mini-moon farewell. He'll live-stream 2020 SO's close approach on his Virtual Telescope website staring at 4 p.m. Central Time, Feb. 1. (5 p.m. Eastern, 3 p.m. Mountain and 2 p.m. Pacific).

Depending on the vagaries of light pressure this relic of the space age will likely return again, but who knows when? See it while you can!

"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.