A mysterious object flew past the Earth yesterday and scientists still aren’t quite sure what it is.

The object, dubbed 2020 SO by astronomers, came within “just” 31,605 miles of our planet at 3:50 a.m. ET on December 1, according to NASA’s Center for Near Earth Object Studies (CNEOS.)

This is an “extremely close,” albeit safe, approach, with the object passing at a distance equivalent to around 13 percent of the average distance between our planet and the moon, astronomer Gianluca Masi from the Virtual Telescope Project told Newsweek.

The object, which is estimated to measure between 15-33 feet across, was discovered by the Pan-STARRS survey based in Maui, Hawaii, on September 17, 2020. This find was confirmed two days later by the Minor Planet Center, which is responsible for the designation of minor bodies in the solar system.

Initial observations suggested that the object was an asteroid. But scientists at CNEOS soon began to suspect that 2020 SO was not a normal asteroid. “We are not sure it is an asteroid—that is, a natural body,” Masi wrote on the Virtual Telescope Project website.

CNEOS director Paul Chodas subsequently suggested that 2020 SO may not be an asteroid at all, tentatively identifying it as the Centaur rocket booster from NASA’s failed Surveyor 2 lunar mission, which launched on September 20, 1966.

He came to this conclusion after turning back the clock and running the object’s orbit backwards using a computer model to work out its past trajectory through space.

Chodas found that the object had already passed relatively close to Earth several times over the past decades, including one moment that indicated it could have actually launched from Earth, according to NASA.

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

The object’s low relative velocity and orbital plane also supported the argument that the object was potentially not of natural origin.

2020 SO was captured by Earth’s gravity on November 8, and calculations show it will remain in orbit around our planet as a temporary satellite until March 2021 before it escapes back into a new orbit around the sun.

As 2020 SO approaches the Earth, astronomers will observe the object in order to determine whether or not it is actually a piece of space junk from the 1960s.