For the most part, Earth and its single moon are locked in a two-body dance as our planet circles the sun. But every so often, a bit of space stuff—otherwise known as a mini-moon—will get caught up in Earth’s gravitational orbit and stick around for a while.

The last mini-moon to visit Earth was 2020 CD3, which circled Earth for a few months before it flew off to orbit the sun in March. Now, Deborah Byrd and Eddie Irizarry write for that scientists have identified another piece of space stuff that is expected to join Earth’s orbit, known as 2020 SO.

Astronomers first spotted 2020 SO on September 17 with the Pan-STARRS1 telescope in Hawaii, reports It’s predicted to enter Earth’s orbit in October or November and might stick around until May next year.

However, as Allen Kim reports for CNN, 2020 SO might not be your typical asteroid. Some astronomers suspect it could be space junk: namely, a leftover booster rocket from the 1960s.

“I suspect this newly discovered object 2020 SO to be an old rocket booster because it is following an orbit about the Sun that is extremely similar to Earth’s, nearly circular, in the same plane, and only slightly farther away the Sun at its farthest point,” Paul Chodas tells CNN.

Chodas directs the Center for Near Earth Object Studies at NASA’s Jet Propulsion Laboratory, the organization that calculates the orbits of near-Earth asteroids, including mini-moons. The orbit of 2020 SO is “precisely the kind of orbit that a rocket stage separated from a lunar mission would follow, once it passes by the Moon and escapes into orbit about the Sun,” says Chodas.

Astronomers won’t be able to confirm details about 2020 SO’s composition until it gets closer to Earth. However, “[i]t’s unlikely that an asteroid could have evolved into an orbit like this, but not impossible,” Chodas says.

Currently, 2020 SO is classified as an Apollo asteroid in the JPL Small-Body Database, reports Michelle Starr for Science Alert. But the object’s velocity is markedly lower than other Apollo asteroids, which sets it apart from the pack. NASA estimates place its speed at a relatively slow 1,880 miles per hour, reports

“What I’m seeing is that it’s just moving too slowly, which reflects its initial velocity,” space archaeologist Alice Gorman of Australia’s Flinders University tells ScienceAlert. “That’s essentially a big giveaway.”

Chodas analyzed 2020 SO’s path and attempted to link it back to previous lunar mission launches, he tells CNN. He notes that the object’s orbit corresponds with the launch of Surveyor 2 on September 20, 1966. The craft was designed to land on the moon, but it crashed, and the rocket used to boost the craft flew off around the sun and into space, where astronomers lost track of it.

Time will tell if 2020 SO is just an out-of-place space rock—or a remnant of a former mission coming back to haunt its home planet.

“In a month or so we will get an indication of whether or not 2020 SO really is a rocket body, since we should start being able to detect the effect of sunlight pressure has on the motion of this object,” Chodas tells CNN.

He adds: “[I]f it really is a rocket body, it will be much less dense than an asteroid and the slight pressure due to sunlight will produce enough change in its motion that we should be able to detect it in the tracking data.”

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