Earth’s ‘Minimoon’ Is About to Leave Us Forever

Late last summer, Earth picked up a new moon. No, you didn’t just miss seeing it in the night sky — this was a so-called “minimoon.” Earth’s gravity occasionally snags passing space rocks, holding them in irregular orbits before they fly off. With the object known as 2020 SO, it was more of a homecoming. Scientists confirmed 2020 SO was actually a discarded rocket booster from the 1960s, but it’s not here to stay. According to astronomers, Earth’s latest artificial satellite is about to become a former satellite as it prepares to zip off into the inky blackness of space.

Calling it a minimoon might seem a bit misleading, but the accepted definition doesn’t require the object to be naturally occurring. 2020 SO made its first close pass of Earth in December, just a day before NASA confirmed it was indeed the long-lost Centaur rocket. After whipping around Earth, 2020 SO took a long elliptical track out past the moon’s orbit, and it’s now on its way back for one final look at home before it’s gone for good.

Scientists knew something was up with 2020 SO as soon as it appeared in telescopes last September. The object’s orbital inclination was almost identical to Earth’s, and it was moving much more slowly than the average near-Earth asteroid. Early on, observers were speculating that 2020 SO was actually a Centaur rocket booster from the 1966 launch of Surveyor 2, a robotic moon lander that sadly crashed into the lunar surface due to a faulty engine. The estimated size of 2020 SO was also a match for the Centaur booster at 21 to 46 feet long (6.4 and 14 meters). The Centaur-D booster was 41.6 feet tall (12.68 meters).

While studying 2020 SO, NASA found that it made several previous approaches to Earth. It came close in 1966 (shortly after it was launched) and again in 1971. This helped the agency nail down the object’s identity.

Astronomers say 2020 SO should pass within 140,000 miles (220,000 kilometers) on February 2nd. This will be much farther away than the last orbit, about half-way between Earth and the moon. After this pass, 2020 SO will pick up enough energy from the gravitational slingshot to escape Earth’s gravity. It will then be bound only to the sun’s gravity, and is, therefore, very unlikely to ever grace us with its presence again. So long, 2020 SO.

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