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 grabs against space rocks, keeping them in irregular orbits before they fly away. With the object known as 2020 SO, it was more of a homecoming. Scientists confirmed that 2020 SO was actually a scrapped rocket booster from the 1960s, but it’s not here to stay. According to astronomers, Earth’s newest artificial satellite is about to become a former satellite as it prepares to fly away into the pitch-black darkness of space.
Calling it a minimoon may seem a bit misleading, but the accepted definition does not require the object to occur naturally. 2020 SO made its first brief passage of Earth in December, just a day before NASA confirmed it was indeed the long-lost Centaur missile. After orbiting Earth, 2020 SO took a long elliptical trail beyond the orbit of the moon, and it’s now on its way back for one last look at home before it’s gone forever.
Scientists knew something was going on with 2020 SO as soon as it appeared in telescopes last September. The object’s orbital tilt was nearly identical to that of Earth, and it moved much more slowly than the average asteroid near Earth. Early on, observers speculated that 2020 SO was actually a Centaur rocket booster from the 1966 launch of Surveyor 2, a robotic lunar lander that sadly crashed into the lunar surface due to a malfunctioning engine. The estimated size of 2020 SO also matched the Centaur booster at 21 to 46 feet long (6.4 and 14 meters). The Centaur-D booster was 41.6 feet long (12.68 meters).
The Centaur missile during the launch of the Surveyor 1 in 1966. The Surveyor 2 used the same model missile, which eventually became 2020 SO.
While studying 2020 SO, NASA found that it had made several previous approximations of Earth. It was closed in 1966 (shortly after it was launched) and again in 1971. This helped the agency establish the object’s identity.
Astronomers say SO 2020 should pass within 220,000 kilometers on Feb. 2. This will be much farther than the last orbit, about halfway between the Earth and the Moon. After this pass, 2020 SO will absorb enough energy from the gravity catapult to escape Earth’s gravity. It will then only be bound by the gravity of the sun and is therefore very unlikely to ever grace us with its presence again. See you, 2020 SO.
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