Earth’s ‘Minimoon’ is about to leave us forever

At the end of last summer, Earth picked up a new moon. No, you didn’t just miss seeing it in the night sky – it was a so called “minimoon”. Earth’s gravity sometimes hangs space rocks, keeping them in irregular orbits before they fly away. With the object known as the 2020 SO, it was more of a homecoming. Scientists have confirmed that 2020 SO was in fact an abandoned rocket thruster from the 1960s, but it’s not here to stay. According to astronomers, Earth’s last man-made satellite is set to become an ancient satellite as it prepares to launch into the inky darkness of space.

Calling it a minimoon might sound a bit misleading, but the accepted definition does not require the object to be natural. 2020 SO made its first close-to-Earth pass in December, just a day before NASA confirmed it was the long-lost Centaur rocket. After delving around Earth, 2020 SO took a long elliptical trail past the orbit of the moon, and he’s now on his way home for one last look at home before he disappears for good.

Scientists knew something was going on with 2020 SO from the moment it appeared in telescopes last September. The object’s orbital tilt was almost identical to that of Earth, and it was moving much slower than the average asteroid close to Earth. At first, observers speculated that 2020 SO was in fact a Centaur rocket thruster since the 1966 launch of Surveyor 2, a robotic lunar lander that unfortunately crashed into the lunar surface due to a faulty motor. The estimated size of the SO 2020 also matched the Centaur booster measuring 21 to 46 feet long (6.4 and 14 meters). The Centaur-D booster was 12.68 meters high.

The Centaur rocket when Surveyor 1 was launched in 1966. The Surveyor 2 used the same rocket model, which eventually became 2020 SO.

During its studies on 2020 SO, NASA discovered that it had already carried out several approaches to Earth. It closed in 1966 (shortly after its launch) and again in 1971. This helped the agency determine the identity of the object.

Astronomers say 2020 SO is expected to drop to less than 220,000 kilometers on February 2. It will be much further than the last orbit, roughly halfway between Earth and the Moon. After this passage, 2020 SO will recover enough energy from the gravitational sling to escape Earth’s gravity. It will then only be related to the gravity of the sun and it is therefore very unlikely that it will honor us with its presence again. So long, 2020 SO.

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