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Last September, astronomers discovered 2020 SO, a small near-Earth object approaching our planet. In November, it was temporarily captured by Earth’s gravity and spent a brief stint in close orbit, becoming what is known as a “mini-moon”. But now his time with us is over and on February 2 he will break free and return to his orbit around the Sun.
But first, the 2020 SW mini-moon will make a final close approach to us about 220,000 kilometers (140,000 miles) from Earth, a little more halfway between Earth and the Moon.
Temporary satellites are quite common. Small asteroids are captured for a brief period before going about their business away from Earth. But since the first detections of 2020 SO, researchers suspected that the new mini-moon was in fact not a natural object, but an artificial one.
It had a low relative speed relative to Earth, and sunlight significantly affected its orbit, a clear way to distinguish small natural and man-made bodies near Earth.
Light can be interpreted as a wave and as a particle. When we look at these particles, photons, we look at massless particles that carry momentum, which means they can make things happen. This is how solar sails work. In the case of 2020 SO, having a large area and relatively light mass, caused the body to show acceleration that could not be explained by gravity.
“The pressure of solar radiation is a non-gravitational force caused by light photons emitted by the Sun hitting a natural or man-made object,” Davide Farnocchia, navigation engineer at JPL, who analyzed the trajectory of 2020 SO for the Center for Near -Earth Object Studies (CNEOS), explained in November. “The resulting acceleration on the object depends on the so-called surface-to-mass ratio, which is higher for small and light objects of low density.”
This was a significant piece of evidence suggesting that 2020 SO was human-made, but another came from a detailed projection of its orbital parameters into the distant past. The object has come close to our planet several times, but in 1966 it was so close that it could have come from Earth. In fact, it corresponded to the launch of the NASA lunar lander.
“One of the possible paths for 2020 therefore brought the object very close to the Earth and the Moon at the end of September 1966”, declared the director of CNEOS, Paul Chodas. “It was like a Eureka moment when a quick check of the lunar mission launch dates showed a match with the Surveyor 2 mission.”
Observations in December, after its first close pass, revealed that it was made of stainless steel, confirming that it was indeed an older part of the upper stage of the Surveyor 2 Centaur rocket, which launched on September 20. 1966.
After tomorrow’s tight passage, the piece of rocket will return to deep space, but it will continue to visit us for decades to come, even if it remains millions of miles away from us.
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