Last September, astronomers discovered 2020 SO, a small object near Earth approaching our planet. In November, it was temporarily caught by Earth’s gravity and spent a short period in dense orbit, becoming a so-called “mini-moon”. But now its time with us is over, and on February 2 it will break free and return to its orbit around the sun.
But first, mini-moon 2020 SO will come close to us one last time at about 220,000 kilometers (140,000 miles) from Earth, a little more than halfway between Earth and the Moon.
Temporary satellites are quite common. Small asteroids are recorded for a short time before taking off, 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 artificial.
It had a low relative speed compared to Earth, and sunlight significantly affected its orbit, a clear way of distinguishing between natural and artificial small bodies in close proximity to Earth.
Light can be interpreted as a wave and as a particle. When we consider these particles, the photons, we are looking at massless particles that carry momentum, which means they can propel things. This is how solar sails work. In the case of 2020, SO, with a large surface area and relatively light mass, led to the body exhibiting an acceleration that could not be explained by gravity.
“Solar radiation pressure is a non-gravitational force caused by photons of light emitted from the sun hitting a natural or artificial object,” Davide Farnocchia, a navigation engineer at JPL who analyzed SO’s trajectory in 2020 for the Center for Near- Earth Object Studies (CNEOS), explained in November. “The resulting acceleration of the object depends on the so-called surface-to-mass ratio, which is greater for small and light objects with a low density.”
This was an important piece of evidence suggesting that 2020 SO was human-made, but another came from a detailed projection of its orbit parameters in the distant past. The object has come close to our planet several times, but in 1966 it was so close that it could actually have come from Earth. In fact, it coincided with the launch of NASA’s lunar lander.
“One of the possible paths for 2020 SO brought the object very close to the Earth and the Moon in late September 1966,” said CNEOS Director Paul Chodas. “It was like a eureka moment when a quick check of the launch dates for lunar missions showed a match with the Surveyor 2 mission.”
December observations, after the first close pass, revealed that it was made of stainless steel, confirming that it was indeed an old piece of the top stage of the Surveyor 2 Centaur rocket, launched on September 20, 1966.
After tomorrow’s narrow passage, the piece of rocket will go back into deep space, but it will continue to visit us for decades to come, despite staying millions of miles away from us.