It wasn’t until just a few years ago that astronomers revealed the Earth’s Moon is a big brother to one or two minimoons or mini-moons – tiny objects that are captured in the planet’s gravity and kept in orbit. In 2016 they discovered 2016 HO3, a 40-meters-(131-feet)-wide rock that may have been in a wide orbit (38 to 100 times the distance of the Moon) for over 100 years. Other minimoons are just temporary satellites – 2020 CD3 found in 2017 is only 12 feet (3.5 meters) wide and on its way out, as was 2006 RH120 which was only in Earth orbit from September 2006 to June 2007. DN160822_03, discovered in 2016, wasn’t so lucky – it ended up crashing to Earth. Astronomers estimate that Earth picks up at least one minimoon per year, so the latest one had to do something to be noticed. It did.
“2020 SO has been classified as an Apollo asteroid in the JPL Small-Body Database - a class of asteroids whose paths cross Earth's orbit. This class of asteroid often has near-Earth encounters. But there are a few clues that 2020 SO is not like the others.
The object is on an orbit that's just a smidge over a year, and on a very low inclination with respect to Earth's orbit; that is, it's not tilted, but on the same orbital path. Its eccentricity - the deviation of the shape of its orbit from a perfect circle - is just a little higher than Earth's. And its velocity is much, much lower than the velocity of an Apollo asteroid.”
Space.com points out why 2020 SO is a strange minimoon, although a low velocity and a near-circular orbit don’t sound like much to get excited about. According to Dr. Paul Chodas, the director of NASA's Center for Near Earth Object Studies, it is.
"I suspect this newly discovered object 2020 SO to be an old rocket booster because it is following an orbit about the Sun that is extremely similar to Earth's, nearly circular, in the same plane, and only slightly farther away the Sun at its farthest point."
Chodas theorized to CNN that 2020 SO may actually be the return of a Centaur rocket used to boost the Surveyor 2 lunar lander into space in September1966 (a course correction failure caused Surveyor 2 to crash on the Moon). The Atlas LV-3C Centaur-D rocket had been discarded and eventually passed by the Moon before entering an orbit around the Sun – never to be seen again. Unless …
“In a month or so we will get an indication of whether or not 2020 SO really is a rocket body, since we should start being able to detect the effect of sunlight pressure has on the motion of this object: if it really is a rocket body, it will be much less dense than an asteroid and the slight pressure due to sunlight will produce enough change in its motion that we should be able to detect it in the tracking data."
In other words, if it starts to wobble, that bauble isn’t a minimoon but a 54-year-old rocket coming back for a visit. If it is, it’s a rarity – only one other rocket stage has ever returned like this … a Saturn V upper stage from the Apollo 12 launch. After all that time, what will this 46-foot-long object look like? Space archaeologist Alice Gorman of Flinders University in Australia shared her thoughts to ScienceAlert.
"It would be interesting to do some reflectance spectroscopy, which would show how rough the surfaces are, how much it's been pitted and decayed from being bombarded by dust and micro meteorites."
So, we're looking for something that looks like an old, battered rocket stage that’s been traveling through space for five decades. Unfortunately, at its closest point while it’s in Earth orbit between October 2020 and May 2021, 2020 SO will be 50,000 km (31,000 miles) away on December 1, 2020, so it will be tough to see . The wobble may be the best sign.
If 2020 SO is manmade, it’s further proof of how hard it is to get rid of space junk. And, if it’s not the Centaur? It’s still one weird minimoon.