Goodbye, mini moon. We hardly knew ya.
Earth had a tiny new moon for about two years, and we didn't even notice until it was on its way out.
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Passing visitors like this get redirected into Earth’s gravity well from time to time. After hanging around for a few months, they are often drawn off as Saturn or Jupiter approach with much deeper gravity wells. Visitors that approach Earth too closely and/or slowly burn up as they enter the atmosphere.
It is probably an errant space rock. Theodore Pruyne, one of the researchers who made the discovery, described it as "a probable 'mini moon,'" not just an bit of space-junk or other human-made object.
Designated 2020 CD3, this temporarily captured object was discovered by researchers the Kitt Peak National Observatory near Tucson, Arizona. Analysis of its orbit suggests it was captured by Earth’s gravity in January 2018 and likely left the Earth-Moon (Luna, the far more permanent moon we know and love) system just as it was being discovered on Feb. 15, 2020.
So, why did it take more than 2 years for us to notice?
The goal of the Catalina Sky Survey involved in the discovery is finding 90% of the objects larger than 460 feet in diameter. 2020 CD3 is between 6 and 16 feet in diameter, about the size of a car.
Objects this small do not reflect a lot of sunlight. It was discovered by chance. There are likely more mini moons orbiting Earth that we've not noticed yet, and they may not be noticed before they move on as well.
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