2020 CD3

2020 CD3 is the second minimoon ever found and the best studied.^[1] A rock about 1.2 m across, it was captured by Earth's gravity in September 2017, completed 11 orbits around us, and was discovered on 15 February 2020, three weeks before it left.^[1][2] What makes an object a minimoon is explained in Minimoons.

Found in its last weeks

The NASA-funded Catalina Sky Survey found it on 15 February 2020 from Mount Lemmon, where the observers were Pruyne, Wierzchoś and Rankin.^[3][4] Two days earlier it had made its last close pass, about 41,000 km above Earth's surface, just outside the ring of geosynchronous satellites at about 36,000 km.^[4] NASA's Center for Near-Earth Object Studies reported that it had been orbiting Earth for at least a year and probably longer, had completed at least three large loops of roughly 70 to 90 days each, reaching about four times the Moon's distance at its farthest, and would escape in early March.^[4] The Minor Planet Center's circular of 25 February 2020 said the object was temporarily bound to Earth, with no link to any known artificial object.^[3]

The full story came from old images. Searching archives, astronomers found it in frames from the Dark Energy Camera and from the Catalina telescopes going back to 9 May 2018.^[2] With the longer track, the orbit could be run backward: Earth captured 2020 CD3 on 15 September 2017, right after it passed 11,974 km from the Moon, and it stayed about two and a half years before crossing out of Earth's Hill sphere, about 0.01 au, on 7 March 2020.^[2] That is a temporarily captured orbiter with 11 completed revolutions, far longer than the nine-month average the models predict, which they allow for objects that pass the Moon closely.^[1]

A natural rock, and a small one

Everything measured says it is natural. The pressure of sunlight on it, read from its motion, matches a solid rock and rules out hollow space debris; its colours match a stony asteroid of the S or V type, and its density is about 2.3 g/cm³.^[1][5] Observations with the Keck I telescope on 23 March 2020, as it was leaving, made it the first minimoon to have its spectrum studied; they show a spin period of about 573 seconds, roughly nine and a half minutes, and a shape about two and a half times longer than it is wide.^[5] Its diameter is about 1.2 m, which puts it among the smallest near-Earth objects ever found.^[1] A lunar origin was considered and ruled out, so it most likely came from the near-Earth asteroid population.^[1]

What it taught astronomers

It was the first minimoon whose measured properties could be checked against theory: its size and its arrival about a decade after 2006 RH120 match the models, while its slow spin does not, which suggests the models of meter-sized asteroids need revision.^[1] Its discovery also showed how hard these objects are to catch: the Mount Lemmon telescope could have detected it for only about 0.03% of its time in orbit.^[1] The Vera C. Rubin Observatory is expected to find many more.^[1]

Where it is now, and the risk list

It is back in orbit around the Sun, with a semi-major axis of 1.03 au and a period of 381 days; JPL's next listed pass is in March 2044 at 0.024 au, about nine times the Moon's distance.^[6] It appears on NASA's Sentry risk list, which is why this app shows a risk block for it, but the numbers mean little: there is about a 1% chance of an impact somewhere between 2061 and 2120, and an object this small would do no damage.^[2]