A second moon for the Earth?

Picture of 2020 CD3 (centre of the image, the bright point source) obtained at the Gemini Observatory on Maunakea in Hawai’i. The image shown here was made from three different images obtained in three different filters. 2020 CD3 stays at the centre of the image, as the telescope follows it during its trajectory. Credit: The International Gemini Observatory/NSF’s National Optical-Infrared Astronomy Research Laboratory/AURA/G. Fedorets

On February 15, 2020, a team of astrophysicists uncovered a small, fast-moving object on the detector of the 1.5m telescope at the summit of Mount Lemmon, Arizona. The telescope is part of a group of three telescopes that image the sky for the Catalina Sky Survey, a NASA mission to study asteroids and comets passing close to Earth.

Following the discovery of the object, now named 2020 CD3, it was shown to be gravitationally bound to the Earth. 2020 CD3 is thus a tiny moon, with a size between 1 and 6 metres. Its orbital characteristics indicate that it belongs to the Arjuna family of asteroids, a class of near-Earth asteroids with an orbit very similar to that of the Earth’s orbit around the Sun.

The orbit of 2020 CD3 around the Earth. Credit : Kacper Wierzchos

This is not the first time Earth has captured an asteroid and turned it into a moon for a short time. 1991 VG and 2006 RH120 are two asteroids that were briefly turned into Earth moons in 1991 and 2006, respectively.

2020 CD3 was captured by Earth between 2012 and 2018. It left Earth’s orbit between March 6 and 7, 2020. It is therefore a fairly long capture. Since the tiny moon was discovered by chance almost as it was leaving its orbit around the Earth, it is very likely that several other tiny moons were never discovered or were overlooked because their trajectories in an image were too similar to those of an artificial satellite.


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