2020 CD3

(also 2020CD3 or CD3 for short) is a tiny near-Earth asteroid (or minimoon) that ordinarily orbits the Sun but makes close approaches to the Earth–Moon system, in which it can temporarily enter Earth orbit through temporary satellite capture (TSC). It was discovered at the Mount Lemmon Observatory by astronomers Theodore Pruyne and Kacper Wierzchoś on 15 February 2020, as part of the Mount Lemmon Survey or Catalina Sky Survey. The asteroid's discovery was announced by the Minor Planet Center on 25 February 2020, after subsequent observations have confirmed that it was orbiting Earth.

It is the second temporary satellite of Earth discovered '' in situ'', after , which was discovered in 2006. Based on its nominal trajectory, was captured by Earth around 2016–2017, and escaped Earth's gravitational sphere of influence around 7 May 2020. will make another close pass to Earth in March 2044, though it will most likely not be captured by Earth due to the greater approach distance.

has an absolute magnitude around 32, indicating that it is very small in size. Assuming that has a low albedo characteristic of dark, carbonaceous C-type asteroids, its diameter is probably around . is classified as an Arjuna asteroid, a subtype of small Earth-crossing Apollo asteroids that have Earth-like orbits.

Discovery

was discovered on 15 February 2020, by astronomers Theodore Pruyne and Kacper Wierzchoś at the Mount Lemmon Observatory. The discovery formed part of the Mount Lemmon Survey designed for discovering near-Earth objects, which is also part of the Catalina Sky Survey conducted at Tucson, Arizona. was found as a faint, 20th magnitude object in the constellation of Virgo, located about from Earth at the time. The observed orbital motion of the object suggested that it may be gravitationally bound to Earth, which prompted further observations to secure and determine its motion.

The object's discovery was reported to the Minor Planet Center's Near-Earth Object Confirmation Page (NEOCP), where a preliminary orbit was calculated from additional observations conducted at several observatories. Follow-up observations of spanned six days since its discovery, and the object was formally announced in a Minor Planet Electronic Circular notice issued by the Minor Planet Center on 25 February 2020. No indication of perturbations by solar radiation pressure was observed, and could not be linked to any known artificial object. Although the evidence implied that is most likely a dense, rocky asteroid, the possibility of the object being an artificial object, such as a dead satellite or rocket booster, had not yet been fully ruled out.

Precovery images of have been identified back to May 2018.

Nomenclature

Upon discovery, the asteroid was given the temporary internal designation C26FED2. After follow up observations confirming the object, it was then given the provisional designation by the Minor Planet Center on 25 February 2020. The provisional designation signifies the object's discovery date and year. The object has not yet been issued a permanent minor planet number by the Minor Planet Center due to its modest observation arc of a couple years and that it has not been observed at enough oppositions.

Orbit

Prior to the temporary capture of , its heliocentric orbit was probably Earth-crossing, either falling into the categories of an Aten-type orbit ( a < 1 AU) or an Apollo-type orbit ( a > 1 AU), with the former considered to be more likely.

Temporary capture

Because has an Earth-like heliocentric orbit, its motion relative to Earth is low, allowing for it to slowly approach the planet and get captured. Nominal orbit solutions for suggest that it was captured by Earth between 2016–2017, and left geocentric orbit by May 2020 according to simulations of its orbit. The geocentric orbit of is chaotic due to the combined effects of tidal forces from the Sun and Earth as well as repeated close encounters with the Moon. The Moon gravitationally perturbs 's geocentric orbit, causing it to be unstable. Over the course of 's orbit around Earth, repeated close encounters with the Moon leads to ejection from its geocentric orbit as the Moon's perturbations can transfer enough momentum for to escape Earth's gravitational influence.

's orbit around Earth is highly variable and eccentric, hence predictions of its past trajectory before mid-2017 are uncertain. Due to the Yarkovsky effect on small asteroids, the first precovery image being from 2018, and numerous approaches to the Earth and Moon, it is unknown if the asteroid was closer than the Moon on Christmas Day 2015.

Between September 2017 and February 2020 it made 12 close approaches to Earth, during which time it was never more than from Earth. According to the JPL Small-Body Database, on 15 September 2017 it passed from the Moon. The closest approach to Earth occurred on 4 April 2019, when it approached to a distance of . The final close approach in 2020 occurred on 13 February 2020 at a distance of about from Earth's surface. The orbital period of around Earth ranged from 70 to 90 days. escaped Earth's hill sphere at roughly in March 2020 and returned to solar orbit on 7 May 2020.

Being captured into a temporary orbit around Earth, is a temporarily captured object or a temporary satellite of Earth. has also been widely referred to in the media as a "mini-moon" of Earth, due to its small size. is the second known temporary captured object discovered ''in situ'' around Earth, with the first being discovered in 2006. Other objects have also been suspected to have once been temporarily captured, including the small near-Earth asteroid 1991 VG and the bolide DN160822 03. Objects that get temporarily captured by Earth are thought to be common, though larger objects over in diameter are believed to be less likely to be captured by Earth and detected by modern telescopes.

Future approaches

will continue orbiting the Sun and will approach Earth on 20 March 2044, from a distance of . It is unlikely that will be captured by Earth in the March 2044 encounter, as the approach distance is too large for capture and outside of Earth's hill sphere. The next encounter will be August 2061, when it is expected to approach Earth from a nominal distance of . After the 2061 encounter the uncertainties in future encounters become much greater. By 2082 close approaches have a 3-sigma uncertainty of

The possibility of impacting Earth has been considered by the Jet Propulsion Laboratory's Sentry risk table. JPL's solution accounts for non-gravitational forces as the multi-decade motion of a very small object is greatly affected by solar heating. Being only a few meters in size, an impact by would pose no threat to Earth as it would most likely fragment and disintegrate upon atmospheric entry. With a cumulative impact probability of 2.5%, it is listed as the most likely object to impact Earth, but because of the harmless size of , it is given a Torino Scale rating of 0 and a cumulative Palermo Scale rating of –5.20 Within the next 100 years, the date with the highest probability of impact is 9 September 2082, which is estimated to have an impact probability of 0.85% and a negligible Palermo Scale rating of –5.66. JPL Horizon's nominal orbit has the asteroid passing from Earth on 8 October 2082 (29 days after the virtual impactor).

Physical characteristics

is estimated to have an absolute magnitude (H) around 31.7, indicating that it is very small in size. Studies reported in November 2020 have determined that the asteroid is about in diameter. The rotation period and albedo of have not been measured due to the limited number of observations. Assuming that the albedo of is similar to those of dark, carbonaceous C-type asteroids, the diameter of is around , comparable to in size to that of a small car. The JPL Sentry risk table estimates to have a mass of , based on the assumption that the asteroid has a diameter of .

See also

* – near-Earth asteroid temporarily captured by Earth after its discovery in 1991
* – the first temporary Earth satellite discovered ''in situ'' 2006
*
* Claimed moons of Earth
* Quasi-satellite

Notes

References

External links

Earth Has A Mini-Moon — But Not for Long!
by Bob King, '' Sky & Telescope'', 2 March 2020

A New Mini-Moon Was Found Orbiting Earth. There Will Be More.
by Rebecca Boyle, '' The New York Times'', 27 Feb 2020

Gemini Telescope Images "Minimoon" Orbiting Earth — in Color!
, OIR Laboratory press release, 27 Feb 2020

Looks like Earth has a new natural moon
by Deborah Byrd, ''EarthSky'', 26 Feb 2020


Minor Planet Center announcement, 25 Feb 2020
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