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 2006 RH120, which was discovered in 2006. Based on its nominal trajectory, 2020 CD3 was captured by Earth around 2016–2017, and has likely escaped Earth's gravitational sphere of influence around May 2020. 2020 CD3 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.
2020 CD3 has an absolute magnitude around 32, indicating that it is very small in size. Assuming that 2020 CD3 has a low albedo characteristic of dark, carbonaceous C-type asteroids, its diameter is probably around 1.9–3.5 metres (6–11 ft). 2020 CD3 is classified as an Arjuna asteroid, a subtype of small Earth-crossing Apollo asteroids that have Earth-like orbits.
Discovery
2020 CD3 was discovered on 15 February 2020, by astronomers Theodore Pruyne and Kacper Wierzchos 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. 2020 CD3 was found as a faint, 20th magnitude object in the constellation of Virgo, located about 0.0019 AU (280,000 km; 180,000 mi) 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 2020 CD3 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 2020 CD3 could not be linked to any known artificial object. Although the evidence implied that 2020 CD3 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.
As of July 2020, no precovery images of 2020 CD3 have been identified. The discoverers of 2020 CD3 have suspected that it is possible that the object may have been imaged by other astronomical surveys prior to its discovery, but had not been identified due to its faintness and highly variable orbit.
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 2020 CD3 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 short observation arc of several days.
Orbit
Prior to the temporary capture of 2020 CD3, 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
The orbit of 2020 CD3 around the Earth. The white band is the orbit of the Moon.
Trajectory and orbits of 2020 CD3 around Earth, from 2015–2020.
Because 2020 CD3 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 2020 CD3 suggest that it was captured by Earth between 2016–2017, and is expected to leave its geocentric orbit by May 2020 according to simulations of its orbit. The geocentric orbit of 2020 CD3 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 2020 CD3's geocentric orbit, causing it to be unstable. Over the course of 2020 CD3's orbit around Earth, repeated close encounters with the Moon will eventually lead to ejection from its geocentric orbit as the Moon's perturbations can transfer enough momentum for 2020 CD3 to escape Earth's gravitational influence.
2020 CD3's orbit around Earth is highly variable and eccentric, hence predictions of its past trajectory are uncertain. According to the JPL Small-Body Database, the closest approach to Earth has already occurred, on 4 April 2019, when it approached to a distance of 13,121 km (8,153 mi). The previous close approach by 2020 CD3 had occurred on 13 February 2020, at a distance of about 41,000 km (25,000 mi) from Earth's surface. The orbital period of 2020 CD3 around Earth is currently around 47 days, though on larger orbits around Earth, 2020 CD3's orbital period can range from 70 to 90 days. However, these estimates are very uncertain due to the chaotic dynamics of 2020 CD3's orbit.
Being captured into a temporary orbit around Earth, 2020 CD3 is a temporarily captured object or a temporary satellite of Earth. 2020 CD3 has also been widely referred to in the media as a "mini-moon" of Earth, due to its small size. 2020 CD3 is the second known temporary captured object discovered in situ around Earth, with the first being 2006 RH120 discovered in 2006. Other objects have also been suspected to have once been temporarily captured, including the small near-Eath asteroid 1991 VG and the bolide DN160822 03. Objects that get temporarily captured by Earth are thought to be common, though larger objects over 0.6 m (2 ft) in diameter are believed to be less likely to be captured by Earth and detected by modern telescopes.
Future Approaches
After its departure from Earth, 2020 CD3 will continue orbiting the Sun and will approach Earth in March 2044, from a nominal distance of 0.0245 AU (3.67 million km; 2.28 million mi). Accounting for uncertainties in its orbit, the minimum approach distance from Earth is expected to be 0.0237 AU (3.55 million km; 2.20 million mi). It is unlikely that 2020 CD3 will be captured by Earth in the March 2044 encounter, as the approach distance is too large for capture. Assuming that 2020 CD3's orbit does not change after the 2044 close approach, the next encounter would be around 2061, when it is expected to approach Earth from a nominal distance of 0.0375 AU (5.61 million km; 3.49 million mi). However, uncertainties in the 2061 approach distance are greater; 2020 CD3's minimum approach distance could be 0.0131 AU (1.96 million km; 1.22 million mi).
The possibility of 2020 CD3 impacting Earth has been considered by the Jet Propulsion Laboratory's Sentry risk table. Being only a few meters in size, an impact by 2020 CD3 would pose no threat to Earth as it would most likely fragment and disintegrate upon atmospheric entry. With a cumulative impact probability of 2.2%, it is listed as the second most likely object to impact Earth, but because of the harmless size of 2020 CD3, it is given a Torino Scale rating of 0 and a cumulative Palermo Scale rating of –5.22. Within the next 100 years, the date with the highest probability of impact is 14 September 2061, which is estimated to have an impact probability of 0.73% (1 in 140) and a negligible Palermo Scale rating of –5.55.
Physical characteristics
2020 CD3 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 1–2 m (3.3–6.6 ft) in diameter. The rotation period and albedo of 2020 CD3 have not been measured due to the limited number of observations. Assuming that the albedo of 2020 CD3 is similar to those of dark, carbonaceous C-type asteroids, the diameter of 2020 CD3 is around 1.9–3.5 m (6–11 ft), comparable in size to that of a small car. The JPL Sentry risk table estimates 2020 CD3 to have a mass of 4,900 kg (10,800 lb), based on the assumption that the asteroid has a diameter of 2 m (6.6 ft).