# 2020 CD3

2020 CD$_3$ (also 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 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 CD$_3$ was captured by Earth around 2016–2017, and escaped Earth's gravitational sphere of influence around 7 May 2020. 2020 CD$_3$ 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 CD$_3$ has an absolute magnitude around 32, indicating that it is very small in size. Assuming that 2020 CD$_3$ 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 CD$_3$ is classified as an Arjuna asteroid, a subtype of small Earth-crossing Apollo asteroids that have Earth-like orbits.

## Discovery

2020 CD$_3$ 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. 2020 CD$_3$ 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 CD$_3$ 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 CD$_3$ could not be linked to any known artificial object. Although the evidence implied that 2020 CD$_3$ 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 2020 CD$_3$ 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 2020 CD$_3$ 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 2020 CD$_3$, 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 2020 CD$_3$ has an Earth-like heliocentric orbit, its motion relative to Earth is low, allowing for it to slowly approach the planet and be captured. Nominal orbit solutions for 2020 CD$_3$ suggest that it was captured by Earth between 2016 and 2017, and left geocentric orbit by May 2020 according to simulations of its orbit. The geocentric orbit of 2020 CD$_3$ 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 CD$_3$'s geocentric orbit, causing it to be unstable. Over the course of 2020 CD$_3$'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 2020 CD$_3$ to escape Earth's gravitational influence.
2020 CD$_3$'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.
Earth approach Christmas Day 2015? :
| Date | JPL Horizons nominal geocentric distance (AU) | NEODyS nominal geocentric distance (AU) | Find_Orb nominal geocentric distance (AU) | ESA NEOCC nominal geocentric distance (AU) | MPC nominal geocentric distance (AU) |
| 2015-Dec-25 | 0.0006 AU (90 thousand km) | 0.0162 AU (2.42 million km) | 0.288 AU (43.1 million km) | 0.345 AU (51.6 million km) | 0.834 AU (124.8 million km) |
Between September 2017 and February 2020 it made 12 close approaches to Earth, during which time it was never more than 0.0112 AU (1.68 million km) from Earth. According to the JPL Small-Body Database, on 15 September 2017 it passed 12,000 km (7,500 mi) from the Moon. The closest approach to Earth occurred on 4 April 2019, when it approached to a distance of 13,104 km (8,142 mi). The final close approach in 2020 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 CD$_3$ around Earth ranged from 70 to 90 days. 2020 CD$_3$ escaped Earth's Hill sphere at roughly 0.01 AU (1.5 million km) in March 2020 and returned to solar orbit on 7 May 2020.
Escaping geocentric orbit 7 May 2020 (assuming the Earth+asteroid are the only objects in the Solar System) :
| Epoch | Earth distance | Geocentric eccentricity | Apogee | Orbital period |
| 2020-May-07 | 0.0189 AU (2.83 million km) | 0.9901 | 2.25 AU (337 million km) | 693.61 years (253,341 d) |
| 2020-May-08 | 0.0191 AU (2.86 million km) | 1.0347 | $\infty$ | $\infty$ |
Being captured into a temporary orbit around Earth, 2020 CD$_3$ is a temporarily captured object or a temporary satellite of Earth. 2020 CD$_3$ has also been widely referred to in the media as a "mini-moon" of Earth, due to its small size. 2020 CD$_3$ 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-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 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

2020 CD$_3$ will continue orbiting the Sun and will approach Earth on 20 March 2044, from a distance of 0.0245 AU (3.67 million km; 2.28 million mi). It is unlikely that 2020 CD$_3$ 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 0.034 AU (5.1 million km; 3.2 million mi). After the 2061 encounter the uncertainties in future encounters become much greater. By 2082 close approaches have a 3-sigma uncertainty of ±7 days.
The possibility of 2020 CD$_3$ 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 2020 CD$_3$ 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 2020 CD$_3$, 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 0.00251 AU (375 thousand km) from Earth on 8 October 2082 (29 days after the virtual impactor).

## Physical characteristics

2020 CD$_3$ 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 CD$_3$ have not been measured due to the limited number of observations. Assuming that the albedo of 2020 CD$_3$ is similar to those of dark, carbonaceous C-type asteroids, the diameter of 2020 CD$_3$ is around 1.9–3.5 m (6–11 ft), comparable to in size to that of a small car. The JPL Sentry risk table estimates 2020 CD$_3$ 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).