# 3200 Phaethon

3200 Phaethon (/ˈfeɪ.əˌθɒn/; previously sometimes spelled Phæton), provisionally designated 1983 TB, is an active Apollo asteroid with an orbit that brings it closer to the Sun than any other named asteroid (though there are numerous unnamed asteroids with smaller perihelia, such as (137924) 2000 BD19). For this reason, it was named after the Greek Hero, Phaëthon, son of the sun god Helios. It is 5.8 km (3.6 mi) in diameter and is the parent body of the Geminids meteor shower of mid-December. With an observation arc of 35+ years, it has a very well determined orbit. The 2017 Earth approach distance of about 10 million km was known with an accuracy of ±700 m.

## Discovery

Phaethon was the first asteroid to be discovered using images from a spacecraft. Simon F. Green and John K. Davies discovered it in images from October 11, 1983, while searching Infrared Astronomical Satellite (IRAS) data for moving objects. It was formally announced on October 14 in IAUC 3878 along with optical confirmation by Charles T. Kowal, who reported it to be asteroidal in appearance. Its provisional designation was 1983 TB, and it later received the numerical designation and name 3200 Phaethon in 1985.

## Orbital characteristics

Phaethon is categorized as an Apollo asteroid, as its orbital semi-major axis is greater than that of the Earth's at 1.27 AU (190 million km; 118 million mi). It is also suspected to be a member of the Pallas family of asteroids.
Its most remarkable distinction is that it approaches the Sun closer than any other named asteroid: its perihelion is only 0.14 AU (20.9 million km; 13.0 million mi) — less than half of Mercury's perihelial distance. It is a Mercury-, Venus-, Earth-, and Mars-crosser as a result of its high orbital eccentricity. The surface temperature at perihelion could reach around 1,025 K (750 °C; 1,390 °F).
Phaethon is a possible candidate for detecting general relativistic and/or solar oblateness effects in its orbital motion due to the frequent close approaches to the Sun. The Apollo asteroids (155140) 2005 UD and (225416) 1999 YC share similar orbits with Phaethon, suggesting a possible common breakup origin.

### Potentially hazardous asteroid

Phaethon is categorized as a potentially hazardous asteroid (PHA), but that does not mean there is a near-term threat of an impact. It is a potentially hazardous asteroid merely as a result of its size (absolute magnitude H ≤ 22) and Earth minimum orbit intersection distance (Earth MOID ≤ 0.05 AU). The Earth minimum orbit intersection distance (E-MOID) is 0.01945 AU (2,910,000 km; 1,808,000 mi), which is defined by the shortest distance between the orbit of Phaethon and the orbit of Earth. With a 30+ year observation arc, the orbit of Phaethon is very well understood with very small uncertainties. Close approaches of Phaethon are well constrained for the next 400 years.
| PHA | Date | Approach distance in lunar distances | Approach distance in lunar distances | Approach distance in lunar distances | Abs. mag (H) | Diameter $^{(C)}$ (m) | Ref $^{(D)}$ |
| PHA | Date | Nominal $^{(B)}$ | Minimum | Maximum | Abs. mag (H) | Diameter $^{(C)}$ (m) | Ref $^{(D)}$ |
| (143651) 2003 QO104 | 1981-05-18 | 2.761 | 2.760 | 2.761 | 16.0 | 1333–4306 | data |
| 2014 LJ21 | 1989-08-01 | 7.034 | 6.843 | 7.224 | 16.0 | 1333–4306 | data |
| 4179 Toutatis | 1992-12-08 | 9.399 | 9.399 | 9.399 | 15.30 | 2440–2450 | data |
| 4179 Toutatis | 2004-09-29 | 4.031 | 4.031 | 4.031 | 15.30 | 2440–2450 | data |
| (159857) 2004 LJ1 | 2038-11-16 | 7.719 | 7.719 | 7.719 | 15.4 | 1746–4394 | data |
| (4953) 1990 MU | 2058-06-05 | 8.986 | 8.984 | 8.988 | 14.1 | 3199–10329 | data |
| 4179 Toutatis | 2069-11-05 | 7.725 | 7.724 | 7.725 | 15.30 | 2440–2450 | data |
| (52768) 1998 OR2 | 2079-04-16 | 4.611 | 4.611 | 4.612 | 15.8 | 1462–4721 | data |
| (415029) 2011 UL21 | 2089-06-25 | 6.936 | 6.935 | 6.938 | 15.7 | 1531–4944 | data |
| 3200 Phaethon | 2093-12-14 | 7.714 | 7.709 | 7.718 | 14.6 | 4900–5300 | data |
| (52768) 1998 OR2 | 2127-04-16 | 6.536 | 6.510 | 6.563 | 15.8 | 1462–4721 | data |
| $^{(A)}$ This list includes near-Earth approaches of less than 10 lunar distances (LD) of objects with H brighter than 16. $^{(B)}$ Nominal geocentric distance from the center of Earth to the center of the object (Earth has a radius of approximately 6,400 km). $^{(C)}$ Diameter: estimated, theoretical mean-diameter based on H and albedo range between X and Y. $^{(D)}$ Reference: data source from the JPL SBDB, with AU converted into LD (1 AU≈390 LD) $^{(E)}$ Color codes: unobserved at close approach observed during close approach upcoming approaches ||||||||

## Physical characteristics

Phaethon is an asteroid with fairly unusual characteristics in that its orbit more closely resembles that of a comet than an asteroid; it has been referred to as a "rock comet". In studies performed by NASA's STEREO spacecraft in 2009 and 2012, rapid brightening and dust tail have been observed. It is possible that the Sun's heat is causing fractures similar to mudcracks in a dry lake bed. This occurs because Phaethon's orbit takes it closer to the Sun than any other named asteroid (0.14 AU at perihelion) causing extreme heating and enough solar radiation pressure to push any separated pieces off the asteroid's surface. Since its discovery, several other objects were found exhibiting mixed cometary and asteroidal features, such as 133P/Elst–Pizarro, leading to a new class of objects dubbed "active asteroids".
In 2018, observations revealed that Phaethon was blue in color. This is extremely rare, as most asteroids tend to be grey or red. In 2020, polarimetric study revealed Phaethon has a surface with steep slopes covered by a mix of regolith with larger pebbles. Phaethon's composition fits the notion of its cometary origin; it is classified as a F-type asteroid because it is composed of dark material or a B-type asteroid because of its blue color. In 2022 it was shown how Phaethon's blue color and its rock-comet-like emission activity can be explained by the effects of the intense solar heating at perihelion causing sublimation of any darkish-red refractory organic, nano-phase iron (nFe0), and pyroxene materials on its surface.
Analysis of a mid-infrared spectral emissivity spectrum from the Spitzer Space Telescope showed Phaethon to be linked to the rare Yamato-type (CY) carbonaceous chondrites. Further analysis of this spectrum confirmed the presence of Mg-rich olivine, carbonates, and Fe-sulfides. These minerals decompose at the temperatures that Phaethon reaches at perihelion, resulting in outgassing, in a process called thermal decomposition. It is believed that this process leads to dust ejection and can explain the formation of the Geminid meteor stream.

## Meteor shower

Shortly after its discovery, Fred Whipple observed that the "orbital elements of 1983 TB shown on IAUC 3879 are virtually coincident with the mean orbital elements of 19 Geminid meteors photographed with the super-Schmidt meteor cameras". In other words, Phaethon is the long-sought parent body of the Geminids meteor shower of mid-December.

## Planned flyby

DESTINY$^+$ (Demonstration and Experiment of Space Technology for INterplanetary voYage Phaethon fLyby dUSt science) is a planned mission to fly by 3200 Phaethon, as well as various minor bodies originating from it. The spacecraft is being developed by the Japanese space agency JAXA and will demonstrate advanced technologies for future deep space exploration. DESTINY$^+$ is planned to be launched no earlier than 2028.

## Close approaches

Phaethon approached to 0.120895 AU (18,085,600 km; 11,237,900 mi) of Earth on December 10, 2007, and was detected by radar at Arecibo. When Phaethon came to perihelion in July 2009, it was found to be brighter than expected. During its approach, the STEREO-A spacecraft detected an unexpected brightening, roughly by a factor of two.

### 2017 approach

On December 16, 2017, at 23:00 UT, Phaethon passed 0.06893169 AU (10,312,034 km; 6,407,601 mi) from Earth (27 lunar distances). The Earth approach distance was known with a 3-sigma precision of ±700 m. This was the best opportunity to date for radar observations by Goldstone and Arecibo, with a resolution of 75 meters/pixel (246 feet/pixel).
The asteroid was bright enough to see in small telescopes, peaking at magnitude 10.8 between December 13–15 while dimming slightly to magnitude 11 on December 16 at closest approach. Arecibo made observations of Phaethon from December 15–19. It will not make an Earth approach closer than the 2017 passage until December 14, 2093, when it will pass 0.01981 AU (2,964,000 km; 1,841,000 mi) from Earth.