3200 Phaethon

3200 Phaethon (previously sometimes spelled Phaeton), provisional designation , 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 ). For this reason, it was named after the Greek myth of Phaëthon, son of the sun god Helios. It is 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 , and it later received the numerical designation and name 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 . 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 — 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 .
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.
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 ) and Earth minimum orbit intersection distance ). The Earth minimum orbit intersection distance (E-MOID) is , 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.
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 recent studies performed by NASA's STEREO spacecraft, dust tails have been observed, and in 2010, Phaethon was detected ejecting dust. 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, which is an extremely rare occurrence among asteroids. In contrast, most asteroids tend to be grey or red. In 2020, polarimentric study revealed Phaeton 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.
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 flyby 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 2024.
Close approaches.
Phaethon approached to 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 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 .
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 from Earth.