3200 Phaethon

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.

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.^[10]

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.^[11] The Apollo asteroids (155140) 2005 UD and (225416) 1999 YC share similar orbits with Phaethon, suggesting a possible common breakup origin.^[12][13]

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".^[16] In studies performed by NASA's STEREO spacecraft in 2009 and 2012, rapid brightening and a visible tail have been observed.^[17][18][19] It is possible that the Sun's heat is causing fractures similar to mudcracks in a dry lake bed.^[20] 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".^[8]

In 2007, observations revealed that Phaethon was blue in color ^[21] corresponding to a B-type asteroid in the Bus taxonomy. In 2020, polarimetric study revealed Phaethon has a surface with steep slopes covered by a mix of regolith with larger pebbles.^[22] Phaethon's composition fits both, the notion of its cometary origin and that of an active asteroid; it is classified as a F-type asteroid because it is composed of dark material^[1][7] or a B-type asteroid because of its blue color.^[23] In 2009,^[21] showed that the spectral shape is similar to that of aqueously altered CI/CM meteorites. Later, in 2022, it was shown ^[24][25] how Phaethon's blue color and its rock-comet-like emission activity could 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.^[26] This link rules out other carbonaceous meteorite types, although the understanding of the origins of CYs are evolving.^[27] Further analysis of this spectrum confirmed the presence of Mg-rich olivine, carbonates, and Fe-sulfides. Until further studies were done in 2023, it was believed that thermal decomposition and outgassing of these minerals during perihelion led to dust ejection, possibly explaining a faint observed tail and the formation of the Geminid meteor stream. However in a 2023 NASA study^[28] imaging from the SOHO spacecraft revealed that the material emitted from the asteroid was sodium gas, with filtered images revealing no sign of dust at all. This cast doubt on the origin of the Geminids being dust and material cast off from Phaethon during periodic solar perihelions, and instead invited speculation of an ancient breakup event.

Shortly after its discovery, Fred Whipple noted that Phaethon's orbital elements were extremely similar to those of the Geminid meteor shower,^[29] indicating that Phaethon was the meteor shower's long-sought parent body.

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.^[30]

Phaethon approached to 0.120895 AU (18,085,600 km; 11,237,900 mi) of Earth on December 10, 2007,^[1] and was detected by radar at Arecibo.^[11] When Phaethon came to perihelion in July 2009, it was found to be brighter than expected.^[31][32] During its approach, the STEREO-A spacecraft detected an unexpected brightening, roughly by a factor of two.^[16]

2010 approach

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).^[1] The Earth approach distance was known with a 3-sigma precision of ±700 m.^[1][a] This was the best opportunity to date for radar observations by Goldstone and Arecibo, with a resolution of 75 meters/pixel (246 feet/pixel).^[11]

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.^[33] Arecibo made observations of Phaethon from December 15–19.^[6] 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.^[1][34]

Path of 3200 Phaethon in the sky during December 2017

Time lapse taken through a telescope in Riga, Latvia (December 10, 2017)

Phaethon at maximum angular velocity, December 15, 2017, 18:47:13–19:24:50 UTC