460P/PanSTARRS

460P/PanSTARRS (also known with the provisional designation ) is a near-Earth object and periodic comet of the Jupiter family, with an orbital period of 5.25 years. In March 2016 it passed at distance of 2.2 million miles (3.5 million km, or 9 lunar distances) from Earth. It was the closest approach by a comet since 1770 and 3rd closest recorded comet to Earth. The close flyby enabled the size of the nucleus to be calculated at about 1 km in diameter, which was much bigger than expected. The comet is very dark, reflecting about 2-3 percent of the visible light, about the same as a charcoal briquette. It has a very similar orbit as numbered comet 252P/LINEAR, and may be related to it (e.g. split off of).

Observational history
The object when discovered on 22 January 2016 by a Pan-STARRS telescope, was thought to be an asteroid and went by the provisional minor planet designation. Astronomer Denis Denisenko noted the body's orbit was very similar to 252P/LINEAR, which led to a follow-up observation by the Lowell Discovery Telescope. The body showed a tail, identifying it as a probable comet and then named. It was also observed by the 4.3 meter aperture Discovery Channel Telescope in Arizona.

The comet made a close flyby to Earth on 22 March 2016, at distance of 2.2 million miles (3.5 million km, or 9 lunar distances) from Earth, which was the closest approach by a comet since 1770 and 3rd closest recorded comet to Earth. That close flyby was an opportunity to study comets more closely. During the closest approach to Earth on 21 and 22 March 2016, the telescopes of Slooh offered views of and 252P/LINEAR. The comet was also observed by radar.

The comet was recovered by R. Weryk in images obtained by the 1.8 m Ritchey–Chrétien telescope of Pan-STARRS on 17 October 2020, when it had an estimated apparent magnitude of 22.6, and was followed up to 5 January 2021. The comet during that apparition was given the provisional number P/2020 U6 (PanSTARRS).

Orbit
460P/PanSTARRS is Jupiter family comet, indicating its aphelion is close to the orbit of Jupiter. The comet has an orbital period on 5.25 years and its perihelion is at 1.0128 AU for the 2021, while it was at 1.009 AU in 2016.

460P/PanSTARRS had the closest known approach of Earth by a comet since 1770, when Lexell's Comet is calculated to have passed within 1.4 million miles (2.2 million kilometers) of Earth, and the third closest approach of a comet to Earth. Another close approach by a comet in modern times was Comet IRAS–Araki–Alcock (aka C/1983 H1) in 1983.

After 's flyby in 2016, it is calculated that is the closest it will be to Earth for at least the next 150 years. The comet approaches to 0.67 AU from Jupiter on 30 January 2034, which will result in an orbital change of the comet, with the perihelion distance increasing to 1.107 AU and the orbital period increasing to 5.43 years. The comet will approach Jupiter again on 6 July 2042, this time at a distance of 0.76 AU. The orbit of the comet will change again, with perihelion distance decreasing to 1.032 AU and the orbital period to 5.25 years. The next close flyby to Earth will take place at a distance of 0.052 AU on 14 March 2048.

The orbits of 460P/PanSTARRS and 252P/LINEAR show similarities, suggesting that 460P/PanSTARRS may have split off from 252P/LINEAR. Comet 252P/LINEAR had its close approach on March 21, 2016, and coming within 3.3 million miles (5.2 million kilometers) made it the 5th closest approach by a comet.

Scientific results
The close flyby in 2016 was an opportunity to study comets more closely. 460P/PanSTARRS was observed by the Goldstone Solar System Radar in California over three days, which revealed various properties including a rotation rate of 35–40 hours along one axis. The radar observations during the Earth flyby detected surface features as small as 8 meters, marking the highest resolution radar image of a comet. was bigger than predicted when observed by astronomical radar; it was estimated to be perhaps 125 meters but was about 1 km in diameter. One reason it was hard to predict its size, is because it reflects less than 3% of the light, a very low albedo. There was no obvious sign of a coma.

NASA's Infrared Telescope Facility (IRTF) also made observations of, revealing that was reflecting only 2-3 percent of visible light. The observations of with the NASA IRTF gave a diameter estimate of 600 – 1200 meters (0.4 mile and 0.75 mile), similar to the diameter reported by the radar observations. The comet was also observed in mid infrared by the Cooled Mid-Infrared Camera and Spectrometer in Subaru Telescope in Hawaii. The observations revealed that it is an inactive body covered by phyllosilicate and organic materials with a diameter of 800 meters.

During the 2016 flyby, activity level was measured both for 460P/PanSTARRS and 252P/LINEAR. 252P/LINEAR, despite having a smaller nucleus with diameter about 600 m, was much more active than 460P/PanSTARRS, about three to four orders of magnitude more. Hyland et al. did not detect gas emission from 460P, while Li et al. did detect CN emission on 17 April 2016 consistent with a production rate of $21.3$ mol/s.

Meteor shower
460P/PanSTARRS may produce meteor shower on 21 March 2023 between the hours of 00–04 UT. The radiant would be located near the bright star Canopus at (R.A. = 6 h ; Dec. = –51°). Meteors would have a slow entry speed of 17 km/s.