# P/2019 LD2 (ATLAS)

P/2019 LD$_2$ (ATLAS) is a Jupiter-family comet discovered by the Asteroid Terrestrial-impact Last Alert System on 10 June 2019. It was initially reported as the first known Jupiter trojan asteroid to display cometary activity, but its classification as a Jupiter trojan was retracted after closer examination and a longer observation arc revealed its orbit to be unstable like a typical Jupiter family comet and implied that its position near the trojans is temporary.

## Discovery

P/2019 LD$_2$ was discovered in images by the Asteroid Terrestrial-impact Last Alert System (ATLAS) at the Mauna Loa Observatory taken on 10 June 2019. Upon discovery, astronomers Alan Fitzsimmons and David Young at Queen's University Belfast suspected a faint coma around P/2019 LD$_2$. Follow-up observations by the Las Cumbres Observatory in 11 and 13 June 2019 confirmed the cometary appearance of P/2019 LD$_2$, which now had a more apparent coma and tail. Later observations by the ATLAS-MLO in April 2020 showed that P/2019 LD$_2$ still retained its cometary appearance, suggesting that it has been continuously active for almost a year.
The discovery of P/2019 LD$_2$'s cometary activity was announced in a press release by the University of Hawaiʻi Institute for Astronomy on 20 May 2020, purporting it as the first known active Jupiter trojan, as it was discovered near Jupiter's L$_4$ Lagrangian point where the Greek camp trojans reside. However, upon closer examination of P/2019 LD$_2$'s orbital dynamics by amateur astronomer Sam Deen, P/2019 LD$_2$ was found to be a Jupiter-family comet with a chaotic orbit instead of a Jupiter trojan. Subsequently, the comet was reclassified and was given the periodic comet designation P/2019 LD$_2$ (ATLAS) by the Minor Planet Center on 22 May 2020.

## Orbit and classification

P/2019 LD$_2$ orbits the Sun at a mean distance of 5.29 AU once every 12.18 years. Its orbit has an eccentricity of 0.135 and an inclination of 11.6 degrees with respect to the ecliptic. The body's observation arc begins with a precovery, published by the Pan-STARRS 1 survey and taken at Haleakala Observatory on 21 May 2018, or 11 months prior to its official discovery observation by the ALTAS-MLO survey.
P/2019 LD$_2$ is a Jupiter-family comet with a Tisserand parameter of 2.94, typical for other Jupiter-family comets. The comet's nominal orbit suggests that it is not in a stable 1:1 resonance with Jupiter as it has made a close approach to the planet on 17 February 2017, at a distance of 0.092 AU (13.8 million km; 8.6 million mi), and will make a similarly close approach in 2028. Unlike the Jupiter trojans, P/2019 LD$_2$ is 21 degrees ahead of Jupiter, and will continue drifting 30 degrees ahead before returning to Jupiter and making close approaches. P/2019 LD$_2$ is now following what looks like a short arc of a quasi-satellite cycle with respect to Jupiter that started in 2017 and will end in 2028.
On 2063 January 23, it will have a very close encounter with Jupiter at 0.016 AU; orbital predictions after this flyby are rather uncertain.

## Physical characteristics

Based on a generic magnitude-to-diameter conversion, P/2019 LD$_2$ measures approximately 14 kilometers in diameter, for an assumed albedo of 0.12 as the median for small Jupiter trojans, and an absolute magnitude of 12.2. However if the comet displays activity, that can lead to the nucleus size to be overestimated. Archival images by DECam from March 2017 indicate that the comet was dimmer than magnitude 23.8 at that time, indicating that the nucleus's radius is less than 1.2 km assuming a 0.05 albedo or less than 0.8 km assuming an 11.2% albedo.
As of May 2020, no rotational light curve of P/2019 LD$_2$ has been obtained from photometric observations. The body's rotation period, pole and shape remain unknown. The visible spectrum does not exhibit any evidence of CN, C2, or C3 emission.
During the approach to perihelion in 2020, the comet shed large-grained (0.1 mm typical) dust grains rich with water ice.