P/2019 LD2 (ATLAS)

Lore & Background

P/2019 LD2 was discovered in images by 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 it. Follow-up observations by the Las Cumbres Observatory on 11 and 13 June 2019 confirmed the cometary appearance, which now had a more apparent coma and tail. Later observations by ATLAS-MLO in April 2020 showed that it still retained its cometary appearance, suggesting continuous activity for almost a year. The discovery of its cometary activity was announced in a press release by the University of Hawaiʻi Institute for Astronomy on 20 May 2020, proposing it as the first known active Jupiter trojan, as it was discovered near Jupiter's L4 Lagrangian point. However, upon closer examination of its orbital dynamics by amateur astronomer Sam Deen, it was found to be a Jupiter-family comet with a chaotic orbit instead of a Jupiter trojan. Subsequently, it was reclassified and given the periodic comet designation P/2019 LD2 (ATLAS) by the Minor Planet Center on 22 May 2020. P/2019 LD2 orbits the Sun at a mean distance of 5.28 AU once every 12.15 years, with an eccentricity of 0.134 and an inclination of 11.6 degrees. Its nominal orbit suggests it is not in a stable 1:1 resonance with Jupiter, having made a close approach to the planet on 17 February 2017 at 0.092 AU, and will make a similarly close approach in 2028. It is 21 degrees ahead of Jupiter and will continue drifting 30 degrees ahead before returning. According to a 2021 study, it will have a very close encounter with Jupiter on 23 January 2063, possibly as close as 0.016 AU, though as of March 2026 JPL gives the minimum and maximum possible close approach distances as 0.052 au and 0.076 au, respectively, with the date as 13 January 2063 (uncertainty ± 1 day).

Reader's Guide

P/2019 LD2 (ATLAS) is notable primarily for its initial misclassification as the first known active Jupiter trojan, a claim that was quickly retracted after amateur astronomer Sam Deen analyzed its orbital dynamics. This episode highlights the importance of long observation arcs and careful orbital analysis in distinguishing between different classes of small Solar System bodies. The comet's discovery and reclassification illustrate the dynamic nature of Jupiter's vicinity, where objects can temporarily appear to be trojans but are actually on chaotic, unstable orbits typical of Jupiter-family comets. Its Tisserand parameter of 2.92 is typical for Jupiter-family comets, and its orbit shows it is following a short arc of a quasi-satellite cycle with respect to Jupiter from 2017 to 2028. The comet's physical characteristics remain uncertain, with diameter estimates ranging from less than 0.8 km radius (based on archival DECam images) to about 6.1 km radius (from Sanglokh Observatory observations), and no rotational light curve has been obtained. During its approach to perihelion in 2020, it shed large-grained dust grains rich with water ice. The case of P/2019 LD2 serves as a cautionary tale in asteroid-comet classification and underscores the value of citizen science contributions to professional astronomy.

Did You Know?

• P/2019 LD2 was initially reported as the first known Jupiter trojan asteroid to display cometary activity, but this classification was retracted after amateur astronomer Sam Deen found its orbit to be chaotic.
• The comet was discovered by the Asteroid Terrestrial-impact Last Alert System (ATLAS) at the Mauna Loa Observatory on 10 June 2019.
• It made a close approach to Jupiter on 17 February 2017 at a distance of 0.092 AU (13.8 million km).
• During its approach to perihelion in 2020, the comet shed large-grained dust grains rich with water ice.

Discovery and the Trojan Misclassification

P/2019 LD2 was first spotted in ATLAS images captured at Mauna Loa Observatory on 10 June 2019. Astronomers Alan Fitzsimmons and David Young at Queen's University Belfast immediately suspected a faint coma, and follow-up observations by Las Cumbres Observatory on 11 and 13 June confirmed a growing coma and tail. The University of Hawaiʻi Institute for Astronomy then issued a press release on 20 May 2020 proclaiming the object the first known active Jupiter trojan, since it had been found near the L4 Lagrangian point where the Greek camp resides. However, amateur astronomer Sam Deen's closer orbital analysis revealed the body was actually a Jupiter-family comet with a chaotic orbit rather than a stable trojan. The Minor Planet Center reclassified it on 22 May 2020, assigning the periodic comet designation P/2019 LD2 (ATLAS). The episode underscored how a temporary orbital position near the trojan population can mask a body's true dynamical identity.

Orbital Mechanics and the Quasi-Satellite Cycle

P/2019 LD2 circles the Sun at a mean distance of 5.28 AU with a period of 12.15 years, an eccentricity of 0.134, and an inclination of 11.6 degrees relative to the ecliptic. Its Tisserand parameter of 2.92 places it squarely among Jupiter-family comets. Rather than sharing Jupiter's orbit in a stable 1:1 resonance, the comet sits 21 degrees ahead of the planet and will continue drifting roughly 30 degrees further before looping back. It made a close approach on 17 February 2017 at 0.092 AU and is expected to do so again in 2028, bookending what appears to be a short quasi-satellite cycle. A 2021 study predicted a very close Jupiter encounter on 23 January 2063, possibly as tight as 0.016 AU. By March 2026, JPL had refined that to 13 January 2063 with a minimum approach of 0.052 AU and a maximum of 0.076 AU, though predictions beyond that flyby remain uncertain.

Physical Characteristics and Cometary Behavior

Estimating P/2019 LD2's true size proves difficult. A generic magnitude-to-diameter conversion assuming a 0.12 albedo and absolute magnitude of 12.2 yields roughly 14 kilometers, but cometary activity can inflate apparent brightness and overstate the nucleus. DECam archival images from March 2017, when the object was dimmer than magnitude 23.8, suggest a radius under 1.2 km assuming a 0.05 albedo or under 0.8 km at 11.2 percent. Broadband observations from Sanglokh Observatory in Tajikistan set a revised upper limit near 6.1 ± 0.1 km. As of May 2020, no rotational light curve had been obtained, leaving the period, pole, and shape unknown. The visible spectrum showed no CN, C2, or C3 emission. Yet during its 2020 perihelion approach, the comet shed water-ice-rich dust grains typically around 0.1 mm in size, confirming genuine cometary outgassing despite its asteroid-like appearance.

Observational History and Ongoing Monitoring

P/2019 LD2's observation arc stretches well before its official discovery. A precovery image from the Pan-STARRS 1 survey at Haleakala Observatory, taken on 21 May 2018, predates the ATLAS-MLO detection by eleven months. After the initial June 2019 detection, Las Cumbres Observatory confirmed the coma and tail within days. Remarkably, ATLAS-MLO images in April 2020 still showed the cometary appearance, implying nearly a year of continuous activity. The University of Hawaiʻi's press release on 20 May 2020 announced the trojan claim, but the reclassification followed just two days later. The body's connections to objects like 6478 Gault and 118401 LINEAR underscore its place in a broader family of active small bodies. Despite these observations, fundamental properties like rotation remain elusive, and the 2063 Jupiter encounter will likely reshape its orbit in unpredictable ways.