308P/Lagerkvist–Carsenty

Lore & Background

The comet was discovered as an asteroid-like object from photographic plates taken during a joint astronomical survey between the Uppsala Astronomical Observatory and the European Southern Observatory on the night of 5 October 1997. At the time, the comet formerly known as P/1997 T3, was a 19th-magnitude object within the constellation Pisces. It was initially thought to be a Jupiter trojan with no apparent activity, until further orbital calculations had shown it was not gravitationally bound to Jupiter itself, but in a centaur-like orbit as far away as Saturn's distance to the Sun.

On 29 July 2014, Erwin Schwab and his colleagues at the Tenerife Observatory later recovered the comet on its second apparition as P/2014 O2, where like its 1997 apparition, it exhibited very little cometary activity. Orbital reconstructions in 2000 by the discoverers themselves had revealed that the comet made a close encounter with Saturn on 12 October 1954, where it approached the giant planet at a distance of 0.0104 AU (1.56 million km). The encounter had turned the orbit of the former centaur into that of a Jupiter-family comet in the present day.

The comet initially did not have a formal name since it was thought to be an asteroid upon discovery. However, by January 1998, the International Astronomical Union (IAU) had decided to rename P/1997 T3 as Lagerkvist–Carsenty, after the two discoverers who recognized its cometary nature before its discovery was announced. The comet received its official numerical designation, 308P, in January 2017.

Reader's Guide

308P/Lagerkvist–Carsenty is significant as a Jupiter-family comet that originated as a centaur before a close encounter with Saturn in 1954 altered its orbit. Its discovery as an asteroid-like object with no apparent activity highlights the difficulty in distinguishing comets from asteroids at large distances. The comet's recovery in 2014 by Erwin Schwab and colleagues confirmed its low activity persists. The naming by the IAU in January 1998 honors the discoverers who recognized its cometary nature. The comet's orbit, with perihelion at 4.2 AU and aphelion at 9.01 AU, places it between Jupiter and Saturn, and it will make two close encounters with Jupiter by 2085 and 2188. Its legacy includes serving as an example of a former centaur transitioning to a short-period comet, similar to 450P/LONEOS.

Discovery and Confirmation of a Hidden Comet

In the winter of 2003, Brian A. Skiff was running routine CCD imaging for the Lowell Observatory Near-Earth-Object Search when an 18th-magnitude smudge caught his eye. On the plates it looked no different from a background star, yet its slight displacement between exposures was enough to trigger a request for follow-up. H. R. Miller at Perkins Observatory and J. Young at Table Mountain Observatory soon resolved the object and reported a faint coma roughly four arcseconds across, accompanied by a wispy tail stretching about ten arcseconds. That confirmation of cometary activity came while the body was already sitting at roughly 5.54 AU from the Sun, a distance where most comets are expected to be dormant. The Minor Planet Center published the first orbital elements on 13 January 2004, and precovery frames from Kitt Peak pushed the detection window back to mid-December 2003. The find added another name to LONEOS's growing list of comets identified through systematic sky surveys rather than visual serendipity.

A Saturn Flyby That Rewrote the Orbit

The comet's present 22-year period is not its original one. Orbital back-integration points to a dramatic gravitational encounter with Saturn in 1992, when the small body passed within just 0.031 AU—about 4.6 million kilometers—of the ringed giant. That close passage shaved a staggering 5.2 AU, or roughly 780 million kilometers, off the semimajor axis, yanking the object into a tighter solar orbit. Long-term numerical work suggests this was the comet's very first foray into the inner Solar System, marking its transition from a centaur-type body wandering the outer regions into what is now classified as a Jupiter-family comet. Its current trajectory is inclined about 10.65 degrees to the ecliptic, with perihelion near 5.45 AU and aphelion reaching 10.44 AU. The story is not finished: on 11 July 2026 the comet will swing past Jupiter at a distance of 0.5 AU, an encounter expected to compress its orbit even further and shorten its period once again.

Reading the Nucleus Through Dust and Gas

Pinpointing the true size of a distant, faintly active comet is notoriously difficult, and 450P/LONEOS proved no exception. A 2011 analysis of its dust output placed the nucleus radius somewhere between 0.5 and 3.5 kilometers—a wide range that reflected the uncertainty inherent in extrapolating from a single apparition. The comet then entered a quiet, inactive phase lasting from 2019 through 2024, during which both the Gemini Observatory and the James Webb Space Telescope captured thermal and reflected-light data. Those observations narrowed the estimate to approximately 1.8 ± 0.5 km, a far more constrained figure. During its 2004 apparition the nucleus produced an asymmetrical coma and a solar-directed anti-tail that stretched up to 150,000 km. By 2022, however, dust production had dropped to roughly 4.8 kilograms per second, a rate attributed primarily to carbon dioxide outgassing triggered when amorphous water ice crystallized at temperatures in the 140-to-160-kelvin range.

Recovery, Designation, and What Comes Next

After nearly two decades of silence, the comet was successfully tracked again in August 2022 using the Gemini North Observatory. Astronomers noted a notable discrepancy: the object was arriving about 4.5 magnitudes fainter than models had predicted, a reminder that long-period comets can behave in ways that challenge even refined activity models. The recovery confirmed the return predicted by its 22-year period and earned the body its second provisional label, P/2022 Q3, complementing the original P/2004 A1. In February 2023 the Minor Planet Center granted it the permanent numerical designation 450P, bestowed in the same batch as eight other comets that had completed their first full orbits. With the 2026 Jupiter encounter now on the calendar, observers will be watching closely to see how the orbit tightens and whether the already modest dust output changes character as the comet settles into an even shorter period around the Sun.