C/2023 H2 (Lemmon)

Lore & Background

The comet was first detected as an asteroidal object with an apparent magnitude of about 21 in images taken by the Mount Lemmon Survey on 23 April 2023. Precovery images from PanSTARRS dating back to 26 March 2023 were later identified. Cometary activity became apparent on 14 May 2023, when a coma about 12 arcseconds across was observed. The comet reached perihelion on 29 October 2023, and on 10 November 2023 it made a close approach to Earth at 0.19 AU, moving nine degrees across the sky in a single day. During this encounter, it brightened to magnitude 6 and displayed a weak ion tail.

The close approach allowed rare measurements of the comet's composition. The water production rate was estimated at (59±7)×10^26 mol/s, unusually low compared to other Oort Cloud comets at similar distances. Production rates for hydroxyl radical and dust were also low. However, the comet exhibited very high mixing ratios of carbon monoxide relative to water (16.3±0.6%), along with high abundances of ethane, methane, hydrogen cyanide, and methanol. These hyper-volatile species likely played a key role in driving the comet's activity.

Reader's Guide

C/2023 H2 (Lemmon) holds significance primarily for the compositional data obtained during its close Earth flyby. The encounter provided a rare opportunity to measure the chemical makeup of an otherwise faint comet. The unusually low water production rate, combined with very high abundances of hyper-volatile species such as carbon monoxide, ethane, and methane, suggests that these compounds may have been the primary drivers of the comet's activity rather than water ice. This challenges typical assumptions about the behavior of Oort Cloud comets at similar heliocentric distances. The comet's unexpected brightening to magnitude 6, far above predictions, further underscores the role of these volatiles in generating activity. The measurements of carbon monoxide at 16.3% relative to water, along with elevated levels of other organic molecules, provide a valuable data point for understanding the diversity of comet compositions. The comet's orbit is noted as similar to that of C/1975 T2 (Suzuki–Saigusa–Mori), placing it in a broader dynamical context.

Did You Know?

• It approached Earth to a distance of 0.19 AU (28 million km) on 10 November 2023.
• The water production rate was unusually low at (59±7)×10^26 mol/s compared to other Oort Cloud comets.
• It displayed a very high mixing ratio of carbon monoxide relative to water at 16.3±0.6%.