Preview of Comet C/2021 A1 (Leonard) and Its Encounter with Venus

Abstract

Long-period comet C/2021 A1 (Leonard) will approach Venus to within 0.029 au on 2021 December 18 and may subsequently graze the planet with its dust trail less than 2 days later. We observed C/2021 A1 with the Lowell Discovery Telescope on 2021 January 13 and March 3, as well as with the Palomar Hale Telescope on 2021 March 20, while the comet was inbound at heliocentric distances of r = 4.97, 4.46, and 4.28 au, respectively. Tail morphology suggests that the dust is optically dominated by similar to 0.1-1 mm radius grains produced in the prior year. Neither narrowband imaging photometry nor spectrophotometry reveal any definitive gas emission, placing 3 sigma upper bounds on CN production of less than or similar to 10(23) molec(-1) at both of the latter two epochs. Trajectory analysis indicates that large (greater than or similar to 1 mm) grains ejected at extremely large heliocentric distances (r greater than or similar to 30 au) are most strongly favored to reach Venus. The flux of such meteors on Venus, and thus their potential direct or indirect observability, is highly uncertain, as the comet's dust production history is poorly constrained at these distances but will likely fall well below the meteor flux from comet C/2013 A1 (Siding Spring)'s closer encounter to Mars in 2014 and thus poses a negligible risk to any spacecraft in orbit around Venus. Dust produced in previous apparitions will not likely contribute substantially to the meteor flux, nor will dust from any future activity apart from an unlikely high-speed (greater than or similar to 0.5 km s(-1)) dust outburst prior to the comet reaching r approximate to 2 au in 2021 September.