JWST Observations of Comet C/1995 O1 (Hale-Bopp) at 46 au

Great comet C/1995 O1 (Hale-Bopp) is a long-period comet and one of the largest cometary nuclei known. Being a bright naked-eye object in 1997, its presence in the night sky spurred many investigations into the comet’s composition, behavior, and dynamical properties. Owing to its large size, detailed studies have been possible over a wide range of heliocentric distances. The comet appeared to be last active in Spitzer Space Telescope mid-infrared observations taken in September 2008 at 27 au from the Sun (Kramer et al. 2014, Icarus 236, 136). Soon thereafter, Szabo et al. (2012, ApJ 761, 8) showed with Hubble Space Telescope, Very Large Telescope, and Herschel Space Telescope data that the comet’s activity was undetectable by September 2009 (28 au) and that the nucleus had a geometric albedo of 8.1±0.9% in the R-band. This albedo is slightly larger than the range observed at other comets (2 to 7%; Kokotanekova et al. 2017, MNRAS 471, 2974), but consistent with the brightest nuclei within the given uncertainty.

To investigate the cause of Hale-Bopp’s high albedo and test the hypothesis that the surface may be brightened by exposed water ice, we observed the comet with the James Webb Space Telescope. Images were taken with the NIRCam instrument through medium-band filters at 1.8 and 3.6 μm on 2022 July 9. The comet was 46.2 au from the Sun and at a phase angle of 1.2 deg. We compare the data to model point spread functions and find no strong evidence for activity in either filter, which implies we are observing the cometary nucleus. Our preliminary photometric analysis confirms the high albedo, 9% at 1.8 μm, but with large uncertainties (±2% full-range) dominated by the unknown rotational context of the nucleus rather than the photometric uncertainties. Astrometry of the comet (~10 mas uncertainty on the 26 mag source) and subsequent orbital update indicates that our observations with NIRSpec in September 2022 will not be adversely affected by ephemeris uncertainties and anticipate that spectral observations from 0.6 to 3 μm will be successful. Preliminary results from the spectroscopy will be presented. We conclude that JWST will be an excellent resource for the study of cometary surfaces.

Support for this work was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127.