An Amateur-Professional International Observing Campaign for the EPOXI Mission: New Insights Into Comets (Abstract)

Volume 40 number 1 (2012)

Karen J. Meech

46-035 Konohiki Street, #3865, Kaneohe, HI 96744; meech@ifa.hawaii.edu

Abstract

(Abstract only) Comets are leftovers from the early solar system and may have played a role in delivering water and organics to the prebiotic Earth. Because comets may preserve a record of the early solar system conditions, they are the focus of small body missions. The EPOXI (Extrasolar Planet Observation and Characterization (EPOCh) + Deep Impact Extended Investigation (DIXI) = EPOXI) flyby of the nucleus of comet 103P/Hartley 2 provided us with physical properties of the nucleus and clear evidence of chemical heterogeneity with CO2-driven jets as a dominant volatile loss mechanism at perihelion compared to subsurface water-ice sublimation. An international Earth-based observation campaign played a complementary role to the in-situ data, providing recovery images of the comet at large distances, physical information about the nucleus size, and from a coordinated multiwavelength program nearly continuous coverage from August 2010 through encounter on 4 November 2010. From the Earth-based campaign it was clear that comet Hartley 2 had a small nucleus (0.57 km radius), with a rotation period near 16.4 hours prior to the onset of activity. As the activity developed the periodicity was found to change significantly over a period of months. The highly active nucleus had long- and short-term gas production variability with peak activity shortly after perihelion. The comet’s activity has been photometrically monitored (as scattered light from the dust coma) from the time of recovery to the present, and the nearly continuous coverage of the comet from August 2010 into 2011 would not have been possible without the amateur contributions. Using these brightness data, we have developed an ice sublimation model to estimate the amount of dust emitted from the comet (and hence the total scattered light) as a function of heliocentric distance as it is driven by a gas flow. The model includes nucleus ices: H2O, CO2, CO, and H2O sublimating from the large chunks seen both from the EPOXI spacecraft and the Arecibo radar observations (Harmon et al. 2011). The model indicates that like other comets, water-ice sublimation began to create an observable dust coma/tail near 4–4.4 AU as the comet approached the sun, but that near perihelion, strong CO2 outgassing in the form of jets (as seen by the spacecraft) was responsible for lifting large ice/dust grains from the surface. CO2 is likely a strong contributor to activity on the outbound leg of the orbit. The models show that the fractional active nucleus area is small for water production (typical of other comets) and that at perihelion most of the water production is likely from the ice grain halo. Sublimation from deeper CO2 reservoirs is likely an important driver of activity for this comet, including out to and beyond aphelion, and this may be a characteristic of unusually active comets—relating to differences in chemistry from either formation or subsequent evolution. This paper will present mission highlights, and emphasize the important role that the amateur observations has in understanding the behavior of this comet.