The nucleus of 103P/Hartley 2, target of the EPOXI mission

TL;DR

This study observed comet 103P/Hartley 2 near aphelion using VLT, confirming its intact nucleus with faint activity, estimating its size and albedo, and providing essential data for the EPOXI mission planning.

Contribution

First optical detection of 103P/Hartley 2's nucleus near aphelion, with size, albedo, and activity measurements to aid mission planning.

Findings

Nucleus radius estimated at ≤1 km.

Detected faint activity fading over time.

Estimated albedo around 6%.

Abstract

103P/Hartley 2 was selected as the target comet for the Deep Impact extended mission, EPOXI, in October 2007. There have been no direct optical observations of the nucleus of this comet, as it has always been highly active when previously observed. We aimed to recover the comet near to aphelion, to a) confirm that it had not broken up and was in the predicted position, b) to provide astrometry and brightness information for mission planning, and c) to continue the characterisation of the nucleus. We observed the comet at heliocentric distances between 5.7 and 5.5 AU, using FORS2 at the VLT, at 4 epochs between May and July 2008. We performed VRI photometry on deep stacked images to look for activity and measure the absolute magnitude and therefore estimate the size of the nucleus. We recovered the comet near the expected position, with a magnitude of m_R = 23.74 \pm 0.06 at the first epoch. The comet had no visible coma, although comparison of the profile with a stellar one showed that there was faint activity, or possibly a contribution to the flux from the dust trail from previous activity. This activity appears to fade at further epochs, implying that this is a continuation of activity past aphelion from the previous apparition rather than an early start to activity before the next perihelion. Our data imply a nucleus radius of \le 1 km for an assumed 4% albedo; we estimate a ~6% albedo. We measure a colour of (V-R) = 0. 26 \pm 0.09.