Letter to the Editor

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

¹ Max Planck Institute for Solar System Research, Max-Planck-Strasse 2, 37191 Katlenburg-Lindau, Germany e-mail: snodgrass@mps.mpg.de
² European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago de Chile, Chile
³ Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
⁴ NASA Astrobiology Institute
⁵ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany

Received: 14 April 2010
Accepted: 10 May 2010

Abstract

Context. 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.

Aims. 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.

Methods. 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.

Results. We recovered the comet near the expected position, with a magnitude of m_R = 23.74 ± 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 ≤1 km for an assumed 4% albedo; we estimate a ~6% albedo. We measure a colour of (V-R) = 0. 26 ± 0.09.