Comet C/2013 A1 (Siding Spring) as seen with the Herschel Space Observatory^⋆,⋆⋆

¹ Konkoly Observatory, MTA Research Centre for Astronomy and Earth Sciences, Konkoly Thege 15-17, 1121 Budapest, Hungary
e-mail: kiss.csaba@csfk.mta.hu
² Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, Giessenbachstr., 85741 Garching, Germany
³ Herschel Science Centre, ESAC, European Space Agency, 28691 Villanueva de la Cañada, Madrid, Spain
⁴ STAR, Stockholm Amateur Astronomers, Drottninggatan 120, 11360 Stockholm, Sweden

Received: 8 October 2014
Accepted: 23 December 2014

Abstract

The thermal emission of comet C/2013 A1 (Siding Spring) was observed on March 31, 2013, at a heliocentric distance of 6.48 au using the PACS photometer camera of the Herschel Space Observatory. The comet was clearly active, showing a coma that could be traced to a distance of ~10′′, i.e. ~50 000 km. Analysis of the radial intensity profiles of the coma provided a dust mass and dust production rate and the derived grain size distribution characteristics indicate an overabundance of large grains in the thermal emission. We estimate that activity started about 6 months before these observations at a heliocentric distance of ~8 au.