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

TL;DR

This study reports Herschel Space Observatory observations of comet C/2013 A1 (Siding Spring), revealing its activity, dust properties, and activity onset at 8 au, with detailed analysis of its coma and dust production.

Contribution

First thermal emission observations of comet C/2013 A1 with Herschel, providing insights into dust composition and activity timing at large heliocentric distance.

Findings

Detected a coma extending to 50,000 km from the nucleus.

Estimated dust production rate and grain size distribution showing large grain overabundance.

Activity likely started around 6 months before observation at 8 au.

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. 50000 km. Analysis of the radial intensity profiles of the coma provided dust mass and dust production rate; 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.