ESA Science & Technology - Herschel's view of Comet Siding Spring

These three images show emission from the dust in the coma surrounding the nucleus of Comet C/2013 A1 – also known as Comet Siding Spring – as observed at three different far-infrared wavelengths with ESA's Herschel space observatory.

Discovered on 3 January 2013, Comet Siding Spring is an Oort Cloud comet on its first journey into the inner Solar System. It will reach perihelion – its closest approach to the Sun – on 25 October 2014 at 1.4 AU (about 210,000,000 km). Having spent most of its life far from the Sun, this comet is much more pristine than periodic comets – those that orbit the Sun every two hundred years or less – and for that reason is particularly interesting to study. 

On 31 March 2013, not long after it was discovered, astronomers observed Comet Siding Spring with Herschel. This was just one month before the observatory exhausted its supply of liquid helium coolant and ceased to collect data. When Herschel observed it, the comet was about 6.5 AU from the Sun. The observations were performed following a proposal for Director’s Discretionary Time from Peter Mattisson from the Stockholm Amateur Astronomers (STAR) in Sweden.

The three panels show the comet at wavelengths of 70 microns (shown in blue), 100 microns (shown in green) and 160 microns (shown in red). Telescopes observing at these long wavelengths see the direct thermal emission from dust in the comet's coma.

The coma is resolved at the two shorter wavelengths (in the left and central panels). Close inspection of these two images reveals that the coma's shape is slightly elongated towards the left – in the direction opposite the Sun. From these images, astronomers estimated that the coma extends some 50,000 km from the comet's nucleus. The structure of the coma can hardly be resolved at the longest wavelength probed by Herschel (in the right panel).

These observations were also used to calculate the total mass of dust in the coma, which amounts to about 300,000,000 kg. At the time of the Herschel observations, the comet appeared to be quite active – astronomers estimated that the activity had begun even prior to the comet's discovery, when it was about 8 AU from the Sun. Observations performed at a later stage with space and ground-based telescopes showed that the comet's activity has increased quite slowly over the past months, which is quite unusual for an Oort Cloud comet. There are even some hints that the comet's activity has declined recently. 

Astronomers have been closely monitoring the activity of Comet Siding Spring because, a few days before perihelion, the comet will have an historic close approach to Mars, passing some 140,000 km from the Red Planet on 19 October 2014. The comet's current moderate activity is good news for the fleet of spacecraft that are operated at Mars by various space agencies (including ESA's Mars Express) because it means a low risk of dust particles hitting the instruments on board. 

Since Oort Cloud comets are discovered with an extremely short notice before perihelion – a few years at most – it is virtually impossible to plan a space mission to fly by such a comet. This is what makes Comet Siding Spring and its closest approach to Mars truly unique, as the spacecraft at Mars will have the chance to observe an Oort Cloud comet from a distance that could not possibly be achieved otherwise.

The analysis of the Herschel images was performed by Cs. Kiss (Konkoly Observatory, Budapest, Hungary), T.G. Müller (Max-Planck-Institut für extraterrestrische Physik, Garching, Germany), M. Kidger (ESAC, European Space Agency, Madrid, Spain), P. Mattisson (STAR, Stockholm Amateur Astronomers, Sweden), and G. Marton (Konkoly Observatory, Budapest, Hungary).