A tale of two comets – Waiting for Siding Spring

This is a tale of two comets. One is a periodic comet, the other a newcomer to the inner Solar System. One has a well known and predictable orbit of six and a half years, the other is yet to complete its first million-year-long loop along a new orbit that has taken it, for the first time, in the vicinity of the Sun. One has been observed and studied for many years, while the other stayed well hidden until it popped up unannounced just over a year ago. These two comets are quite different. The predictable one is 67P/Churyumov-Gerasimenko (67P/C-G), a comet that was discovered in 1969 and is currently being visited and studied up close by ESA’s Rosetta spacecraft. With an orbital period of under 20 years and a low inclination, 67P/C-G is classed as a “Jupiter-family” comet — we know of many comets belonging to this family. The other comet is C/2013 A1, also known as comet Siding Spring after the observatory in Australia where it was first observed in January 2013. (Note: another comet discovered in 2007 at the same observatory is also known as Siding Spring: comet C/2007 Q3.) Comet Siding Spring is currently on its first journey to the inner Solar System; it has spent most of its life in the Oort cloud, a gigantic reservoir of comets that surrounds the Solar System and stretches outwards to roughly 100,000 AU from the Sun — where 1 AU is the Earth-Sun distance. Comets in the Oort cloud are dormant, but even a small perturbation can modify their orbits and kick them into the inner Solar System; this is what happened to comet Siding Spring a few million years ago. When this comet was first discovered, it was 7.2 AU from the Sun, placing it somewhere between […]



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