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.


Comet Siding Spring observed on 19 September 2014 using the European Southern Observatory’s Very Large Telescope (VLT). At the time, the comet was at 1 AU from the Earth and 1.494 AU from the Sun. Credit: ESO/A. Guilbert-Lepoutre

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.


The different orbits of two periodic comets, 67P/C-G and 1P/Halley, and the Oort cloud comet Siding Spring. Credit: ESA

When this comet was first discovered, it was 7.2 AU from the Sun, placing it somewhere between the orbits of Jupiter and Saturn. It will reach its closest point to the Sun, at about 1.4 AU, around the orbit of Mars, on 25 October 2014, before heading back out towards the outskirts of the Solar System.

Comets are fossils from the Solar System’s early days, and still contain some of the primordial material that made up our system’s planets. While periodic comets, with their frequent passes close to the Sun, may have evolved and lost part of the original material, Oort cloud comets are more pristine, having spent billions of years untouched, far away from the Sun. Their material has been much less processed, making them an even richer trove of information about the formation of the Solar System 4.6 billion years ago.

However, Oort cloud comets are extremely unpredictable and, unlike their dependable Jupiter-family relatives, they can come from any direction in the sky. When distant from the Sun, Oort cloud comets are very dim and difficult to observe, so they are usually discovered with extremely short notice before they whirl around to begin their trip back to oblivion. Astronomers can study them from ground-based observatories during this short interval – a few years at most – but it is virtually impossible to plan a space mission to fly near to such a comet.

This is where comet Siding Spring stands out. One week prior to its closest approach to the Sun, the comet will also pass extremely close to Mars. In fact, after its discovery, scientists even considered the possibility of a collision with the Red Planet – an option that was excluded as soon as the comet’s orbit was determined with greater accuracy. The encounter will be of a very close kind; on 19 October, comet Siding Spring will pass about 140 000 km from Mars.

What makes this close approach truly historic is the presence of various spacecraft currently operating at Mars. Siding Spring’s close encounter offers these craft an unforeseen perk: to perform a flyby of an Oort cloud comet.

The distance that will separate comet Siding Spring from Mars and the fleet of spacecraft around it does not even begin to compare to the mere tens of kilometres that are currently between Rosetta and comet 67P/C-G [*]. Spacecraft that flew by comets in past decades have also reached much closer distances, ranging from a few tens to a few hundreds of kilometres.

Those were all periodic comets though, and this kind of flyby is not an option for an unpredictable Oort cloud comet. The spacecraft on and around Mars have a truly unique opportunity to study such a comet from a distance that could hardly ever be achieved otherwise. The orbiters will observe Siding Spring’s nucleus and its enveloping coma, as well as monitoring the Martian atmosphere during the comet’s approach.


Artist’s impression of the Mars Express spacecraft in orbit. Credit: ESA/Medialab

Joining the observing campaign as part of ESA’s Solar System fleet will be Mars Express, which is currently orbiting the Red Planet. This spacecraft has started observing comet Siding Spring last Monday, and many more observations are planned for the coming days just before and after closest approach. The comet’s activity is currently moderate, meaning that the risk of dust grains impacting Mars Express is extremely low — this will allow the spacecraft to operate safely during the comet’s closest approach to Mars.

The High Resolution Stereo Camera (HRSC) on Mars Express will image the comet’s nucleus, although it is unlikely to resolve it fully. The camera will also be used to observe the coma and to study the behaviour of the comet’s trailing debris of small meteors. Other instruments on Mars Express will perform measurements to study the composition of the coma and to learn how the cometary material interacts with both the Martian atmosphere and the wind of charged particles streaming outwards from the Sun.

Read more about Europe’s Mars orbiter observing Siding Spring from the perspective of Spacecraft Operations in the Mars Express blog. Updates will be posted on the blog leading to closest approach, and a webcast from the European Space Operations Centre, Darmstadt, Germany, will be streamed live 19 October, starting at 19:50 CEST.

Other missions that will participate in the observing campaign are NASA’s Mars Reconnaissance Orbiter, Mars Odyssey orbiter, and the Mars Atmosphere and Volatile EvolutioN Mission (MAVEN), and the Indian Space Research Organisation’s Mars Orbiter Mission – the last two having only reached the planet in late September 2014. NASA’s two rovers, Curiosity and Opportunity, will also observe the comet from the surface of Mars.

This rare opportunity to get up close and personal with an Oort cloud comet will allow astronomers to collect different, but complementary, information to that gleaned from observing periodic comets. This is not a tale of one comet, or even of two – it is the tale of our Solar System and its history. The more comets we explore, the more clues we can gather to piece it all together.

[*] While the current orbit of comet 67P/C-G takes it quite close to the Sun, with closest approach (perihelion) at about 1.3 AU and an orbital period of only six and a half years, this has not always been the case. In fact, analysis of the comet’s orbital evolution suggests that 67P/C-G has only been on this orbit since 1959, when a close encounter with Jupiter brought it closer to the Sun; prior to that, its perihelion was at just under 2.8 AU. This comet has quite a history of close interactions with Jupiter: orbital calculations show that its perihelion was as distant as 4 AU until 1840. This means that 67P/C-G is only a recent dweller of the inner Solar System, and so its material remains reasonably unprocessed.