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.
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 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 the Martian atmosphere during the comet’s approach.
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.
Europe’s Mars orbiter and its scientific instruments will have a front-row seat on Sunday when Comet Siding Spring grazes the Red Planet, skimming past at a little more than a third of the Moon’s distance from Earth.
Comet Siding Spring, discovered in January 2013, is less than a kilometre across and will pass Mars at 56 km/s, closing to within 139 500 km at 18:27 GMT (20:27 CEST) on 19 October.
Updates will be posted here in the Mars Express blog and a webcast from ESA’s Space Operations Centre, ESOC, in Darmstadt, Germany, will be streamed live 19 October, starting at 17:50 GMT (19:50 CEST).
OK, well… you can’t actually see Mars Express – but it’s there! And you can see the Red Planet and Comet C/2013 A1 Siding Spring very, very well in this image taken by Kos Coronaios, 12 October, at 17:30 UTC. Great work, Kos!
An excellent photo showing comet Siding Spring and Mars in the same image, with the close flyby location marked with red arrows. Many thanks and well done to Rob Kaufman, Victoria, Australia.
This image shared via the Facebook page of the Coordinated Investigations Of Comets (CIOC) Siding Spring group – thanks guys!
A short time lapse of Comet 2013 A1 (Siding Spring); imaged between 20:25 and 10:03pm AEST, 16 October 2014, acquired by Jeanette Lamb, Brisbane, Australia. Great work, Jeanette!
This time-lapse shared via the Facebook page of the Coordinated Investigations Of Comets (CIOC) Siding Spring group – thanks guys!
Mars Express timeline around Siding Spring
- ERT: Earth receive time
- LOS: Loss of signal
- AOS: Acquisition of signal
- Beacon: Continuous ‘beacon’ signal transmitted via MEX low gain antenna to confirm spacecraft functioning as expected
On 14 October at 16:18 CEST, Comet Siding Spring will be 25,001,331 km from the centre of Mars, 24,997,942 km from the surface of Mars (mean) and 24,997,399 km from Mars Express, closing at a speed of -55.88 km/second.
For the Comet Siding Spring encounter this weekend, the Mars Express mission team have decided to not execute the initial contingency plan of pointing the high gain antenna toward the expected direction of comet particles and switching off all non-essential systems. Instead, we will be taking advantage of this remarkable opportunity to perform science observations of the comet – yet with a few precautions.
Siding Spring will pass by Mars at 139 500 km – a little more than
1/3 the distance between Earth and our Moon.
The final decision was made at the beginning of October. As we had two plans ready – one dubbed ‘nominal science’ and one ‘contingency’ – we were able to make this decision close to the encounter date and take advantage of the latest estimates of comet activity.
In creating the weekly observation plan for the spacecraft, several teams and many different factors are involved. Not only do we have to decide on where to point the spacecraft but we also have to take into account the amount of data generated, ground station availability, power requirements, thermal conditions, pointing restrictions and several others. Each of these is a lot of work in itself and trying to have two plans that were as similar as possible kept our mission planning team quite busy. This is why we have to plan everything far in advance.
So, the plan we will use is much like our standard weekly operations, but with a few additional precautions:
- NASA’s DSN deep space tracking network are providing excellent support and will be allocating all of their antennas at both their Madrid and Goldstone sites purely to support the ESA, NASA and ISRO Mars missions. As well as using two of their 34m antennas to perform dumps of science data, we will be using the two giant 70m antennas to track a beacon signal that will be transmitted from one of the Mars Express low gain antennas (this was originally part of our contingency plan, but will be used in this now-routine science plan to give us an indication that all is well on the spacecraft).
- We will also perform additional checks to the spacecraft systems. Once we get our first data after the comet encounter, engineers will go through all subsystems to be sure that everything is nominal and compare system status values to default values recorded earlier.
- The Data Management Systems Engineers will dump all the data from our computer systems on Monday to be sure that nothing has become corrupted and our Power/Thermal Engineers will perform a test the week after flyby to measure any change in the performance of our solar arrays.
We are not expecting to find any problems but rather perform these tests so as to be sure.
MEX science during Siding Spring
Presentation by H. Svedhem, ESA’s mission scientist for MEX, detailing the plans for science during Siding Spring flyby this week and next.
This presentation was given at the Comet Siding Spring (C/2013 A1) Science Workshop, NASA JPL, 19 September 2014.
The recording of the workshop features Hakan here:
From 1:17:00 to end
Editor’s note: Those who have been following our blog will know that the MEX Flight Control Team at ESOC have been actively preparing for the flyby of comet C/2013 A1/Siding Spring on 19 October. Initial estimates gave the possibility that Mars Express might have to contend with a large particle flux – and that several (2? 3?) very high-speed (~56 km/sec!) particles might bash into the spacecraft. Happily, additional observations by ground and space telescopes (including the ESA/NASA Hubble Space Telescope) have allowed initial estimates to be refined and the risk is now understood to be much lower – and perhaps even as low as zero. In today’s blog post, the team explain how this (happy!) real-life, real-time development is affecting their preparations for fly-by.
Late last year, estimates given in scientific papers estimated that over the duration of the encounter, the number of large cometary particles per square metre would be around 1. As MEX’s area in the most protected attitude is about 3m2, we could then expect about 3 potentially significant impacts. Not good!
By the middle of this summer, published estimates (based on new images and additional modelling) were indicating a flux of around 10-6 particles per m2, which, for Mars Express, very roughly equates to a 1-in-300,000 chance of being hit. It’s starting to look like our comet C/2013 A1/Siding Spring will manifest itself as a more friendly passer-by than initially thought and that it won’t be hurling clouds of large particles at unthinkable speeds towards Mars and its man-made satellites.
So why the big change?