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Karl Battams

Comet Siding Spring: Exciting times

Posted by Karl Battams

06-10-2014 15:11 CDT

Topics: comets, comet Siding Spring, Mars

This article originally appeared on Karl Battams' blog and is reposted here with permission.

We're now less than two weeks away from Comet Siding Spring buzzing ridiculously close to Mars, and the excitement is building - in both good and bad ways...

Of course we're all tremendously thrilled at the prospect of the comet flying so close to so many wonderful spacecraft at Mars. We're scientists, and we live and breathe for stuff like this. If you've followed either of our two workshops, you'll know that pretty much the entire Martian fleet of spacecraft and rovers are going to be attempting to take observations of the comet. Granted they are not at all designed for looking at comets but many of the methods and instruments they use to study Mars and its atmosphere will be almost equally at home studying a comet at it zips past them.

One of my personal favorites is MAVEN, which I really hope will see some great interactions between the Martian atmosphere and the comet's extended gassy atmosphere. Just as exciting is the prospect of the Mars Reconnaissance Orbiter's HiRISE camera being able to actually resolve (i.e. determine the shape) of the nucleus of the comet! Of course, we've seen comet nuclei before - indeed, ESA's Rosetta is returning jaw-dropping images of comet 67P - but comet Siding Spring is a little different. All of the comet nuclei we've observed so far have been periodic comets - objects that stay relatively close to our solar system and swing predictably past the Sun every few or few dozen years. They've been doing this for centuries or millennia, which means their surface has undergone substantial changes since their formation a few billion years ago.

Scale comparison of comets visited by spacecraft as of 2014

Image credits: Halley: Russian Academy of Sciences / Ted Stryk. Borrelly: NASA / JPL / Ted Stryk. Tempel 1 and Hartley 2: NASA / JPL / UMD. Churyumov-Gerasimenko: ESA / Rosetta / NavCam / Emily Lakdawalla. Wild 2: NASA / JPL. Montage by Emily Lakdawalla.

Scale comparison of comets visited by spacecraft

Comet nuclei we've observed so far have all been periodic comets. Are Oort Cloud comets substantially different? We don't know, but hope to find out...

Comet Siding Spring is different - it's an Oort Cloud comet, dynamically new, and on its first ever foray into our solar system. This means it is largely pristine and will likely not have undergone any major changes since it formed. We've never seen one of these comets up close. Never. We don't know exactly what to expect.

The comet nuclei we've looked at so far have all been extremely black. Ridiculously so, in fact, like the blackest coal you can imagine, and we tend to assume that this is how all comet nuclei are.

But is it really true that all comets like that? Could this blackening perhaps be something that happens after long periods exposed to solar radiation (sunlight), much like a steak left too long on the grill? Could it perhaps be that fresh Oort Cloud comets like Siding Spring have a much brighter surface? We truly don't know, so this will be another fascinating result.

I do want to forewarn you that the images HiRISE will take will not offer anywhere near the level of detail you're used to seeing from true comet encounter missions. We don't know how big Siding Spring's nucleus is but assuming it's a typical size of a couple of kilometers (or miles) or so, then it should cover maybe 7 or 8 pixels on the HiRISE camera. That's enough to give us an idea of overall shape, but not enough to show surface details. It will allow us to figure out exactly how big it is though, so that's another bonus. Either way, it should be an amazing encounter for these and many other reasons.


OK, I hate to do this but we've been here before. Remember that whole Comet ISON incident? We all know how that ended. No kidding, I have met face-to-face, and apologized to, children who literally cried themselves to sleep on the day comet ISON fizzled out. I don't think anyone will have quite that level of emotional investment this time around, but I want to throw out some warnings early on. Comet Siding Spring - just like Comet ISON - is an Oort Cloud comet and right now it is behaving exactly as Oort Cloud comets do: like a big ol' unpredictable pain in the you-know-what!

Last week Matthew gave a great account on the current behavior of the comet, which can be simply summarized as "unpredictable". For some reason, over the past couple of weeks it has dropped quite significantly in brightness. We knew it would drop a little at this time, but this seems more dramatic than we'd have though. I urge you to read Matthew's post for all the details and ramifications, but the bottom line is that we are more-or-less clueless right now on what it will do over the next couple of weeks. It could dramatically brighten, it could stay the same, it could fade rapidly, or it could even completely fall apart! (Sound familiar?) I raise this point because some of the potential science we've highlighted does rely on the comet being an active object when near Mars. If it fizzles, we should still get some results but MAVEN, for instance, would definitely benefit from the comet being more active (gassy) than not.

Thus, once again and just like this time last year, the CIOC finds itself promising great science but with a big fat disclaimer attached. That's the nature of our business though, and if we didn't stretch ourselves to attempt the extraordinary, then we simply wouldn't be doing our jobs. I'm not going to offer predictions here. In 2011 I said comet Lovejoy would be destroyed before reaching the Sun (it survived - sort of...), and last year I said ISON would probably survive past the Sun (it didn't). This is why I don't visit casinos. But in 2011 and again last year, I did also promise amazing science either way, and I absolutely nailed that one. We learned - and continue to learn - a ridiculous amount about comet ISON from all of our observations.

So once more, here and now, I am going to happily go on record and say that we WILL get fantastic science out of this encounter, regardless of what the comet does over the next 13-days. (And if you stick around on this website and the blogs, and you'll be among the first to hear about it!)

Keep up-to-date on the latest Siding Spring and sungrazing comet news via my @SungrazerComets Twitter feed. All opinions stated on there, and in these blog posts, are entirely my own, and not necessarily those of NASA or the Naval Research Lab.

See other posts from October 2014


Or read more blog entries about: comets, comet Siding Spring, Mars


messy: 10/06/2014 06:04 CDT

The difference between this thing and ISON is that ISON disintegrated AFTER it passed close to the Sun. Sliding Spring hasn't gotten anywhere near the Sun yet.

Karl Battams: 10/07/2014 05:45 CDT

@messy: Yes and no... It's certainly not on anywhere near as perilous an orbit as ISON (which did disintegrate *before* perihelion, not after), but Siding Spring is an Oort Cloud comet, and they frequently do weird things. Siding Spring passes perihelion in a couple of weeks, but out at 1.4AU (i.e. near Mars) so it doesn't have anywhere near so much solar radiation to worry about. But nonetheless it has been drifting around the Oort Cloud quite happily for over 4-billion years without ever experiencing pretty much *any* solar radiation. Now it's suddenly getting a lot of it, and it really might not like that... Anyway, my parallel to ISON was two fold: 1. Misbehaving Oort Cloud comet; 2. A high-profile campaign in which we've made huge promises that rely on a well-behaved comet (see #1). They are indeed very different objects in terms of orbits and risk factors, but there are enough parallels to bring back old nightmares for me :)

Ford Prefect: 10/07/2014 03:48 CDT

How exactly do we know that this comet has never visited the inner Solar System?

Stephen: 10/08/2014 09:27 CDT

Presumably, we know that Siding Spring hasn't been in the inner solar system before from it's orbit. I haven't looked up the orbit, but it could be hyperbolic, which also means that when it leaves, it's not coming back. BTW: minor typo: "than we'd have though." => "than we'd have thought." The NASA and ESA spacecraft plan to hide behind Mars for the worst of the debris. Has the IRSO said what they'll do with MOM?

TimR: 10/09/2014 04:04 CDT

Reading Mathews pages on Siding Spring including the one on its reduced brightness, I lean towards the 2nd explanation for the drop in brightness: "2. There may have been a population of icy grains in Siding Spring’s coma that were released at larger distances and survived until now but are starting to disappear due to the increasing heat." Modeling estimates could be made for this one. His team can estimate at what distances from the Sun various common volatiles will sublimate. They surely know this for many but the modeling must account for particle size (cross section). So its more complicated. The first explanation remains more conjecture because there is no idea as to the ratio of dust (non-volatiles) to volatiles in the nucleus. It would be difficult to model with any accuracy. However, the second can be estimated, i.e. distances where the most common volatiles sublimate. I'd like to know of a paper that lists the distances for these volatiles. The problem is surely more complicated because there are a lot of different compounds but one might be able to group them by type or just temperature and commonality. The question could also be addressed by making cross-sectional light curves of the comets coma and tail. Where has the brightness dropped? In the tail - where? Or the coma? A fifth explanation might be that, short of catastrophic, the nucleus has had outbursts of dark particulates (dust) that is absorbing and cutting light reflection from the coma. Again having a cross-sectional light curve would be a great thing.

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