Here we are - just one day before the "big event". In many ways it brings back memories for me from last year, when I was perched atop Kitt Peak in Arizona, waiting for comet ISON to have its shining moment in the Sun.
Indeed, there are parallels here. Like ISON, comet Siding Spring is an Oort Cloud comet and what we refer to as "dynamically new". This means that - to the very best of our knowledge - this comet has never been in the inner solar system, instead having spent the overwhelming bulk of its 4-billion year existence in a frozen expanse, half-way to the next nearest star. (Aside: I'm frequently asked, "How do you know it came from the Oort Cloud?" The simple answer is that we know its current orbit, so we can just trace that back a few million years and see where it came from. )
Another similarity, albeit just a coincidence, is that both ISON and Siding Spring have been on very unusual trajectories that placed them very close to planets or, in ISON's case, the Sun. Here, we have Siding Spring which is passing some 16-times closer to Mars than any comet has to Earth in modern record. Comet ISON wasn't quite that close to any solar system body, but did pass surprisingly close to Venus, Mercury, Mars and the Sun. Considering the vast emptiness of space, these really are a remarkable orbits, but nonetheless just coincidence.
And finally - again, like comet ISON - we have watched nervously in these final couple of weeks before Siding Spring's close approach to Mars (and its perihelion a few days later on October 25) as the comet has suddenly and seemingly dramatically faded in brightness. This in particular has left us scratching our collective heads but we have come to the conclusion that, while indeed it has dropped some in brightness, this drop isn't quite a extreme as the light-curve plots imply. I'd recommend you read Matthew's recent blog post for more information on that.
So where does all this leave us? As I write, we're only just over 24-hours away from the comet's gassy coma engulfing Mars as the comet roars past the Red Planet at 56km/s (126,000mph). Does this brightness drop jeopardize all of the Mars-based observations we've been hearing about? Is the comet still even active? Could it fall apart? Would it change the situation and threaten Mars if it did fall apart now? They're good questions that I've been asked in recent days in media interviews and via email and Twitter. So again, just like I did last year with ISON, let me try and calm a few of these last minute nerves, and be explicit on what we do and don't know.
Let's get this out of the way. Yes - absolutely yes. Despite fading recently, we are still receiving word from numerous observers that the comet is still producing water, gas and dust, just as active comets do. Indeed, in the animation opposite you can see that the comet still has a nice tail, gassy coma, and obvious bright central condensation - all classic signs of a happy, healthy comet. I think we all got tired of the "F-word" last year, but I'm going to use here, if only once: Comet Siding Spring has not fizzled.
This is difficult to answer because we honestly never knew how the observations would turn out anyway, regardless of the comet's brightness. The Mars spacecraft are not designed to look at comets and with the exception of a couple of grainy images of comet ISON last year (under very different viewing circumstances), they've never looked at a comet. So we've been relying on modeling and best-guesses all the way along here. That said, I think I can make some general statements that should be largely true.
At this stage, MRO HiRISE will very likely see something, and the images it returns should look quite a bit better than the ISON images did last year. One of our stretch goals for HiRISE in this Campaign was that maybe it would be able to actually resolve the shape of the comet's nucleus. This would be an awesome first - we've never seen an Oort Cloud nucleus before! In good news, Siding Spring's apparent drop in activity might actually be an advantage here as a less active nucleus means less ice and dust blocking the view of the nucleus. Time will tell on that but I'm sure they'll see something either way.
Missions like NASA's MAVEN, India's MOM and ESA's Mars Express would prefer a more active comet as they are primarily looking at various molecules surrounding the comet and interacting with the Martian atmosphere (though MOM and MEX are attempting direct imaging too, I believe). The comet is still producing water and gases, so I imagine that they will get some kind of results, but perhaps conditions are not quite as favorable as before.
For the rovers on the Martian surface, seeing the comet was always going to be a challenge due to the viewing geometry of the comet and power constraint on the rovers. In general I'd say that brighter is probably better for them but the comet will be so close to Mars that it may not make a huge difference. The challenge always was, and will remain, more of an operational one. So we must hold out hope, but not expectation. (I'll just disclaim that I'm not on any of the Mars science teams so I would definitely urge media in particular to contact those teams directly for elaboration and latest plans/results.)
Bottom line is that the Mars missions are attempting to do something that they're absolutely not designed to do, so any kind of a result from any of the missions should be considered a huge win for science! And I honestly do think we'll win pretty big on this one.
At this point, we see absolutely no indication of the comet fragmenting. Could it fragment? Sure. Comets do that sometimes, and Oort Cloud comets seem particularly fond of it. But Siding Spring does not experience any particularly hostile environments that could induce fragmentation. It's not close enough to the Sun to feel tidal forces, and Mars' gravity just isn't strong enough to factor in. Granted, the comet is now very near perihelion (its closest point to the Sun), reaching that just a few days after Mars approach, so it is experiencing the most solar radiation that it has in 4+ billion years. But I still don't see that causing any fragmentation.
But, for the sake of argument, let's pretend the comet fragmented last night. What would we see now? Well likely it would increase in brightness as interior volatiles are exposed to solar radiation, so that would actually be a good thing. Water and gas production would increase, too. But even in the case of a very dramatic fragmentation, none of those chunks would pose any risk to Mars. It's simply too close now, and there's no way the chunks could move far enough out of their current orbit to come anywhere near Mars. The physics of the situation simply don't allow that. Likewise, dust grains released in any fragmentation event also would not be any threat to the spacecraft. So we absolutely can strike this off our list of concerns.
Clearly we have plenty of unknowns in this situation; science is fun like that! But in my personal opinion, things look really good for a very successful Mars-based observing campaign, and now we just wait excitedly for results. And it's this point that I want to end on, because things are a little different this year.
The Mars spacecraft do not send data down to Earth in real-time, so unlike last year when SOHO's realtime solar images gave the world a ring-side view of comet ISON, we have no such data stream from Mars. There is no timeline that I know of from the various Mars mission on when results will be announced, but I'd imagine that it will be at least a day or two, and perhaps longer for newer missions like MOM and MAVEN who are still learning the ins and outs of their spacecraft, instruments and data. Please be patient! There are a couple of events going on this weekend that you can follow, however.
First, I saw that the Virtual Telescope is going to be showing live views of the comet and expert commentary on Sunday October 19th. Likewise, Slooh will be holding a similar event, so that's another event you can follow. Finally, ESA will hold a live webcast from the Space Operations Centre, ESOC, in Darmstadt, Germany, which will be streamed live 19 October, starting at 17:50 GMT (19:50 CEST). There may be more than this so keep an eye on social media to look for similar events occurring this weekend.
We'll update this site as soon as we hear results, so don't stray to far from here. I'll also be tweeting anything I hear over the current days, as I'm sure numerous other will be. So sit back, relax, and wait to see what fabulous new results come from this extraordinary interplanetary observing campaign!