Comet Siding Spring: Risk Assessment

Umm... should those two be quite that close together??

If you've been following this site at all, you'll know that on October 19, 2014, Comet Siding Spring is going to have an extremely close encounter with the planet Mars. Not so much so that it risks hitting the planet - we know it will remain a good distance. And we can lose the foil hats and stop worrying about the comet being thrown into a different orbit by Mars' gravity. The comet will barely even notice the Martian gravity, if at all. The concern is this: much like children, if there's one thing that comets do well, it's leaving a mess in their wake.

As comets hurtle through space, they constantly release gases, ice, dust and rocks, creating the characteristic gassy coma and long tail (or tails) with which we are so familiar. Generally this is of no concern to us as comets tend to keep their distance. But for a while, comet Siding Spring has left a certain few folks with rather sweaty palms.

When the discovery of the comet - and its slightly worrying orbit - were announced, the mission operators and scientists working on the several spacecraft that we currently have orbiting and roving Mars became naturally a tad concerned. They have billions of dollars of very delicate instrumentation floating/roving out in space, and a rock-and-dust spewing comet is about to buzz right past them at tens of kilometers per second. The very fine dust might not pose much risk but a marble-sized rock at those velocities could conceivably rip straight through a spacecraft. Typically that is tremendously bad news for said spacecraft (and rock).

Thus we, the CIOC, were tasked with organizing an observing campaign that would achieve two main goals:

• 1. Learn as much about the comet as possible and assess the risk to Martian spacecraft posed by its passage past the Red Planet; and
• 2. Take advantage of this close passage to hopefully get some pretty awesome images of a comet - from Mars - and combine them with a bunch of ground based observations to learn just a little more about the building blocks of our solar system.

Right now I want to focus on that first point, and save the second for another day.

For a few months, scientists have been performing modeling studies and analyses of comet Siding Spring, trying to paint the best picture possible of the risk the comet poses to planetary spacecraft. About a week or so ago, the preliminary conclusions were announced, and NASA HQ have given us this PDF copy of the presentation [7.4MB] to share with you all. Feel free to download and peruse at your leisure. It's generally understandable for most readers but in places the terminology gets a tad heavy, so what I want to do is highlight some of the key conclusions and put things into Layman's terms.

But first... let's get the bottom line out of the way right now: it seems most likely that our Martian spacecraft and rovers will be absolutely fine. [You can stop holding your breath.] Now for some details.

Early this year, a couple of teams of comet experts were assembled to model the comet's encounter with Mars and estimate the size and number of particles that the near-Mars environment would encounter, and predict the timing and duration of said event. After a series of meetings, the teams' findings were presented and peer reviewed [because that's how science works!] in May 2014. Scientific publications are now in the works, but that PDF I just pointed at is the basic summary of their combined findings.

In a situation like this, lots of considerations had to be made:

• How active is the comet? How much dust and ice is it producing? Obviously an active comet is much more worrying than a very quiet one.
• How big are the released particles? Perhaps the dust is really fine powdery stuff. Perhaps it's really big and chunky. Typically it's a mixture but the ratio of the two is sometimes important.
• How fast are the particles moving from the nucleus? If the dust/rocks are being lifted off by volatiles (ices) vaporizing, they might not be getting pushed away quickly. In that case the material will stay close to the comet and pass nowhere near Mars.
• What size particles are likely to reach out as far as Mars? If it's only really fine powdery dust that will get pushed out as far as Mars, that's much less of a concern than if bug chunks are getting flung out there.

These are just some of the questions/uncertainties that had to be addressed, and each of these "free parameters" has some range of realistic values, but combining all those values leads to really fuzzy answers. So the teams took advantage of ground-based observations to narrow down lots of these uncertainties. Measurements of gas production rates (affecting the release of dust) and the sizeof the dust coma (giving information about the dust production and size) were obtained by a handful of spacecraft in late 2013. These definitely helped the team, but part of the teams plan was - and remains - to continue to use the latest observations available to tweak and constrain their models. That's exactly why we have this Observing Campaign! Ground-based observations can get fed right into these computer models, and make the uncertainties (error bars) increasingly small.

So what do the modeling results currently say? Well, according to the PDF (and I don't have any newer results than this), we can say a few things about the comet's passage by Mars.

First, one of the key factors here appears to be the velocity with which the particles leave the nucleus. Sounds obvious, I know, but such velocities do vary a lot from comet-to-comet. Thankfully, observations imply that most of the large and potentially damaging dust is leaving the comet nucleus at less than 1 meter per second (m/s), with a best estimate of about 0.4m/s. If that's so, it's definitely good news as it puts Mars outside of the comet's dusty coma. But scientists do not like to just toss out a 'best guess' and walk away, so the teams also looked at more extreme cases assuming that a jet (a powerful streaming outburst on a comet) suddenly erupts and pushes lots of particles out further. In either case, the results still don't look scary for Mars.

I don't want to get bogged down in details but it's important to understand something about comet dust and how it behaves. Any dust grain - from micron up to pebble-size - is potentially hazardous at cometary velocities, but these different grain sizes behave very differently once they leave the nucleus. Bigger chunks tend to stay close to the comet and its orbit whereas the really fine stuff very quickly gets pushed away by radiation pressure (literally the pressure of sunlight on the particle). One of the team's key findings was that only grains in the millimeter size range have the right combination of radiation pressure and release velocity to conceivably be in the Martian vicinity in the minutes or hours following the comet passing by. So, with that in mind, the next consideration is how many of the particles are expected.

The PDF frequently uses two different terms to describe the particles rates: fluence, and flux. These terms are related but mean two slightly different things. The fluence is the total number of particles encountered by some unit of area during the event. So think of a 1-meter square box with some number of dust particles inside it: that's the fluence. The term flux refers to the number of particles that pass through some unit of area during some period of time. So now imagine someone has a bag of dust outside of our 1-meter box and they are using a fan to blow dust through the box. The rate at which the dust travels through the box is the flux.

The two are naturally related but the distinction between them is important, as we really want to know not only the number of particles to expect near Mars at any given time, but also the rate at which they'll be appearing (and hence the duration of the event). Accordingly, on page 15 of that presentation/report, there's this very nice pair of plots:

Note that "SS" means the rate expected from Siding Spring, "MM" is the typical meteoric background' rate (i.e. what you'd expect on a normal day), "Leo" refers to the Leonid and "Per" the Perseid meteor showers at Earth, and... umm... I'm not entirely certain what the other ones are but I'll update this as soon as I do! "Lst" is the Leonid storm from 1999-2002, and "OD" is orbital debris. What this plot shows is that the total number of particles expected during the encounter (fluence, on the right) is really very small compared to what Earth sees every year from the Perseids, for example. But the difference is that the Perseids happen over many hours whereas the SS particles will happen in a very short duration - couple of hours maybe - and thus the flux is very high during that time. Hopefully that makes sense. Crucially though, the takeaway message is that while Mars will experience a high rate of particles during the encounter, the actual number of particles it sees will be really relatively low compared to what typically floats around in space anyway.

Of course this leads me to ask myself this: would I rather have someone throw one rock at me per minute for an entire hour, or 60 rocks at me in just one minute? Personally I'd rather get it over with in one minute - and then perhaps address the more pressing question of why they were throwing rocks to begin with. But I digress. The conclusions of the report mostly paint a reassuring picture, with very few physically realistic scenarios pushing a lot of particles out near our Mars fleet. That said, there is a cautionary note buried near the end of the report: "Due to their large size and high velocity relative to Mars, damage from particle impacts is less easily mitigated by spacecraft re-orientation." What this means is that should there be any big chunks heading towards a spacecraft, there's perhaps not a lot that can be done. Hunkering down and bracing itself (aka "spacecraft re-orientation") might not make a big difference to 56km/second (125,000mph) rock! There's pretty much no way we can predict this though. If it happens, then it happens, but it sounds like the Siding Spring threat is really not much more than the general day-to-day threat of space rocks.

So with this uncertainty in mind, the report thus concludes on a note that applied just as well to comet ISON last year as it does to comet Siding Spring this year:
"Warning: Comets are famously variable and modeling their activity remains somewhat uncertain. Further observation of Comet Siding Spring is prudent."

Prudent indeed.

Many thanks to Yan Fernandez and Tony Farnham for their inputs to this post!

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.