Following my last blog post about the risk assessment of comet Siding Spring buzzing Mars in October, the Planetary Society kindly reposted the entry on their site as well. If you look at their version of my post, you'll see that Editor Emily Lakdawalla has made an excellent observation near the end of the page. She notes that:
"One thing that Mars spacecraft can do is phase their orbits such that they are behind Mars when the worst of the coma passes. Mars Odyssey performed that maneuver yesterday, JPL plans to adjust the orbits of Mars Reconnaissance Orbiter and MAVEN, and ESA is making sure Mars Express is safe, too, ISRO's Mars Orbiter Mission is planning for the encounter, too.
In my post I had stated 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!". This still holds true: should a spacecraft be stuck out somewhere and directly in the line of debris, there truly is very little to be done. Thankfully - and I simply to forgot to mention this - Emily is exactly correct in that the spacecraft operators can take preemptive action to minimize the chance of finding themselves in this sticky spot. The spacecraft teams can actually change the orbits of their spacecraft very slightly so that during the comet's closest approach, they are essentially in the shadow of the planet and safe from the peak of the storm.
If you look at slide #12 of the Risk Assessment PDF [7.4MB] you will see a chart that shows the duration of the debris shower, and when the peak is expected to occur. The whole event looks like it won't last for more than about two hours - perhaps less - and the highest-risk peak is only about 30-minutes or so. After that, the trajectory of both Mars and the comet trail take them off in different directions and the risk is gone. Thus as long as the spacecraft can be "behind" Mars during that relatively brief window, the risk to them really does drop to a very small number indeed as the planet's atmosphere will comfortably soak up any particles that reach it. (My understanding is that the particles would not have any chance of penetrating down to ground-level and risking the rovers.)
So now we have even more reasons to be unconcerned by this encounter and instead look forward to getting some very unique and spectacular images of the comet in October! This will be the subject of a future blog post I'm planning to write after our CIOC Workshop on Monday.