Is Comet Siding Spring going to turn our Mars probes into shredded tinfoil?

Shiver in your shoes, Martians! This month – specifically, 19 October at 18:28 Zulu – Comet C/2013 A1 ‘Siding Spring’ makes its closest approach to Mars. The nucleus, a few kilometres in diameter, will come a smidgeon under 120,000km from the red planet.

Mars from the Siding Spring nucleus at closest approach - a picture I made with my trusty Celestia astronomy package.
Mars from the Siding Spring nucleus at closest approach – a picture I made with my trusty Celestia astronomy package.

That’s close. Though not as close as once feared. When the comet was first discovered by Robert H. McNaught in January 2013, using the 20-inch Upssala Schmit telescope at Siding Spring observatory in New South Wales, it was thought likely to hit Mars. It was only later, after multiple observations and cross-checks, that the orbit was refined.

Good news is that this is a tremendous opportunity – and there’s a fleet of orbiting satellites up there for the purpose.  Two, the US MAVEN and India’s Mars Orbit Mission (MOM) – arrived just last week. That puts a lot of instruments in close proximity, and the Indians have plans to use MOM to check for methane on the comet as it brushes past. The Mars Reconnaissance Orbiter will use its HIRISE camera to look at the comet nucleus and activity. Mars Odyssey will check out the coma. MAVEN will make a range of observations with eight different instruments. Even the rovers on the ground, Curiosity and Opportunity, will point their cameras at the sky – Curiosity’s ChemCam, which can pick up the composition; and Opportunity’s PanCam, which will give us a visual from the surface of Mars.

More shenanigans from my Celestia software. This is a view looking from inside the coma towards Mars and the Sun at closest approach.
More shenanigans from my Celestia software. This is a view looking from inside the coma towards Mars and the Sun at closest approach.

Bad news is that this fleet of satellites took years to get up there, cost billions of dollars – and are basically irreplaceable. The nucleus won’t get near Mars. But the coma of dust and debris surrounding it will. Estimates are that during the several hours it takes Mars to pass through the comet’s coma, the planet will be peppered with about five years’ worth of normal meteor activity. It’s all small stuff – nothing more than 1cm diameter, most of them only fractions of a millimetre. But the relative speed is 56 km/sec (200,000 km/h). That’s – uh – impressive. At that speed a 1 gram mass has a kinetic energy of 15,680,000 joules, or 4.35 kwH. In human terms? Enough to run a domestic fan heater on high for a couple of hours. Woah! And that’s just one particle. There are going to be a LOT of particles skidding past Mars.

More Celestia fun; a picture I made of planetary orbits at the moment of Siding Spring's Mars encounter.
More Celestia fun; a picture I made of planetary orbits at the moment of Siding Spring’s Mars encounter.

Precautions have included adjusting orbits so the probes will be on the opposite side of the planet from the comet 100 minutes after closest encounter, when the dust is estimated to reach its highest density. The MRO shifted its orbital parameters to that end on 2 July, while Odyssey did so on 5 August and MAVEN on 9 October. Still, that’s not a complete fix – they’ll travel back around into the danger zone soon enough. Other precautions include pointing the spacecraft so more delicate components are shielded by less crucial elements. And MAVEN will be put into a partial shut-down mode. Once the danger’s past, they’ll restart the science.

By 22 October, according to mission timelines, it’ll all be over. And, if the cometary debris hasn’t shredded them into tinfoil, they’ll be back to their normal work exploring the red planet.

Is Earth in any danger? None whatsoever. Even if we were at closest approach to Mars, the comet wouldn’t affect us – but as it happens, we’re nearly a quarter-turn away from Mars in any case, just at the moment. That’s not the issue – the issue is the several billion dollars worth of science equipment we’ve got around Mars at the moment, its survival – and the science we’ll get from them during this once-in-a-lifetime opportunity.

Copyright © Matthew Wright 2014


3 thoughts on “Is Comet Siding Spring going to turn our Mars probes into shredded tinfoil?

  1. Great stuff, Matthew. I sure hope the satellites survive. It’s good to know that if a comet the size of mount Everest misses us, I can still watch the Monday Night Football game, but probably not because the smaller debris would obliterate the satellite feed.😦

    I did a little digging, and noted that some 120mm take rounds hit with around 12.1 MJ of energy, almost as much energy as that 1g of mass cruising through space. The kinetic energies in space are truly astounding.

    1. It’s awesome how literally astronomical the energies get in space. This comet is exceptionally fast – eight times Earth orbital velocity. At those impact speeds the kinetic impact is compounded by plasma effects on whatever it hits. Ouch, big time. It will be interesting to see how the Mars probes survive. I’d expect some damage.

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