Comet Siding Spring (C/2013 A1), discovered in January and now inbound from the Oort Cloud, promises to put on a spectacular show in October 2014 — for the spacecraft on and around Mars.
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We’re just a couple weeks away from getting decent naked-eye views of Comet PanSTARRS, and later this year we might be treated to even better views of Comet ISON.
But to see a really amazing spectacle, comet-lovers are imagining how they might magically transport themselves to the ruddy plains of Mars late next year.

Mars and Comet Siding Spring

How close will Comet Siding Spring (C/2013 A1) comet to Mars in October 2014? Early predictions suggest a collision is possible.
JPL Horizons
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That’s because Comet Siding Spring (C/2013 A1) is going to pass very close to the Red Planet on October 19, 2014. Right now celestial dynamicists at the Jet Propulsion Laboratory think the comet should miss the planet by about 32,000 miles (50,000 km), but there’s still about a 1-in-10,000 chance of an outright collision.
You can follow the comet’s interplanetary motion using the excellent orbit simulator provided by JPL’s Horizons website. (Here’s a comical adaptation of that site’s output by Rob Kaufman of Australia’s Bright Astronomy Club.)

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Ace comet sleuth Rob McNaught discovered this object on January 3rd using the Uppsala Schmidt Telescope at Siding Spring Observatory in Australia. (Notably, this came just 10 days before a wildfire overran part of the observatory, destroying his home and others’.) Almost immediately, astronomers in Arizona found the object in Catalina Sky Survey images taken a month earlier.
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Those observations, combined with others chipped in by observatories elsewhere, show that the comet has a highly inclined, retrograde orbit that will come no closer than 1.4 astronomical units (140 million miles) to the Sun. That’s nowhere near Earth, but 1.4 a.u. is just about at the orbit of Mars.
A few days ago Russian amateur (and comet discoverer) Leonid Elenin identified the comet’s close approach to the Red Planet next year. Turns out he wasn’t the first to notice. “The JPL small-body system automatically checks for close approaches to all planets and the Moon,” comments dynamicist Paul Chodas, “and we’ve been monitoring this impact probability for several weeks now.”
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As they draw closer together, the two bodies will be racing toward one another at 35 miles (56 km) per second, and early estimates suggest that the nucleus of C/2013 A1 could be 30 or 40 miles across. So a collision would mean a very bad day (er, sol) on Mars. Purdue impact specialist H. Jay Melosh ran the numbers using a powerful crater calculator he developed. Since no one yet knows how big the nucleus actually is, he ran simulations using diameters of 5 and 30 miles (8 and 50 km).
Both simulations yielded craters at least 6 miles (10 km) deep. The smaller assumption creates a basin 100 miles (160 km) across. But the larger one’s outcome is off the charts: an enormous pit more than 500 miles (800 km) across. Size-wise, this would be one of the top 10 impacts ever on the Red Planet!
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“These would be really big holes in the ground,” Melosh says. “But since the average recurrence interval for such large craters is very long, we would have to be very, very lucky to have such a thing happen in our lifetimes.”

Comet Siding Spring as seen from Mars

This plot shows how Comet Siding Spring (C/2013 A1) will appear as seen from Mars. The “best observing” occurs when the comet’s total brightness is at least magnitude 12 and its elongation from the Sun is 30° or larger. Click on the image for a larger view.
Jon Giorgini / NASA / JPL
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Hit or miss, Comet Siding Spring is going to put on one helluva show as seen from Mars. As calculated by small-body aficionado Bill Gray, the comet will approach Mars from the south and sweep into its northern-hemisphere skies over just a few hours. “It probably won’t reach the magnitude -8.8 shown in the ephemeris,” he cautions. “Still, maybe it’ll be bright enough (and suitably placed) for something on Mars to get a nice picture or two.”

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That possibility hasn’t escaped the notice of NASA mission managers. Right now three craft are circling the Red Planet right now (Mars Reconnaissance Orbiter, Mars Odyssey, and ESA’s Mars Express), and both Curiosity and Opportunity are roving its surface. Potentially any of them might be commanded to take some comet pictures.
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For example, one of Curiosity’s Mast Cameras has a 100-mm focal length and color capability. If the comet performs as expected, says Michael Ravine of Malin Space Science Systems, which built the cameras, some imaging will definitely be planned. “I’m imagining the bright comet spectacularly positioned over a landscape that includes Mount Sharp,” he muses.
Or what about MRO? It’s got a bunch of “imaging assets.” So I put the question to MRO project scientist Richard Zurek.
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“Just off the top of my head,” Zurek replies, “there are several instruments that could come into play: MARCI with its multi-color imaging (including in the ultraviolet), CTX with its moderately high resolution in a wide field of view, and CRISM and MCS can add information in the near and mid infrared. Most of this would be focused on the coma for CTX and and the rest on the tail, depending on comet’s evolution. And of course we would try something with HiRISE with its very high spatial resolution (possibly many pixels on the nucleus), assuming the slew rates of the spacecraft are reasonable.
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But C/2013 A1 poses danger for these craft as well. The comet should pass by close enough to completely immerse Mars in its gas-and-dust coma. Who knows what might rain down onto the Martian surface?
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One way or another, it should be quite a spectacle!