Discovered on January 3, C/2013 A1 Siding Spring was typical of comet finds: a dim moving speck on a photograph. However, recent observations suggest that Comet Siding Spring could be extremely interesting indeed – its current orbit brings it only 37,000 km from Mars.
The discovery conditions of C/2013 A1 indicated that the comet was an intrinsically bright object on a retrograde orbit through the inner Solar System. Translated: a large object moving at very high speeds relative to the planets. The brightness of the comet suggests an object between 10-50 km in diameter, similar to that of Comet Hale-Bopp, which dazzled the Northern Hemisphere for 18 months in 1996-1997. If we were to apply the Torino Impact Scale to Mars, C/2013 A1 would be rated a 7.
Comet Siding Spring has garnered further interest after an orbital analysis by Leonid Elenin (ISON-NM Observatory). In a blog post on February 23, Elenin found that C/2013 A1 approached within 103,000 km (~64,000 miles) of Mars’s surface on October 19, 2014. Further, he noted that the chances of an impact could not be ruled out. Today, Elenin updated that analysis with new observations of the comet’s position. The new orbit brings the approach even closer - only 37,000 km (~23,000 miles) from the surface of the red planet. Despite the new observations, uncertainty regarding the impact was not eliminated.
Should the C/2013 A1 impact Mars next October, scientists will have a front row seat for one of the rarest of all geological events – an impact capable of causing a mass extinction had it been aimed at Earth instead. As the comet makes its closest approach to Mars, it will be racing outward from the sun at approximately 56 km/s relative to Mars. If the comet were to impact Mars, the energy released would be on the order of trillions of megatons, while the resulting crater could be up to 500km across and 2.5km deep depending on the exact size of the comet.
Even if the impact scenario is unlikely, the comet’s close approach means that Mars will almost certainly be within the coma, the comet’s tenuous atmosphere. Elenin suggests a slightly less apocalyptic scenario in this case: “Having a very tenuous atmosphere, the surface of the red planet will be subject to intensive bombardments by microparticles which, among other things, might cause malfunction of the space probes currently there.” The intensity of micrometeorite bombardment will depend on several factors, such as the exact proximity of the approach and how dusty C/2013 A1 is.
At its closest Comet Siding Spring will be a spectacular sight from the Martian surface. Elenin suggests that the brightness of the comet may surpass -8 magnitude for Martian observers, bright enough to make the comet visible in the daytime sky. As a result, Curiosity’s MastCam and navigation cameras may produce some spectacular images of the comet from the surface. Mars Reconnaissance Orbiter’s HiRise imager may also be able to contribute valuable observations at closest approach. The HiRise imager’s resolution of 1 microradian means that at a distance of 37,000km, objects should have a resolution of 37 m/pixel, meaning the nucleus will be easily resolvable.
While these imaging campaigns may be possible, the priorities of mission planners at NASA and ESA will be to take protective measures first. Even spacecraft specially designed to study comets can be heavily damaged by micrometeor bombardment. Giotto, which was designed to study Halley’s Comet in 1986, took two impacts from small particles in the span of half an hour. The first disrupted communications with Earth after it knocked the probe off of its spin axis, while the second destroyed the probe’s camera. While Giotto soldiered on to rendezvous with another comet in 1992, a similar bombardment in Martian orbit may knock veteran probes like the ESA’s Mars Express and NASA’s Mars Reconnaissance Orbiter out of commission completely.
Whatever the case, Comet Siding Spring will surely be capturing much more interest in the months to come as further observations continue to refine its orbit. Due to the uncertainties currently resulting from the comet’s great distance from the sun, the chance of an impact is not likely to be confirmed or dispelled for months to come. I’ll keep you updated on the situation in the months ahead.
Main Image: Comet McNaught as seen by the ESO in Chile in 2007. The view from the surface of Mars of Siding Spring could be pretty similar. Image Credit: ESO
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