When the comet Siding Spring was detected early in 2013, initial predictions suggested that there was a "non-negligible chance" that it would hit Mars the following year. Thought to be up to half a mile wide, the comet ended up missing the Red Planet by a cosmic hair's breadth, much to the disappointment of computational geophysicist Cathy Plesko.
"I think I was the most excited person on the planet!" Plesko recalled recently. "Because if it would have hit Mars, I would have had a published thesis with predictions about how that was going to go."
Plesko is based at the Los Alamos National Laboratory in New Mexico and works with a team of scientists in a number of institutions who are focused on impacts on planetary bodies and simulations of impact mitigation techniques. An impact on Mars would have been an incredible stroke of luck - her 2009 Ph.D. research was about the effects of comet impacts on Mars. If Siding Spring had slammed into the Martian surface in 2014, the event would have put her published theories to the test.
For the first time in human history, we would have had a ringside view of the devastation unleashed during a massive impact on a planetary body. The sheer energy generated by the cometary impact would have been witnessed not only by countless telescopes on Earth, but also by an armada of Mars satellites that would have watched and recorded what it's actually like for an atmosphere to get hit by a major comet. What's more, NASA's Mars rovers Curiosity and Opportunity would have felt what it's like to get hit by a comet, making measurements of the effects on the ground - that is, if they survived.
It didn't happen, but the historic flyby did underscore the potentially devastating threat hanging over life on Earth. What if Siding Spring had been aimed at Earth instead of Mars?
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Though much of our concern is currently focused on the threat of asteroid impacts, Plesko has her sights set on the more unpredictable foe.
"The real 'gotcha' we need to worry about - that's a much lower probability, but a much bigger danger - would be a short-warning time incoming comet," she told Seeker after presenting her research at the December meeting of the American Geophysical Union (AGU) in San Francisco, Calif.
Keep in mind that Siding Spring was discovered in January 2013 and the Mars flyby happened in the October of 2014 - only 22 months lapsed from discovery to flyby. If the comet had been headed toward Earth, there wouldn't have been enough time to plan, build and launch a mission to intercept it.
"The point with Siding Spring was that it wouldn't have taken much of a trajectory change for it to have been pointed at the Earth," said Plesko. "If it had been pointed at Earth, we wouldn't have known until it was very close whether or not it was going to hit Earth. We would have had to have mounted a response."
Black Cat in a Coal Bin
Before an appropriate response can be worked out, however, we must first understand why comets are such an unpredictable phenomenon.
When surveying the solar system for errant space rocks, astronomers typically look around the equatorial plane. This is the imaginary disk around which the planets orbit the sun. There are some erratic asteroids that, through a collision or some gravitational interaction in the past, orbit well beyond the equatorial plane, but for the most part we have a good idea where a large incoming asteroid is going to come from. And when we do detect a particularly scary space rock, we'd have years, decades or even centuries of notice before we started losing sleep over it.
Long period comets, on the other hand, do not originate from the equatorial plane. They populate a distant region well beyond the planets and even beyond the heliosphere, the sun's magnetic sphere of influence. Called the Oort Cloud, this hypothetical region surrounds the solar system like a shell, and is thought to be up to one light-year distant. Any one of these comets could careen into the inner solar system from any angle; from above, below, from the side, or from the glare of the sun. This makes them very hard to survey.