When comet C/2013 A1 (Siding Springs) was discovered earlier this year, there was a chance — albeit small — that it could hit the planet Mars in October of 2014. Now, based on updated observations, astronomers have all but completely ruled out an impact.
When astronomers spot a 'new' object in the solar system, they not only continue to observe it after they first see it, but they also look back to see if they can find it in past images as well. The point is to gather as many observations as possible, so that they can narrow down its path as much as possible. Until they have traced its complete path or orbit, the object's movement through space is represented by a 3-dimensional 'cone of uncertainty'.
Until recently, although the planet Mars wasn't smack-dab in the centre of C/2013 A1's cone, it was still inside the edge of the cone. The maximum distance the comet could pass by the planet was a few hundred thousand kilometers, but the minimum distance was still zero, so there was a chance that there could be an impact. The most recent observations have narrowed down that comet's path even further, though, and Mars has now slipped out of the cone.
According to a NASA statement from April 12th: "Based on data through April 7, 2013, the latest orbital plot places the comet's closest approach to Mars slightly closer than previous estimates, at about 68,000 miles (110,000 kilometers). At the same time, the new data set now significantly reduces the probability the comet will impact the Red Planet, from about 1 in 8,000 to about 1 in 120,000."
Mars isn't free and clear now, though. At least not yet.
Comets are very unpredictable. As they get closer to the Sun and heat up, the side that faces towards the Sun heats up enough that it becomes a landscape of explosive geysers. These geysers can act like rocket thrusters, and with the comet nucleus rotating as well as flying through space, and the exact strength of these geysers being completely random, the exact motion of the comet just can't be completely narrowed down.
So, it's very likely now that C/2013 A1 (Siding Springs) will just slip past Mars and give Curiosity and Opportunity a great show in the process. However, the centre of the comet's cone of uncertainty keeps getting closer to the planet and the minimum distance for the comet passby being only about 8,000 kms, so with around 18 months still left to go before it arrives at Mars, there's a lot of time for it to close that distance again.
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