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So we could in theory deflect an object away from a collision towards Earth. How hard is it to make this comet hit Mars for sure?

Just a thought.

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If it hits Mars, it could help to terraform the planet. The CO2 ice caps would melt and heat the atmosphere. It would melt a significant amount of frozen water in the soil and thicken the Martian air with water vapor too.

Wikipedia is predicting a 1 in 1250 chance of hitting Mars. Outgassing as the comet nears the sun could change its trajectory significantly also.

http://en.wikipedia.org/wiki/C/2013_A1

Making the comet hit should be much harder than deflecting one. Becaues planets are such small targets in the grand scheme of things.

The trajectory of this comet makes things especially difficult. It is moving fast, travelling in the opposite direction of all the planets and is coming in at a large angle relative to the plane of the solar system. From Phil Plait's post it will be moving at over 55km/s relative to Mars during the flyby/impact. Simply intercepting this comet is basically impossible today.

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