There's been a lot of speculation recently about what would happen if a large meteor or comet were to strike the Earth but, come October 2014, we may see a very real example of such an impact as a newly discovered comet has an extremely close encounter with the planet Mars.

Comet C/2013 A1 (Siding Spring) was discovered on January 3rd by veteran comet hunter Robert H. McNaught at the Siding Spring Observatory in New South Wales, Australia. Upon learning of this discovery, astronomers at the Catalina Sky Survey dug through their database and found that they had collected images of the comet back on December 8th. Using the combined observations, the astronomers pieced together what they could of the comet's orbit, and even with that limited information they calculated that the object was going to fly within 100,000 kilometres of Mars on October 19th, 2014.

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That's a pretty wide pass, relatively speaking. Asteroid 2012 DA14 came within 27,000 kilometers of the Earth back on February 15th.

However, as is always the case with finding new objects floating around in our solar system, with more observations come better estimates of the path that those objects will take as they whiz through space.

As of Wednesday, February 27th, fresh observations of C/2013 A1 have shifted its path over 60,000 kilometres closer to Mars, and it is now expected to pass within 37,000 kilometres of the planet's surface!

That would probably be the end of the story if we were talking about an asteroid or meteor, and we'd answer the question in the headline with a resounding 'No!'. However, a comet is a different matter entirely — made of different matter, that is. Ice (well, mostly).

Where C/2103 A1 is now, over one billion kilometres from the Sun, it may as well be an asteroid or meteor, since it's just a big, mostly-inert hunk of ice and rock flying through space. However, as the comet gets closer to the Sun and starts to heat up, that ice is going to start turning back into gas. Whatever side of the comet that is facing the sun is going to be turned into an icescape of erupting geysers, and those geysers are going to push on the comet like thruster engines, slowly nudging it off its original course. Where it will be pushed, though, is anyone's guess right now, because if the hunk of ice isn't already rotating, it will be once the geysers start up, and that will add more uncertainty to any calculations of its orbit. It may be pushed further away from Mars, or it may be pushed into a direct collision course with the planet.

So, what will it be like if the comet does hit Mars? The word cataclysmic comes to mind.

It's hard to narrow down the size of C/2013 A1 at the moment, due to its distance and the small cloud of gas (the coma) it's generating around it, but it is estimated at being anywhere from 8 to 50 kilometres wide, and it's traveling at about 55 km/s. Even at the smaller end of the scale, an object that large would hit the planet with the force of about 100 million megatons, blasting a crater over 100 kilometres wide into the Martian surface. If you ramp that up to the bigger end of the scale, the blast becomes more like 20 billion megatons and the crater would be near 600 kilometres wide!

Either way, the effects would be devastating, both for the planet and for the various satellites and rovers we have investigating Mars right now. Even if the rovers weren't anywhere near ground-zero, and the satellites were spared a direct hit, the impact would blast tons of rock into space and throw up a dust storm that would fill the planet's atmosphere for months or even years after. Curiosity, with its nuclear power supply, could most likely continue on, if it wasn't physically damaged by any aspect of the impact, but Opportunity would lose power as the dust blotted out the Sun, and most (if not all) of the satellites in orbit would be destroyed by the newly orbiting debris field.

Could there be an 'up' side to this scenario?

It would certainly give us the opportunity to observe such an event real-time, to get a better idea of how such an impact would affect the Earth. It could also introduce an incredible amount of water and gases to Mars' environment, which (once the dust settles) could actually have a beneficial effect on any plans we might still have for colonizing the Red Planet in the future.

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