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Mars could be hit by a comet with the power of a billion megatons next year, astronomers claim.

Comet C/2013 A1 was discovered between Jupiter and Saturn in January by Robert McNaught at Australia's Siding Spring Observatory and was forecast to pass within 37,000km of Mars in October 2014.

But according to a new recalculation, the comet may hit our nearest planetary neighbour after all.

Researcher Leonid Elenin said there is now a slightly higher chance of the impact occurring.

The movements of comets are difficult to predict, because as they approach the sun their structure is affected by increases in temperature which can throw it off course, according to the Huffington Post.

The report says that if the comet hit Mars, at a speed of 56 km/second, it would leave a crater about 500km wide and 2km deep.

The Mars Curiosity rover and the Mars Reconnaissance Orbiter could capture a view of the ball of ice and dust as it passes or hits the planet.

NASA puts the most likely "close-approach" distance between the comet and Mars at about 100,000km.

Astronomer Phil Plait told The Economist that given the unusual speed of the comet, its impact should yield a blast equivalent to that of a billion megatons of TNT.

"It would be an event on the same sort of scale as the impact that drove the dinosaurs extinct 65 million years ago," Mr Plait said. "If it really is that big, and if the comet were to hit the side of Mars facing Earth then the blast could well be visible to the naked eye, even in daylight."

The cratering process after such an impact would be interesting to geologists and astrobiologists, according to The Economist.

There is a lot of ice frozen into the Martian crust and the heat of an enormous impact would melt a huge amount of it.

If, as some believe, there are microbes living deep under the Martian surface, such a burst of warm, wet conditions over a substantial chunk of the planet would give them a brief chance to thrive at and close to the surface before the planet refroze," the report said. "Parts of the surface and subsurface in the impact region, if there is an impact, will stay warm for decades."

NASA spokesman Donald Yeomans said "unless this comet completely fizzles, it should be extraordinary as seen with Mars-based assets".

"And if the comet passes close enough to the planet it may allow a natural experiment," he said.

"Over the past decade there has been much discussion of the possibility that there might be methane on Mars, possibly produced by the aforementioned subterranean microbes.

"Various observers claim to have seen evidence for the gas, but theoretical arguments cast serious doubt on their results.

"One of the questions in play is how fast the Martian environment can oxidise organic compounds (such as methane) which get pumped or dumped into it.

"A very close encounter with a comet might result in a measurable pulse of organic matter being introduced into the upper atmosphere; its fate would be interesting to track."