A large comet due to pass close to Mars has an outside chance of actually smashing into the red planet.
If it does hit, the impact will be colossal.
Comet Siding Spring, discovered in early January has a nucleus estimated to be anywhere between 8km and 50km in diameter, and is travelling at 56km per second.
If it were to hit Mars, Space.com reported the collision could create an impact crater on the planet up to 10 times the diameter of the comet's nucleus and up to 2km deep.
At Slate Magazine, astronomer Phil Plaitt estimated a 15km-diameter comet would hit with an explosive yield of roughly one billion megatons, equal to a million billion tons of TNT exploding. It would be an explosion about 25 million times larger than the largest nuclear weapon ever tested on Earth.
The latest trajectory of the comet generated by Nasa's near-Earth object programme office indicated it would pass within 300,000km of Mars, and could pass within 50,000km of the surface, in October 2014.
"At present, Mars lies within the range of possible paths for the comet and the possibility of an impact cannot be excluded," Nasa said.
But it added that the probability of an impact was less than one in 600, and future observations were expected to provide data that would completely rule out a Mars impact.
Slate's Plaitt said if the comet were to hit Mars, almost certainly all robotic probes in orbit and on the surface would be lost.
"An impact that size would blast debris all over the planet, and the rovers could be damaged or destroyed. Even something in orbit wouldn’t be safe; the ejecta would come screaming off the planet and sent every which way in orbit around Mars. It would be like orbiting into a shotgun blast," Plaitt said.
With the comet travelling faster than the Sun's escape velocity, assuming it survived the encounter with Mars, it would head out into deep space, almost certainly never to return.
Nasa said it was estimated the comet had been on a journey of more than a million years, coming in from the solar system's distant Oort cloud.
The comet could be complete with the volatile gases that comets which made regular repeat visits often lacked due to their frequent returns to the Sun's neighbourhood.
It was possible the comet, named after Siding Spring Observatory in Australia, where it was discovered by Rob McNaught, may be bright enough that it could be viewed through binoculars or a small telescope from the southern hemisphere in September 2014.
Without an impact, Mars may be out of the line of fire of even the nebulous cloud, called a coma, which enveloped comets.
Answering a question from Planetary Society blogger Emily Lakdawalla, astronomer Mike Kelley said the comet's tail would be more or less pointed away from Mars during the close encounter.
If the comet passed about 100,000km from Mars, he was not predicting anything spectacular for the planet. If the comet ended up getting much closer, then he might have to reconsider.
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