Mars is in danger of being struck by an asteroid at the end of next month, astronomers have calculated.

The newly discovered space rock known as 2007 WD5 has a one in 75 chance of colliding with the planet on January 30. While the probability of an impact is only slim, the odds have been cut from one in 350 when the object was first identified, and they are much shorter than is usual for new asteroids.

If 2007 WD5, which is about 100 metres in diameter, does strike Mars on January 30, it would cause an explosion equivalent to several megatonnes of TNT.

“These odds are extremely unusual,” said Steve Chesley, an astronomer with the Near Earth Object Program at NASA's Jet Propulsion Laboratory in Pasadena, California.

“We frequently work with really long odds when we track threatening asteroids. We know that it's going to fly by Mars and most likely going to miss, but there's a possibility of an impact.”

If the asteroid does hit, it would give astronomers a rare opportunity to study the effects of such a strike. The object is broadly similar in size to the one that hit Tunguska in Siberia in 1908, which felled an estimated 80 million trees over 810 square miles. Had the Tunguska rock hit a city, it would have wiped it out.

The impact, however, would be tiny in comparison to that of the asteroid that struck Chicxulub in Mexico 65 million years ago, which caused a worldwide cataclysm that is thought to have wiped out the dinosaurs. The new asteroid is 100 times smaller than the 10km diameter rock that caused that event.

The likely impact would be on the threshold of visibility from the largest of Earth's observatories, but its effects would readily be seen by probes orbiting the Red Planet such as the European Space Agency's Mars Express. It will not be visible with domestic telescopes or the naked eye.

The asteroid would probably hit a spot near the Martian equator, close to the point where Nasa's Opportunity rover has been exploring since 2004. Opportunity is safe, however, as it lies outside the projected impact zone.

The chances of an impact being seen with large terrestrial telescopes are greater than usual because Mars is currently very close to Earth. On Tuesday, the planet was just 55 million miles away, the closest it will be until 2016.

Mars is currently the brightest object in the night sky after the Moon.

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