The US space agency's Near Earth Object Program (NEOP) revealed that the asteroid's exact course was difficult to predict, but said it could slam into Mars on January 30, leaving a crater measuring an estimated 1 kilometer across.

If the asteroid, which has been named 2007 WD5, missed Mars as expected it could return to swing past Earth years or decades later, but there was no indication of a threat to the planet, scientists said.

A collision with Mars would be likely to send an enormous dust cloud into the planet's atmosphere.

The exact path of the asteroid, which was discovered in November by the NASA-funded Catalina Sky Survey near Tucson, Arizona, was becoming increasingly difficult to observe because it was receding from the Earth, scientists said.

The asteroid, believed to measure around 50 meters (160 feet) across, had already passed within 7.5 million kilometers (5 million miles) of Earth in early November.

NEOP scientists at the Jet Propulsion Laboratory in La Canada Flintridge in California told the Los Angeles Times they were excited by the possibility of an asteroid striking Mars, describing it as "wildly unusual."

"We're used to dealing with odds like one-in-a-million," said astronomer Steve Chesley. "Something with a one-in-a-hundred chance makes us sit up straight in our chairs."

Any strike on Mars would be comparable to the Tunguska asteroid hit in Siberia, Russia in 1908, which felled 80 million trees over 2,150 square kilometers (830 square miles).

Depending on where the asteroid struck, NASA spacecraft, including the Mars Reconnaissance Orbiter and two surface rovers -- Opportunity and Spirit -- might have a ringside seat, offering a "scientific bonanza," Chesley said.

"Normally, we're rooting against the asteroid (if it is threatening Earth)," Chesley said. "This time we're rooting for the asteroid to hit."

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