The asteroid, believed to measure around 50 metres across, had already passed within 7.5 million kilometres of Earth in early November.
 
Nasa termed the possible impact with Mars an "unlikely event" in a news release on Friday, but Scientists from Nasa's Near Earth Object Programme were reported in the Los Angeles Times as saying they were excited by the possibility.
 
"We're used to dealing with odds like one-in-a-million," said Steve Chesley, an astronomer, "Something with a one-in-a-hundred chance makes us sit up straight in our chairs."
 
A collision with Mars would be likely to send an enormous dust cloud into the planet's atmosphere.
 
If the asteroid, 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.