Their Delta II rocket was due to blast off from Cape Canaveral in Florida on Sunday, but storms put that back until Monday.

Those storms still haven't cleared, so the launch has been delayed once more.

The scientists say they'll have a third go on Tuesday.

Delta II is carrying Spirit - the first of two US robots designed to explore the red planet.

Mars Express

The second robot - Inspiration - is set to blast off later this month.

The two robots were named by a nine-year-old schoolgirl, Sofi Collis from Arizona.

The US missions follow quickly behind Europe's Mars Express, which blasted off carrying the British-built Beagle 2 lander a week ago.

Good position

The rush to Mars has come about because the planet is currently in a good position for an approach from Earth.