At the end of a 500 million km trip which has taken seven months, the robots MER-A and MER-B are due to be parachuted down in special probes at opposite sides of Mars.
The Mars Expedition Rover project has cost the National Aeronautics and Space Administration $800 million, and 250 specialists and researchers from the US space agency are participating.
Two probes, Spirit and Opportunity launched on 10 June and 7 July respectively, are carrying the robots.
The first of the robots was due to descend towards Mars late on Saturday at a speed of 5.4 km per second. The second will make its descent three weeks later.
For the robots' descent, NASA has recycled the system it used successfully in 1997 on the Mars Pathfinder.
"The risk is real, but so is the potential reward of using these advanced rovers to improve our understanding of how planets work"
Eight and a half km off the ground, a parachute will be deployed, and the thermal shield protecting the probe will be cast off.
Tiny reactors will be used for braking and stabilisation as the probe lands.
Eight seconds before touchdown, large cushions of air should be inflated around the probe, allowing it to bounce a dozen times upon landing before coming to a halt.
Once the cushions deflate, the outer shell open up and the robot inside will begin to deploy its solar panels two hours after landing.
At that stage, a panoramic camera will be deployed to take the first colour pictures in an environment with temperatures of up to 360 degrees.
If all goes well the first pictures will reach Earth the day after the first Rover's arrival on Mars.
However, NASA has warned extensively about how tough its mission is, with a risk of failure if winds are too strong upon arrival or if the robot lands in the wrong spot.
"The risk is real, but so is the potential reward of using these advanced rovers to improve our understanding of how planets work," said Ed Weiler, NASA's associate administrator for space science.
The European Space Agency lost
contact with Beagle 2
"I don't know what else humans could have done to make these two rovers successful."
Of 30 attempted Mars missions over the past 40 years, just 12 have succeeded.
The European Space Agency's Beagle 2 robot was supposed to land on Mars on 25 December, but NASA's rivals have since lost contact with the probe.