The space agency launched a first rover earlier this month. The second rover is set for launch late Saturday night from the Kennedy Space Center for the six-month journey.
Forecasters put the chance of good weather at 60 percent.
The six-wheeled robot, named Opportunity, was set for launch Thursday from Cape Canaveral, Fla., but was delayed. NASA has through July 15 to launch the 400-pound rover to ensure its Jan. 25 arrival at Mars.
Its launch was delayed a day by setbacks to the earlier liftoff of the first rover, Spirit. NASA launched Spirit on June 10.
Spirit is scheduled to reach Mars on Jan. 4.
On Friday, according to a NASA statement, a flight team at NASA's Jet Propulsion Laboratory, Pasadena, Calif., fired navigational rockets on Spirit for the first time to "fine-tune its flight path toward Mars."
The $800 million double mission aims to send the robots roaming across the surface of Mars to prospect for minerals that would indicate whether the planet could have harbored life.
But experts caution that Mars landings are very risky. Out of nine attempted Mars landings, only three have made it. That's one reason the space agency is sending two probes this time