Nasa is sending a rover to Mars in the hope of finding evidence of alien life.
The next-generation Perseverance rover will liftoff from Florida's Cape Canaveral before arriving at the red planet on Friday, where it look for traces of life that could once have been present on our nearest neighbour.
The space agency's mission – which has cost $2.4 billion – will launch at 7.50am local eastern time, or 12.50pm in the UK.
The car-sized six-wheeled robotic rover, which will launch atop an Atlas 5 rocket from the Boeing-Lockheed joint venture United Launch Alliance, also is scheduled to deploy a mini helicopter on Mars and test out equipment for future human missions to the fourth planet from the sun.
The weather forecast from the Air Force's 45th Weather Squadron put chances of an undisturbed liftoff at 80 percent, reporting a slim chance that thick clouds would form over the launchpad and delay the launch.
"This is the ninth time we've landed on Mars, so we do have experience with it," NASA Administrator Jim Bridenstine told Reuters in an interview on Wednesday.
Perseverance is due to land at the base of an 820-foot-deep (250 meters) crater called Jezero, a former lake from 3.5 billion years ago that scientists suspect could bear evidence of potential past microbial life on Mars. Scientists have long debated whether Mars - once a much more hospitable place than it is today - ever harbored life.
One of the most complex manoeuvres in Perseverance's journey will be what mission engineers call the "seven minutes of terror," when the robot endures extreme heat and speeds during its descent through the Martian atmosphere, deploying a set of supersonic parachutes before igniting mini rocket engines to gently touch down on the planet's surface.
It is the latest launch from Earth to Mars during a busy month of July, following probes sent by the United Arab Emirates and China.
Aboard Perseverance is a four-pound (1.8 kg) autonomous helicopter named Ingenuity that is due to test powered flight on Mars for the first time.
Since NASA's first Mars rover Sojourner landed in 1997, the agency has sent two others - Spirit and Opportunity - that have explored the geology of expansive Martian plains and detected signs of past water formations, among other discoveries. NASA also has successfully sent three landers - Pathfinder, Phoenix, InSight.
The United States has plans to send astronauts to Mars in the 2030s under a program that envisions using a return to the moon as a testing platform for human missions before making the more ambitious crewed journey to Mars.
Perseverance will conduct an experiment to convert elements of the carbon dioxide-rich Martian atmosphere into propellant for future rockets launching off the planet's surface, or to produce breathable oxygen for future astronauts.
Additional reporting by Reuters
