The weather in Florida is set fair for the launch of the American space agency's (Nasa) big new Mars rover, reports BBC
The one-tonne Perseverance robot is heading to the Red Planet to search for life and to begin the process of returning rocks to Earth for analysis.
An Atlas rocket will send the vehicle on its way from Cape Canaveral Air Force Station.
It'll be the third mission despatched to Mars this month, after launches by the UAE and China.
Lift-off is timed for the start of a two-hour window that opens at 07:50 local time (12:50 BST; 11:50 GMT).
The cruise to Mars takes seven months. When it arrives, Perseverance will attempt to land in a more-than 40km-wide, near-equatorial bowl called Jezero Crater. Satellite images suggest this held a lake billions of years ago.
Scientists say the rocks that formed in this environment stand a good chance of retaining evidence of past microbial activity - if ever that existed on the planet.
Perseverance will spend at least one Martian year (equivalent to roughly two Earth years) investigating the possibility. Unlike the previous four rovers Nasa has sent to Mars, its new machine is equipped to directly detect life - either current or in fossilised form.
But any evidence it uncovers will almost certainly have its sceptics, which is why researchers want to bring whatever Perseverance finds back home for the deeper analysis only sophisticated laboratories on Earth can perform.
The rover will package its most interesting rock discoveries in small tubes. An elaborate mix of future missions will then launch later this decade to try to retrieve these samples.