A bold attempt to land a one-ton rover as big as a Mini Cooper on Mars will end in triumph or disaster today.
Scientists hope to receive the signal confirming that the six-wheeled robot, Curiosity, is safely on the planet's surface at 06.31 UK time.
Two thirds of Mars missions to date have failed, including Britain's ill-fated Beagle 2 lander which was lost on Christmas Day 2003.
But none has been as complex and daring as the Mars Science Laboratory (MSL) mission, which aims to deliver the largest rover to land on the Red Planet.
Curiosity, costing £1.59 billion, is twice as long and five times as heavy as the twin rovers Spirit and Opportunity which landed there in 2004. Because of its size and weight, getting the vehicle on to the Martian surface presented a major challenge to scientists at the American space agency Nasa.
The dramatic solution involves dropping the robot on nylon tethers from a hovering "sky crane".
John Bridges, from the University of Leicester, one of two British scientists leading teams on the mission, said: "I'm cautiously optimistic. Space exploration is not for the faint-hearted. The previous rover landing used inflatable bouncing bags. Curiosity's just too heavy for that, so they developed the sky crane technique."
Curiosity's target is Gale Crater, near the Martian equator, where billions of years ago there may have been a large lake.
The rover is due to land close to a Mount Sharp, a 5.5 kilometre peak in the middle of the crater with clay deposits round its base.
Curiosity bristles with sophisticated instruments designed to discover if Gale Crater could ever have supported simple life.
For one Martian year - 98 Earth weeks - the rover will explore its surroundings using a robot arm to scoop up soil and drill into rock.
It also carries its own laser gun for "zapping" rocks up to 30 feet away. The laser will vaporise tiny amounts of material in a flash of light that can be analysed to reveal chemical data.
As well as carrying a stereo camera for panoramic shots, Curiosity has a magnifying imager that can reveal details smaller than the width of a human hair.
Samples will be analysed using a state-of-the-art onboard laboratory.
The landing site bears geological signs of past water, including what appears to be a lake bed on the floor of the crater. Channels that may have been carved by flowing water have also been identified.
Dr Bridges said: "There's this idea that Mars was warm and wet long ago, but we don't know how long there were standing bodies of water on Mars, whether they were short-lived or lasted hundreds of millions of years. That's important to the question of whether life ever existed there."
An Atlas V rocket carrying Curiosity blasted off from Cape Canaveral, Florida, last November. The journey to Mars crossed 352 million miles of space.
Tomorrow's landing will begin with the capsule containing the rover first being slowed by friction as it enters the Martian atmosphere at 13,200mph, and then a supersonic parachute.
Closer to the ground, the "sky crane" descent stage is released, braking its fall with retro rockets.
Hovering above the surface, it will drop Curiosity on to the planet at the end of three 25ft nylon tethers.
Once the rover has touched down, the descent stage will break off and crash a safe distance away.