PASADENA, UNITED STATES // The Mars rover Curiosity - on a quest for signs the Red Planet once hosted the building blocks of life - is streaking into the home stretch this morning of its eight-month voyage in what Nasa describes as its most challenging landing attempt ever.
Curiosity, the first mobile science laboratory ever sent to a distant world, is scheduled to touch down inside a vast, ancient impact crater at 9.31am.
Mission control engineers at the jet propulsion laboratory (JPL) near Los Angeles acknowledge that delivering the one-tonne, six-wheeled, nuclear-powered vehicle in one piece is a risky proposition, with zero margin for error.
But on the eve of Curiosity's rendezvous with Mars, JPL's team said the spacecraft and its systems were functioning flawlessly, and forecasts called for favourable Martian weather over the landing zone.
After a journey from Earth of more than 567 million kilometres, engineers say they are hopeful the rover, the size of a small sports car, will land precisely as planned near the foot of a tall mountain rising from the floor of Gale Crater in Mars's southern hemisphere.
Flight controllers anticipate clear and calm conditions for touchdown, slated to occur in the Martian late afternoon. There may be some haze in the planet's pink skies from ice clouds, typical for this time of year, with temperatures at about -12°C.
"We're on target to fly through the eye of the needle," Arthur Amador, the Mars Science Laboratory mission manager, said on Saturday, as Curiosity hurtled to within 4.5 million km of its destination.
Facing deep cuts in its science budget after the cancellation of the space shuttle programme - Nasa's centrepiece for 30 years - the agency has much at stake in the $2.5 billion (Dh9bn) mission.
Mars is the chief component of Nasa's long-term deep space exploration plans. Curiosity, the space agency's first astro-biology mission since the 1970s-era Viking probes, is designed to search for evidence that the planet most similar to Earth may have once harboured ingredients necessary for microbial life to evolve.
The rover is equipped with an array of chemistry and geology instruments capable of analysing samples of soil, rocks and atmosphere.
Nearing the end of its journey encased in a capsule-like shell, Curiosity was essentially flying on automatic pilot with a backup computer also on standby. Two hours before atmospheric entry, mission control will send its very last transmission to Curiosity, a "parameter update" giving the craft its exact position in space.
After that, controllers will have little to do but anxiously track Curiosity's progress as it flies into Mars's upper atmosphere at 20,921 kph - 17 times the speed of sound - and begins a descent and landing sequence Nasa refers to as "the seven minutes of terror".
Curiosity's fate will then hinge on a complex series of manoeuvres that include a giant parachute deployment and a never-before-used jet-powered "sky crane" that must descend to the right spot over the planet, lower the rover to the ground on nylon tethers, cut the cords and fly away.
"This is the most challenging landing we've ever attempted," said Doug McCuistion, Nasa's Mars Exploration Programme director. If everything works according to plan, controllers at JPL will know within a minute or two that the Curiosity is safely on the ground, alerted by a terse radio transmission relayed to Earth from the Mars orbiter Odyssey flying overhead.
If no landing signal comes, it could take hours or days for scientists to learn if radio communications with Curiosity were merely disrupted or that it crashed or burnt up during descent.