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The Mars rover Curiosity, the most sophisticated mobile science lab ever sent to another world, hurtled closer to the Red Planet on Saturday, on track "to fly through the eye of the needle" for a precise, safe landing on Sunday night, NASA officials said.
Mission control engineers at the Jet Propulsion Laboratory near Los Angeles acknowledge that delivering the one-ton, six-wheeled, nuclear-powered rover in one piece is a highly risky proposition under the best of circumstances.
But JPL's team said the spacecraft and its systems were all healthy and performing flawlessly, and that weather forecasts over the landing zone on Mars were favorable, as Curiosity streaked to within 4.5 million (km 2.8 million miles ) of its destination.
NASA, facing deep cuts in its science budget and struggling to regain its footing after cancellation of the space shuttle program, the agency's centerpiece for 30 years, has a lot riding on a successful Mars landing.
Mars is the chief component of NASA's long-term deep space exploration plans. Curiosity is designed primarily to search for evidence that the planet most similar to Earth may have once harbored ingredients necessary for microbial life to evolve.
After an eight-month voyage of more than 567 million km ( 350 million miles), engineers said they were hopeful that the rover will land precisely as planned near the foot of a tall mountain rising from the floor of a vast impact basin called Gale Crater.
"We're on target to fly through the eye of the needle," Arthur Amador, the Mars Science Laboratory mission manager, told reporters at a briefing about 36 hours before landing time.
Touchdown is scheduled for 10:31 pm Sunday Pacific time (1:31 am EDT on Monday).
With Curiosity in the final stretch of its journey encased in a capsule-like shell, the spacecraft is essentially flying on automatic pilot, guided by a computer packed with pre-programmed instructions.
So precise has the vessel's approach to Mars been that NASA engineers passed on one last opportunity to perform a trajectory adjustment by remote control on Friday.
On Sunday, mission control will activate the craft's backup computer, ensuring that it will assume onboard command of the vessel should the primary computer fail during entry into the Martian atmosphere and its tricky descent to the surface.
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' upper atmosphere at 13,000 miles per hour, 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 maneuvers 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 Program 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 the rover were merely disrupted or that it crashed or burned up during descent.
Flight controllers anticipate clear and calm conditions at Gale Crater for landing, which will 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 -12C (10F).
From 248 million km (154 million miles) away, 1,400 scientists, engineers and guests are expected to tensely wait at JPL to learn Curiosity's fate. Another 5,000 people will be watching from the nearby California Institute of Technology, the academic home of JPL.
A NASA Television broadcast from mission control will take over the giant Toshiba screens in Times Square in New York City.