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A nuclear-powered rover as big as a compact car is set to begin a nine-month journey to Mars this weekend to learn if the planet is or ever was suitable for life.

The launch of NASA's $2.5 billion Mars Science Laboratory aboard an unmanned United Launch Alliance Atlas 5 rocket is set for 10:02 a.m. EST (1502 GMT) on Saturday from Cape Canaveral Air Force Station, located just south of the Kennedy Space Center.

The mission is the first since NASA's 1970s-era Viking program to directly tackle the age-old question of whether there is life in the universe beyond Earth.

"This is the most complicated mission we have attempted on the surface of Mars," Peter Theisinger, Mars Science Lab project manager with NASA prime contractor Lockheed Martin, told reporters at a pre-launch press conference on Wednesday.

The consensus of scientists after experiments by the twin Viking landers was that life did not exist on Mars. Two decades later, NASA embarked on a new strategy to find signs of past water on Mars, realizing the question of life could not be examined without a better understanding of the planet's environment.

"Everything we know about life and what makes a livable environment is peculiar to Earth," said astrobiologist Pamela Conrad of NASA's Jet Propulsion Laboratory in Pasadena, Calif., and a deputy lead scientist for the mission.

"What things look like on Mars are a function of not only the initial set of ingredients that Mars had when it was made, but the processes that have affected Mars," she said.

NEW MARS ROVER

Without a large enough moon to stabilize its tilt, Mars has undergone dramatic climate changes over the eons as its spin axis wobbled closer or farther from the sun.

The history of what happened on Mars during those times is chemically locked in its rocks, including whether liquid water and other ingredients believed necessary for life existed on the planet's surface, and if so, for how long.

In 2004, the golf cart-sized rovers Spirit and Opportunity landed on opposite sides of Mars' equator to tackle the question of water.

Their three-month missions grew to seven years, with Spirit succumbing to the harsh winter in the past year and Opportunity beginning a search in a new area filled with water-formed clays. Both rovers found signs that water mingled with rocks during Mars' past.

The new rover, nicknamed Curiosity, shifts the hunt to other elements key to life, particularly organics.

"One of the ingredients of life is water," said Mary Voytek, director of NASA's astrobiology program. "We're now looking to see if we can find other conditions that are necessary for life by defining habitability or what does it take in the environment to support life."

The spacecraft, which is designed to last two years, is outfitted with 10 tools to analyze one particularly alluring site on Mars called Gale Crater. The site is a 96-mile (154-kilometer) wide basin that has a layered mountain of deposits stretching 3 miles above its floor, twice as tall as the layers of rock in the Grand Canyon.

Scientists do not know how the mound formed but suspect it is the eroded remains of sediment that once completely filled the crater.

SKY CRANE DELIVERY

Curiosity's toolkit includes a robotic arm with a drill, onboard chemistry labs to analyze powdered samples and a laser that can pulverize rock and soil samples from a distance of 20 feet away.

If all goes as planned, Curiosity will be lowered to the floor of Gale Crater in August 2012 by a new landing system called a sky crane. Previously, NASA used airbags or thruster jets to cushion a probe's touchdown on Mars but the 1,980-pound (900-kilogram) Curiosity needed a beefier system.

"There are a lot of people who look at that and say, 'What are you thinking?'" Theisinger said. "We put together a test program that successfully validated that from a design standpoint it will work. If something decides to break at that point in time, we're in trouble but we've done everything we can think of to do."

The rover, which is twice as long and about three times heavier than the Spirit and Opportunity rovers, also needed more power for driving at night and operating its science instruments.

Instead of solar power, Curiosity is equipped with a plutonium battery that generates electricity from the heat of radioactive decay.

Similar systems have been used since the earliest days of the space program, including the Apollo moon missions, the Voyager and Viking probes and more recently in the Cassini spacecraft now circling Saturn and NASA's Pluto-bound New Horizons mission.

Radiation monitors have been installed through the area around the Cape Canaveral launch site in case of an accident, though the device has been designed to withstand impacts and explosions, said Randall Scott, director of NASA's radiological control center at the Kennedy Space Center.

Meteorologists were predicting good weather for Saturday's launch. Earth and Mars will be favorably aligned for launch until December 18.