By Irene Klotz
CAPE CANAVERAL, Florida (Reuters) - An unmanned Atlas 5 rocket blasted off from Florida on Saturday, launching a $2.5 billion nuclear-powered NASA rover toward Mars to look for clues on what could sustain life on the Red Planet.
The 20-story-tall booster built by United Launch Alliance lifted off from its seaside launch pad at Cape Canaveral Air Force Station at 10:02 a.m. EST (3:02 p.m. GMT).
It soared through partly cloudy skies into space, carrying NASA's Mars Science Laboratory on a 354-million mile (556 million km), nearly nine-month journey to the planet.
"I think this mission is an important next step in NASA's overall goal to address the issue of life in the universe," lead scientist John Grotzinger, with the California Institute of Technology, told reporters shortly after the launch.
The car-sized rover, nicknamed Curiosity, is expected to touch down on August 6, 2012, to begin two years of detailed analysis of a 96-mile (154-km) wide impact basin near the Martian equator called Gale Crater.
The goal is to determine if Mars has or ever had environments to support life. It is the first astrobiology mission to Mars since the 1970s-era Viking probes.
Scientists chose the landing site because it has a three-mile-high (4.8-km high) mountain of what appears from orbital imagery and mineral analysis to be layers of rock piled up like the Grand Canyon, each layer testifying to a different period in Mars' history.
The rover has 17 cameras and 10 science instruments, including chemistry labs, to identify elements in soil and rock samples to be dug up by the probe's drill-tipped robotic arm.
The base of the crater's mountain has clays, evidence of a prolonged wet environment, and what appears to be minerals such as sulfates that likely were deposited as water evaporated.
Water is considered to be a key element for life, but not the only one.
Previous Mars probes, including the rovers Spirit and Opportunity, searched for signs of past surface water.
"We are not a life-detection mission," Grotzinger said. "We have no ability to detect life present on the surface of Mars. It's an intermediate mission between the search for water and future missions, which may undertake life detection."
With Curiosity, which is twice as long and three times heavier than its predecessors, NASA shifts its focus to look for other ingredients for life, including possibly organic carbon, the building block for life on Earth.
"It's a long shot, but we're going to try," Grotzinger said.
Launch is generally considered the riskiest part of a mission, but Curiosity's landing on Mars will not be without drama.
The 1,980-pound (898 kg) rover is too big for the airbag or thruster-rocket landings used on previous Mars probes, so engineers designed a rocket-powered "sky-crane" to gently lower Curiosity to the crater floor via a 43-foot (13-meter) cable.
"We call it the 'six-minutes of terror,'" said Doug McCuistion, director of NASA's Mars Exploration Program, referring to the landing. "It is pretty scary, but my confidence level is really high."
Curiosity is powered by heat from the radioactive decay of plutonium. It is designed to last one Martian year, or 687 Earth days.