By Irene Klotz

CAPE CANAVERAL, Fla. (Reuters) - An Atlas 5 rocket blasted off from Florida on Thursday carrying a space probe on NASA's first quest to collect samples from an asteroid and return them to Earth in hopes of learning more about the origins of life.

The United Launch Alliance booster lifted off at 7:05 p.m. EDT (2305 GMT) from Cape Canaveral Air Force Station. Perched on top of the 19-story rocket was NASA’s robot explorer Osiris-Rex, built by Lockheed Martin <LMT.N> to carry out the seven-year, $1 billion mission to and from the near-Earth asteroid Bennu.

United Launch Alliance is a private partnership between Lockheed and Boeing <BA.N>.

The 3,300-pound (1,500 kg) solar-powered probe is expected to take two years to reach its destination, a dark, rocky mass roughly a third of a mile wide and shaped like giant acorn orbiting the sun at roughly the same distance as Earth.

Scientists believe Bennu is covered with organic compounds dating back to the earliest days of the solar system.

“You can think of these asteroids as literally prebiotic chemical factories that were producing building blocks of life 4.5 billion years ago, before Earth formed, before life started here,” NASA astrobiologist Daniel Glavin said before launch.

Once it settles into orbit around Bennu in 2018, the Osiris-Rex probe will spend up to two more years mapping the asteroid's surface and taking inventory of its chemical and mineral composition.

Scientists will ultimately select a promising site to sample and command Osiris-Rex to fly close enough to Bennu so that the probe's 11-foot-long (3.4 meter) robot arm can touch the asteroid's surface. A sampling container will then release a swirl of nitrogen gas, which will stir up gravel and soil for collection.

After gathering at least 2 ounces (60 grams) of material, Osiris-Rex will fly back to Earth, jettisoning a capsule bearing the asteroid-sample container for a parachute descent and landing in the Utah desert in September 2023.

Osiris-Rex is the latest in a series of missions to asteroids that began with the 1991 flyby of asteroid Gaspra by NASA's Jupiter-bound Galileo spacecraft.

Only one other spacecraft, Japan’s Hayabusa, has previously returned samples from an asteroid to Earth, but due to a series of problems it collected less than a milligram of material. A follow-on mission, Hayabusa 2, is underway, with a return to Earth planned for December 2020.

The U.S. space agency also hopes Osiris-Rex will demonstrate the advanced imaging and mapping techniques needed for future science missions and for upcoming commercial asteroid-mining expeditions.

Although Bennu occupies the same approximate orbital distance from the sun, it poses little threat to Earth. NASA estimates that there is a one-in-2,700 chance that Bennu might hit Earth sometime between 2175 and 2199.

(Reporting by Irene Klotz at Cape Canaveral; Editing by Steve Gorman and Tom Brown)