NASA has fired a rocket carrying a robot space probe to collect samples from an asteroid which it is feared could one day hit Earth, in a bid to learn more about the origins of life.

The 19-storey rocket blasted off from Florida on Thursday and soared in to space at 22,000 mph - more than 28 times the speed of sound.

It will take the Osiris-Rex Atlas 5 rocket two years to reach its destination - a dark, rocky mass roughly a third of a mile wide and shaped like a giant acorn orbiting the sun at roughly the same distance as Earth.

Bennu, which crosses the planet's orbit every six years, travels around the sun at 63,000 mph and will pass between Earth and the moon in 2135.

NASA estimates that there is a one-in-2,700 chance that Bennu mighthit Earth sometime between 2175 and 2199.

The asteroid is thought to be 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," said NASA astrobiologist Daniel Glavin.

Once it settles into orbit around Bennu in 2018, Osiris-Rex will spend up to two more years mapping the asteroid's surface.

After gathering at least 2 ounces (60 grams) of material, Osiris-Rex will fly back to Earth, landing in the Utah desert in September 2023.