The mission will take seven years, from Thursday night's planned liftoff from Cape Canaveral to the return of the asteroid samples in 2023, and cover an incredible 4 billion miles (6 ½ billion kilometers) through space.

It promises to be the biggest cosmic bounty since the Apollo moon rocks, hand-picked and delivered by astronauts in the late 1960s and early 1970s.

NASA has already brought back comet dust and specks of solar wind. And Japan already did it at an asteroid a decade ago, and is en route to a second space rock encounter.

But we're talking tiny grains in these cases. NASA's robotic asteroid hunter, Osiris-Rex, is designed to scoop up pebbles and rock the size of gravel anywhere from one to four or even five handfuls in a single swipe.

"We are going out to explore an unknown world," said principal scientist Dante Lauretta of the University of Arizona at Tucson.

"We're going to map it in great detail. It will be the most well characterized asteroid in our solar system by the time we're through with it."

Thanks to observations from the Hubble and Spitzer space telescopes and ground observatories, scientists already know the roundish Bennu (BEN'-oo) is about 1,600 feet (487 meters) across at its bulging middle and the color of coal, indicative of carbon richness.

It's believed to have formed 4.5 billion years ago, a remnant of the solar system's building blocks. As such, it may still hold clues as to the origin of life on Earth and, possibly, elsewhere in the solar system.

The name Bennu comes from the heron of Egyptian mythology. Osiris was an Egyptian god; Osiris-Rex is an acronym for origins, spectral interpretation, resource identification, security-regolith explorer.

There's also a practical side to the more than $800 million mission: planetary defense. Bennu is one of many near-Earth asteroids that occasionally cross paths with our planet. The more scientists know about these potentially hazardous rocks, the better the chance of tracking them and, worst comes to worst, bracing for them.

Bennu wouldn't obliterate Earth or wipe out life, just carve out a huge crater. Lauretta said the odds of a direct hit by Bennu are low less than one-tenth of 1 percent and not until about 150 years from now.

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