An historic NASA mission embarking Thursday — the first U.S. attempt to sample an asteroid — may point to the origins of the solar system.

It's a big mission for a small sample, heading three quarters of a million miles into space to one of several thousand near-Earth asteroids, on a seven-year mission.

Asteroid Bennu was chosen because it's not too far from our planet and because it's a big one: 500 meters across, about the size of a small mountain.

Smaller asteroids tend to spin quicker, throwing off any loose debris on the surface.

"About one year after launch, and going around the sun once, we will do a gravity assist and change the plane of the spacecraft, put it in the same plane as the asteroid Bennu," said OSIRIS-REx Project Manager Michael Donnelly.

Two years after launch, in August 2018, the OSIRIS-REx will arrive at Bennu.

But the critical moments won't happen for two more years, when in July 2020, the craft's 10-foot-long arm will dip down, and for just 3 seconds, bounce off the surface to collect just a couple of ounces as a sample.

"We have great science ahead of us. I'm really excited to get to this milestone, launched to its journey to Bennu and back," OSIRIS-REx Principal Investigator Dante Lauretta said.

Then comes the trip home, getting back to Earth in September 2023. The sample will be delivered into the hands of scientists to study.

The Atlas V rocket carrying the OSIRIS-REx spacecraft is set to lift off Thursday between 7:05 p.m. and 9 p.m. from Cape Canaveral Air Force Station.

Right now, there's an 80 percent chance of good weather.