About a dozen years ago, the notion of bringing a pristine asteroid sample back to Earth was one of those “wouldn’t it be cool if…” things nerdy scientists kicked around over a few beers. But after one of those bar-table discussions Dante Lauretta and a few of his colleagues decided they might just be onto something. So they spent seven years writing and making proposals, and finally won enough support from their colleagues that NASA agreed to fund the concept.

The idea has since become a spacecraft, OSIRIS-REx, now resting in a hangar at Kennedy Space Center. The rocket that will launch it into deep space on Sept. 8 stands nearby. If all goes well for OSIRIS-REx over the next two years, it will fall into orbit around the 500-meter asteroid Bennu, and spend the next 700 days carefully studying the asteroid to determine where best to try to grab a sample for a return to Earth.

Only then, sometime in July, 2020, will the real challenge begin. How, exactly, does one reach out and grab some pebbles off the surface of an asteroid and live to tell the tale? Ars spoke with Lauretta, a planetary scientist at the University of Arizona and the mission’s principal investigator, to get the details.