Set to boost NASA's OSIRIS-REx spacecraft to the Bennu asteroid, United Launch Alliance (ULA) is engaging in a variety of "dress rehearsal" actions—the full name of the spacecraft is quite a mouthful, hence, the acronym—Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer.
At Space Launch Complex 41, which is located at the Cape Canaveral Air Force Station in Florida, the booster and the Centaur upper stage of ULA's Atlas V went through a "wet dress rehearsal," where the launch vehicle vented the craft's gaseous propellant.
OSIRIS-REx is scheduled to closely survey the asteroid prior to obtaining a sample from the surface and to then return a small, two ounce sample of material back to Earth for study. The liftoff is targeted for September 8th.