NASA, Boeing and United Launch Alliance had to scrub another attempt to send up astronauts Butch Wilmore and Suni Williams on the Crew Flight Test mission aboard the CST-100 Starliner on its first human spaceflight.

The duo saw the scrub on June 1 after previously attempting to launch on May 6 from Cape Canaveral Space Force Station’s Space Launch Complex 41 atop a ULA Atlas V rocket for a trip to the International Space Station.

Watch live: Boeing’s Starliner set for launch today on historic human spaceflight

When they do launch, the plan is for the pair to test out manual maneuvers and the spacecraft’s environmental controls on both the way up and ride down. NASA plans to keep them on the ISS for about eight days, aiming for a return to Earth for a landing in the desert in the southwestern United States.