NASA astronauts are set to test-drive Boeing’s Starliner spacecraft during its first manned mission to the International Space Station on Monday, May 6, after a decade of mechanical challenges. The CST-100 Starliner test mission, long delayed and over $1 billion over budget, will carry two NASA astronauts to the International Space Station in its final test before regular missions can begin.

The crewed flight test aims to validate Boeing’s capabilities for regular trips to the ISS, competing again SpaceX, under NASA’s Commercial Crew Program. Boeing has encountered setbacks with Starliner, including a failed uncrewed test flight in 2019 due to software issues and challenges with the vehicle’s parachute system.

“The first crewed flight of a new spacecraft is an absolutely critical milestone,” NASA associate administrator Jim Free told reporters. “The lives of our crew members, Suni Williams and Butch Wilmore, are at stake.”

Despite these hurdles, NASA has expressed confidence in the spacecraft’s improvements and Boeing’s commitment to safety.

“Knowing firsthand the ins and outs of NASA, this launch would not be happening if there wasn’t an enormous amount of confidence in the Starliner achieving its objectives,” Jim Bridenstine, the previous NASA administrator, told Reuters.

NASA’s Commercial Crew Program enables companies to offer spacecraft services to private customers. SpaceX has utilized this opportunity, selling seats on its Crew Dragon for an estimated $55 million each, while Boeing has not yet done so with its Starliner, which costs about $90 million per seat, according to NASA’s inspector general.