NASA astronauts Barry "Butch" Wilmore, from left, Eric Boe and Suni Williams watch as Aerojet Rocketdyne test team engineers direct the test-firing of an RL10 engine at the company's facility in West Palm Beach, Florida. The engine will be one of two used for the Centaur upper stage during a United Launch Alliance Atlas V mission to launch Boeing's CST-100 Starliner on a flight test carrying a crew. The engine was test-fired as part of acceptance testing to confirm the engine is ready for flight.