NASA on Monday slashed the number of astronaut missions on Boeing's Starliner contract and said the spacecraft's next mission to the International Space Station will fly without a crew, reducing the scope of a program hobbled by engineering woes and outpaced by Elon Musk's SpaceX.

NASA and Boeing are revising the Starliner spacecraft contract to reduce the number of missions from six to four, the U.S. space agency said in a statement on Monday.