(Reuters) - Boeing's Starliner capsule faces a crucial test on Thursday when it is expected to dock with the International Space Station (ISS) on the spacecraft's first trip to orbit carrying astronauts, as the aerospace giant looks to sharpen its competition with Elon Musk's SpaceX.

The CST-100 Starliner, with astronauts Barry "Butch" Wilmore and Sunita "Suni" Williams aboard, was launched from the Cape Canaveral Space Force Station in Florida on Wednesday, strapped to an Atlas V rocket furnished and flown by the Boeing-Lockheed Martin joint venture United Launch Alliance (ULA).

The reusable gumdrop-shaped capsule and its crew have a rendezvous with the ISS. It is scheduled at 12:15 p.m. ET (1615 GMT) to dock autonomously with the ISS, which orbits some 250 miles (400 km) above Earth. It is due to stay docked for about eight days, then safely return the two astronauts to Earth, among other flight objectives.

Its launch on Wednesday followed years of technical problems, various delays and a successful 2022 test mission to the orbital laboratory without astronauts aboard.

Boeing intends for Starliner - seeded with NASA funding - to compete with SpaceX's Crew Dragon capsule, which since 2020 has been the U.S. space agency's only vehicle for sending ISS crew members to orbit from U.S. soil. The mission is a test flight required before NASA can certify Starliner for routine astronaut missions.

The seven-seat Starliner's inaugural crew includes two veteran NASA astronauts in Wilmore, 61, a retired U.S. Navy captain and fighter pilot, and Williams, 58, a former Navy helicopter test pilot with experience flying more than 30 different aircraft.

Getting Starliner to this point has been a fraught process for Boeing under its $4.2 billion fixed-priced contract with NASA, which wants the redundancy of two different U.S. rides to the ISS. The Starliner is several years behind schedule and more than $1.5 billion over budget. Meanwhile, Boeing's commercial airplane operations have been rocked by crises involving its 737 MAX jetliners.

(Reporting by Joey Roulette in Washington; Editing by Will Dunham)

By Joey Roulette