Boeing's new Starliner capsule flew its first crew of astronauts to space on Wednesday from Florida in a much-delayed test mission to the International Space Station, a milestone in the aerospace giant's ambitions to step up its competition with Elon Musk's SpaceX.

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

The gumdrop-shaped capsule and its crew was headed for a rendezvous with the International Space Station (ISS) following years of technical problems, delays and a successful 2022 test mission to the orbital laboratory without astronauts aboard.

On Wednesday, the Atlas V's engines thundered to life in flaming clouds of exhaust and coolant-water vapor as the spacecraft roared off its launch pad into sunny skies from Florida's Atlantic Coast.

The rocket's upper stage separated from its lower section about four minutes into flight, followed by Starliner's separation from the second stage. Now on its own, the spacecraft fired onboard thrusters to start pushing itself into orbit, mission managers said, a process that starts its 24-hour catch-up journey with the ISS, which orbits some 250 miles (400 km) above Earth.

Starliner will need to execute precise maneuvers to dock with the ISS as planned on Thursday, demonstrate it can stay docked for about eight days, then safely return the two astronauts to Earth.

Boeing intends for Starliner to compete with SpaceX's Crew Dragon capsule, which since 2020 has been NASA's only vehicle for sending ISS crew members to orbit from US soil.

Mark Nappi, Boeing's Starliner program chief, told a news conference that tens of thousands of things have to go right during Starliner's launch, and that on Wednesday "it all lined up."

"We had just a perfect countdown and launch. ... We're on orbit with Butch and Suni, it's been a longtime coming. And we're really, really proud," Nappi said.

Boeing shares were up less than 1% in early afternoon trade.

"Starliner ascends to the heavens," NASA chief Bill Nelson wrote on social media, congratulating Boeing, ULA and the US space agency.

Nelson called the launch "a milestone achievement for the future of spaceflight. Butch and Suni - safe travels through the stars. See you back home."

Last-minute issues had nixed the Starliner's first two crewed launch attempts. A May 6 countdown was halted two hours before liftoff over three issues that required weeks of extra scrutiny. Another try last Saturday was halted less than four minutes before liftoff because of a launchpad computer glitch.

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

They have spent a combined 500 days in space over the course of two ISS missions each. Wilmore is the designated commander for the flight, with Williams in the pilot seat.

Boeing, with its commercial airplane operations rocked by a series of crises involving its 737 MAX jetliners, needs a win in space for its Starliner venture, already several years behind schedule and more than $1.5 billion over budget.

The longtime NASA contractor has built modules for the decades-old ISS and rockets designed to loft astronauts toward the moon. But Boeing never before built its own operational spacecraft, a feat complicated by years of software issues, technical glitches and management shakeups on the Starliner program.

Meanwhile, SpaceX's Crew Dragon has become a dependable taxi to orbit for NASA. That capsule and Starliner are among the first in a new generation of privately built spacecraft - seeded with NASA funding - designed to fly astronauts to low-Earth orbit and the moon under the agency's Artemis program.
Starliner would compete head-to-head with SpaceX's Crew Dragon capsule, which since 2020 has been NASA's only vehicle for sending ISS crew members to orbit from US soil.

"Congratulations on a successful launch," Musk wrote on social media.

Although Starliner is designed to fly autonomously, the crew can assume control of it if necessary. The test flight calls for Wilmore and Williams to practice maneuvering manually en route to ISS.

Wilmore and Williams are due to join ISS's current seven resident crew members before riding the capsule back to Earth for a parachute and airbag-assisted landing in the US Desert Southwest - a first for a crewed NASA mission.

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 US rides to the ISS, an outpost expected to retire around 2030.

Meanwhile, SpaceX's fourth test launch of its Starship rocket system, another step in its quest to build a reusable satellite launcher and moon lander, is due for liftoff on Thursday from Texas.