The U.S. space agency and SpaceX, the private rocket company founded by billionaire Elon Musk, cited a technical glitch concerning the ignition fluid used to start the spacecraft's engines.

The countdown had seemed to be progressing smoothly until about two and a half minutes before blastoff, when NASA announced on its live webcast that the launch of the four crew members on a six-month science mission would be postponed.

The SpaceX Falcon 9 rocket topped with a Crew Dragon capsule had been scheduled for liftoff at 1:45 a.m. EST (0645 GMT) from NASA's Kennedy Space Center in Cape Canaveral, Florida.

The first backup launch opportunity for the mission was set for early Tuesday, about 24 hours from the initial attempt to get the rocket off the ground.

Neither NASA nor SpaceX immediately said how long it might actually take before they would be ready to try again. Eleventh-hour launch scrubs are fairly routine in the highly complex and risky endeavor of human spaceflight.

Had Monday's launch been a success, it was expected to take the crew about 25 hours to reach their destination at the International Space Station (ISS), a laboratory orbiting about 250 miles (420 km) above Earth

Designated Crew 6, the mission will carry the sixth long-duration ISS team that NASA has flown aboard SpaceX since Musk's California-based company began sending American astronauts to orbit in May 2020.

The latest ISS crew is led by mission commander Stephen Bowen, 59, a one-time U.S. Navy submarine officer who has logged more than 40 days in orbit as a veteran of three space shuttle flights and seven spacewalks.

Fellow NASA astronaut Warren "Woody" Hoburg, 37, an engineer and commercial aviator designated as the Crew 6 pilot, will be making his first spaceflight.

The Crew 6 mission also is notable for its inclusion of UAE astronaut Sultan Alneyadi, 41, only the second person from his country to fly to space and the first to launch from U.S. soil as part of a long-duration space station team. UAE's first-ever astronaut launched to orbit in 2019 aboard a Russian spacecraft.

Rounding out the four-man Crew 6 is Russian cosmonaut Andrey Fedyaev, 41, who like Alneyadi is an engineer and spaceflight rookie designated as a mission specialist for the team.

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