Last-minute computer trouble nixed today’s launch attempt for Boeing’s first astronaut flight, the latest in a string of delays over the years.

Two Nasa astronauts were strapped in the company’s Starliner capsule when the countdown was automatically halted at three minutes and 50 seconds by the computer system that controls the final minutes before lift-off.

With only a split second to take off, there was no time to fix the latest problem and the launch was called off.

Nasa astronauts Suni Williams (left) and Butch Wilmore talk to family members at Space Launch Complex 41 in Cape Canaveral, Florida. Photo / AP

Technicians raced to the pad to help astronauts Butch Wilmore and Suni Williams out of the capsule atop the fully fuelled Atlas V rocket at Cape Canaveral Space Force Station. Within an hour of the launch abort, the hatch was reopened.

The team can’t get to the computers to troubleshoot the problem until the rocket is drained of all its fuel, said Tory Bruno, CEO for the rocket maker, United Launch Alliance.

Bruno said one of the three redundant computers located near the rocket at the pad was sluggish. All three must work properly to proceed with a launch, he said.

Depending on what needs to be fixed, the next launch attempt could be as early as Wednesday, local time. If it doesn’t blast off this coming week, then that would be it until mid-June in order to move the rocket off the pad and replace batteries.

“This is the business that we’re in,” Boeing’s Mark Nappi said. “Everything’s got to work perfectly.”

It was the second launch attempt. The first try on May 6 was delayed for leak checks and rocket repairs.

Nasa astronauts Butch Wilmore (left) and Suni Williams (right) were scheduled to liftoff on the Boeing Starliner capsule for a trip to the International Space Station. Photo / AP

Nasa wants a back-up to SpaceX, which has been flying astronauts since 2020.

Boeing should have launched its first crew around the same time as SpaceX, but its first test flight with no one on board in 2019 was plagued by severe software issues and never made it to the space station.

A redo in 2022 fared better, but parachute problems caused more delays. A small helium leak in the capsule’s propulsion system last month came on top of a rocket valve issue.

Boeing's Starliner capsule, atop an Atlas V rocket, sits on the launch pad during sunrise at Space Launch Complex 41 in Cape Canaveral, Florida. Photo / AP

More valve trouble cropped up two hours before today’s planned lift-off, but the team used a back-up circuit to get the ground-equipment valves working to top off the fuel for the rocket’s upper stage. Launch controllers were relieved to keep pushing ahead, but the computer system known as the ground launch sequencer ended the effort.

“Of course, this is emotionally disappointing,” Nasa astronaut Mike Fincke, the back-up pilot, said from neighbouring Kennedy Space Centre shortly after the countdown was halted.

But he said delays are part of space flight. “We’re going to have a great launch in our future.”