Preparations for the company’s trial cargo run to the International Space Station proceeded smoothly until 4:55 a.m. EDT (08:55 GMT) when an onboard computer aborted the launch.

“Liftoff … we’ve had a cutoff. Liftoff did not occur,” said NASA launch commentator George Diller, caught off guard by the sudden, last-minute turn of events.

A computer monitoring the rocket’s nine engines detected a climbing pressure reading in one engine’s chambers and halted the launch 0.5 seconds before liftoff, SpaceX president Gwynne Shotwell told reporters.

“Just like a pilot at the end of a runway revs the engines and looks at the gauges. We were revving the engines, we were looking at the gauges and we decided not to fly,” Shotwell said, adding that the problem was unlikely to be a sensor issue.

The company’s next launch opportunity is at 3:44 a.m. EDT (07:44 GMT) on Tuesday.

It is trying to send the unmanned rocket, carrying a Dragon cargo capsule, to the International Space Station, and would be the first private company to do this.

SpaceX is one of two firms hired by NASA to fly cargo to the $100 billion orbital outpost, which is owned by the United States, Russia, Europe,Japan and Canada.

Since the U.S. space shuttles were retired last year, NASA has had no way to reach the station and is dependent on its partner countries to fly cargo and crew. It hopes to change that by buying rides commercially from U.S. companies.