A mission to send the first unmanned commercial spacecraft to the International Space Station was aborted less than a second before liftoff today after a computer detected an engine problem.

Space Exploration Technologies Corp.’s Falcon 9 rocket, carrying the Hawthorne, California-based company’s Dragon capsule, attempted to lift off at 4:55 a.m. from Cape Canaveral, Florida.

The launch was rescheduled for May 22, with a back-up date the following day.

The closely held company, known as SpaceX and led by billionaire Elon Musk, was trying to make history by docking its vehicle with the station. The U.S. retired its own shuttle fleet last year and relies on other countries for rides to space. It wants the private sector to take over the job of ferrying supplies to and from the lab.

“This is not a failure,” Gwynne Shotwell, president of SpaceX, said during a National Aeronautics and Space Administration press conference after the launch attempt. “We aborted with purpose. It would be a failure if we were to have lifted off with an engine trending in this direction.”

Engine Five

The engines ignited and rumbled momentarily before going silent. An on-board computer aborted the launch a half-second before liftoff after detecting high pressure in engine five, possibly caused by a fuel valve malfunction, Shotwell said.

“The software did what it was supposed to do — aborted engine five, and it went through the remaining engine shutdown,” she said. The Falcon 9 rocket needs all nine engines for liftoff and seven to achieve orbit.

The problem was similar to one that occurred during the maiden flight of the Falcon 9 in 2010, Shotwell said. That mission was also scrubbed, though only temporarily.

SpaceX planned to have technicians on the launch pad today to inspect the engine.

NASA, which has worked with SpaceX on the launch, streamed the pre-dawn liftoff live on its website. The attempted launch followed almost three years of delays and featured an overhauled capsule that was “fundamentally a new spacecraft,” Shotwell said yesterday.

International Partners

It was to be the third flight of the Falcon 9 rocket. Three-quarters of the world’s families of rockets had at least one failure in the first three flights, according to research cited in a press kit distributed by SpaceX days before the launch.

Nine out of 10 launch failures are caused by engine malfunctions, stage separation mishaps or avionics problems, according to the 31-page document.

Only spacecraft developed by the governments of the U.S., Europe, Japan and Russia have docked with the station, which orbits about 240 miles above Earth. None of those vehicles has the ability to return significant amounts of science experiments to Earth, as the manned shuttle did and as the Dragon capsule is designed to do.

The spacecraft held more than 1,000 pounds of cargo such as food, clothing and lab equipment as well as student science experiments. The supplies were deemed “non-critical” to the space station mission.

Shuttle Retired

NASA retired the shuttle in July before companies such as SpaceX and Orbital Sciences Corp., based in Dulles, Virginia, proved they could resupply the station. The Obama administration a year earlier canceled a program to develop a shuttle successor, betting the private sector would offer lower costs.

SpaceX and Orbital have been awarded a total of almost $700 million in NASA contracts to develop the ability to deliver cargo to the station, and another $3.5 billion for 20 resupply missions slated to begin this fall.

The decision to rely on commercial technology to carry supplies starting in 2012 and astronauts in 2017 “is inherently risky because the vehicles are not yet proven and are experiencing delays in development” according to a March report by the Government Accountability Office, Congress’s investigative arm.

NASA doesn’t have agreements with its Russian, Japanese and European partners beyond 2016, according to the report. If those deals and the commercial launches don’t work out, “the situation could lead to a potential cargo shortfall,” the GAO said.

SpaceX’s Dragon capsule was scheduled to separate from the rocket about 10 minutes after liftoff, according to Shotwell.

It was to spend the first 24 hours essentially catching up to the station. Early May 21, it was to perform a fly-by at a distance of 1.5 miles to test sensors and flight systems before attempting to dock early May 22.

The mission follows a planned May 7 liftoff that was postponed to test software. The flight to the station was initially projected for June 2009.