UPDATE* 9:35 p.m. EDT - Inspections of the number five engine found a check valve that allows a nitrogen purge of the engine prior to ignition had stuck open. This stuck valve was the root cause of the high pressure in engine five detected moments before launch. The valve will be replaced and the SpaceX team will make further evaluations of the data on Sunday. If further inspections look good, the next launch attempt will be at 3:44 a.m. EDT on Tuesday.*

CAPE CANAVERAL, Florida – This morning's scheduled launch of the SpaceX Falcon 9 rocket was scrubbed with less than a second remaining on the countdown clock due to unusually high pressure in one of the engines.

In the pre-dawn darkness at Cape Canaveral in Florida, everything was looking good for a 4:55 a.m. EDT liftoff with all eyes focused on SpaceX's Launch Complex 40. About 15 minutes before launch, SpaceX founder Elon Musk announced via Twitter the software used for liftoff had "initiated master countdown script 'Auto Sequence: Yoda.'"

Excitement was building as the countdown made its classic downward progression from 10 seconds and veteran NASA announcer George Diller made it all the way to "T-0 seconds." With a flash of light emanating from the bottom of the rocket Diller continued with "liftoahhhhffff…." before his voice trailed off as it was clear the rocket was going nowhere. "We've had a cutoff, liftoff did not occur" Diller added.

The ignition sequence for the nine Merlin rocket engines had started, but the pressure in engine five was trending high. Once the limit was hit, launch software took over and aborted the liftoff. In a post abort press conference, SpaceX president Gwynne Shotwell said the problem looked like an actual issue with the engine and was not one of the sensor or software issues that had led to delays in the schedule earlier this year during simulations.

"We can not blame the software guys for this one" Shotwell noted.

The abort actually happened at T-0.5 seconds after all engines had "started nominally" and only engine five was trending high. The problematic engine is located in the center of the engine layout pictured above in the hangar before the launch attempt.

One of launch procedures SpaceX uses is to hold the rocket on the launch pad for a few seconds after ignition to make sure everything is working properly. Shotwell said today's abort is a perfect example of why the procedure is used. She said it is analogous to an airline pilot lining up on the runway and holding the brakes as the power levers are pushed forward, "we were revving the engines, we were looking at the gauges, we decided not to fly."

Aborts are nothing new to the rocket launching world. The decision not to launch is considered critical to the safety and capabilities of any launch vehicle. Shotwell emphasized the upstart space company did not suffer a setback other than a few days on the schedule. She says the cause of the problem is not known other than it was likely due to a lack of sufficient fuel in the combustion chamber.

"This is not a failure, we aborted, with purpose" Shotwell told reporters. "It would be a failure if we had lifted off with an engine trending in this direction."

Because of orbital mechanics and the path of the International Space Station, the next opportunity for a liftoff will be on Tuesday at 3:44 a.m. EDT. The SpaceX team must wait until ISS is in a proper orbital path to minimize the amount of propellant needed to chase down and get in phase with the station. The team wants to preserve as much fuel as possible for the complex maneuvering that will be needed for the demonstrations required by NASA.

This isn't the first time engine five on a Falcon 9 has caused a problem. On the first launch of the Falcon 9 in June 2010, engine five experienced a higher pressure as well leading to an initial abort.

Shotwell said initial inspection of the engine this morning showed the necessary valves appeared to be working properly and technicians must now make a more detailed inspection of the engine before determining the root cause of the high pressure.

SpaceX makes its own rocket engines at its Hawthorne, California factory. The company is investigating the possibility of fixing the engine on the rocket, or if needed, swapping out engine five from the Falcon 9 rocket in the adjacent hangar that is slated for the next flight later this year.