In the final seconds before liftoff, SpaceX aborted its planned launch of a Falcon 9 rocket—slated to carry the first commercial spacecraft to visit the International Space Station (ISS)—due to an engine problem.

"We are scrubbed for today," a SpaceX official said during a live online broadcast of the launch. "We won't be planning to launch again this morning. We only had a one-second window, and we've obviously missed that."

The Falcon 9 was supposed to lift off Saturday at 4:55 a.m. ET from Cape Canaveral Air Force Station in Florida carrying an unmanned version of the private firm's Dragon space capsule. But moments before liftoff the rocket automatically aborted the launch.

Preliminary information indicates that the scrub was due to slightly high pressure in the combustion chamber of one of the rocket's nine launch engines.

"We light the Falcon 9 engines, but the computer checks all the engine telemetry to make sure that we're good to fly before we release the vehicle," the SpaceX official explained. "Obviously we detected something that was wrong in one of the limits that we compared the data against, so we aborted the launch."

The company says the next launch window is scheduled for 3:44 a.m. ET on Tuesday.

"We got a little bit more work to go, but we'll get that right and we'll come back at it," the SpaceX official said.

Laucnh to Be Historic Milestone

If all goes as planned for next week's launch, the gumdrop-shaped Dragon capsule—stuffed with cargo for the ISS crew—will separate from the rocket components approximately ten minutes after launch.

Once in orbit, Dragon will undergo a gauntlet of test maneuvers and systems demonstrations before being allowed to berth with the space station on the fourth day after launch.

Even "if it fails one of the steps—depending on which step it fails and what its fuel reserves are at the time—it may have the opportunity to try again," said Robert Pearlman, editor of the space-history and artifacts website collectSPACE.com.

Pearlman called the Dragon test mission a potentially historic milestone for the future of spaceflight. (Also see "Virgin Galactic Unveils WhiteKnightTwo Space Plane.")

"It could set the stage for not just a series of cargo deliveries," he said, "but for American astronaut deliveries to the space station, as well as eventually establish a commercial spaceflight industry here in the United States outside of just satellite launches."

Company spokesperson Kirstin Brost Grantham added that "everyone at SpaceX is excited for this mission. You take on a certain amount of risk when you are trying to be the first to do something this big, but we can't wait to try."

How to Catch a Dragon

One of Dragon's most crucial tests will be to determine whether its piloting software can follow instructions from astronauts aboard the ISS in case they decide to abort the vehicle's approach to the space station.

"If Dragon is near the space station and something goes wrong, you want the astronauts to be able to say 'abort' and have the spacecraft leave without damaging anything," Brost Grantham said.

If Dragon passes the abort test, a robotic arm attached to the ISS will snatch the capsule and bring it in for berthing. In the future, Dragon will be able to dock with the space station under its own power, Brost Grantham said.

Before it berths, collectSPACE.com's Pearlman added, Dragon will have to perform one last test.

"Upon final approach, when it comes within a distance that the [robotic] arm can reach it, it will have to demonstrate that it can actually stay there and be stable in orbit without causing trouble to the arm or the station or itself."

Dragon to Bring Back Experiments, Old Gloves

Because this mission is primarily a test demonstration, Dragon will not be carrying any essential items to the space station or back to Earth.

For example, the launch manifest includes clothing for astronauts, extra food, and laptop batteries. The return payload will include hardware from completed space station experiments and old space suit gloves from past ISS inhabitants.

If SpaceX passes this test, NASA will give the company permission to fulfill its $1.6-billion-dollar contract to ferry supplies—and eventually astronauts—to and from the ISS.

That's a big deal, Pearlman said, because, ever since NASA's space shuttles were retired last July, the agency has had no way of returning significant amounts of material back to Earth.

For now NASA can transport items to the space station using automated transfer vehicles operated by the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA).

But these transporters are designed for one-way trips only and burn up on reentry, making return cargo trips impossible.

The Russian Soyuz spacecraft—currently NASA's only way of transporting astronauts to the ISS—does have the ability to return to Earth, but "its primary function is to return humans, not payload," Pearlman said.

Manned Private Flights by 2015?

SpaceX's Brost Grantham added that while the company is confident in its vehicles and software, success is not guaranteed.

"Every step of this mission is incredibly complicated," she said. "Whatever happens, we're going to learn more from this mission. We know that, because it's a test flight, it's inherently risky ... so it's entirely possible we're unable to accomplish all of our objectives, but we will learn from that."

SpaceX predicts that the first manned Dragon flights could occur as early as 2015. But before that happens, the spacecraft will need to be outfitted with more environmental controls and crew accommodations, something the company is already working on, according to Brost Grantham.

"A few months ago we had NASA astronauts come out and do a test of the accommodations to make sure it meets their needs," she said.

SpaceX will also have to demonstrate a successful launch-abort system for Dragon before NASA will allow it to be used as a crew-transport vehicle.

"If there's an emergency, there will be these superthrusters on the side of the spacecraft that turn on and will immediately carry the astronauts a safe distance from the rocket," Brost Grantham said.

So far, NASA and SpaceX appear to have a good working relationship, collectSPACE.com's Pearlman added.