Wednesday's twice-around-the-world test flight of SpaceX's Dragon spacecraft is fraught with risk, and the company's wealthy founder and chief executive predicts about a 60 percent chance the mission meets all of its objectives.

The Falcon 9 rocket is poised for launch. Credit: Stephen Clark/Spaceflight Now

"Maybe I'm being a little pessimistic," said Elon Musk, the technology mogul responsible for PayPal and the Tesla electric car company. "Maybe it's a 70 percent chance," he said this week in an interview with Spaceflight Now.

Musk founded SpaceX in 2002 aiming to revolutionize space travel. Although he admits learning hard lessons about the risk of spaceflight, his long-term objectives remain the same.

"One of the over-arching objectives of SpaceX is to try to make spaceflight as routine as air flight, even though the technical challenges associated with spaceflight are much greater than with air flight," Musk said.

Known for setting measured expectations before rocket launches, Musk said he expects "2-in-3" odds of complete mission success, meaning the Falcon 9 booster puts the Dragon capsule in the correct orbit, the craft functions as planned, then the ship lands on target in the Pacific Ocean.

"I think we have a very good chance of the rocket succeeding. As long as something didn't go wrong in the manufacture or building process of the rocket, it should perform same as last time. We've actually corrected a few minor bugs," Musk said. "Nevertheless, it is only the second launch. For some bizarre reason, statistically speaking, second launches of new rockets don't have a great track record. I hope we buck that trend. I would say it's something on the order of 90 percent likely that the rocket will succeed."

The Falcon 9 rocket hit an orbital bull's-eye on its first flight in June, reaching a 150-mile-high orbit despite several minor bugs in its roll control system on the way into space. SpaceX corrected those issues for this launch.

But that flight only tested the rocket. The Dragon spacecraft, which Musk describes as more complex than the Falcon 9, has never operated in space before.

"In terms of complete success for the spacecraft, I would say it's probably a 60 percent chance," Musk said. "Complete success meaning that it does everything it's supposed to do, that it lands fully intact and it's just where we expected."

Wednesday's mission is sponsored by NASA, which is counting on SpaceX to partially replace the space shuttle's cargo-carrying abilities after the space planes are retired in 2011.

The Dragon spacecraft is designed to deliver up to 7,300 pounds of supplies to the International Space Station in a single flight. Station crews will offload the cargo and stow experiments, used equipment and other items back inside the Dragon, which has the ability to return 5,500 pounds of mass to Earth.

Artist's concept of a Dragon spacecraft approaching the International Space Station. Credit: SpaceX

After the shuttle fleet is retired, Dragon vehicles will be the only option for returning sizable cargo from the space station to Earth.

The dry cargo will be stuffed inside a pressurized volume of 240 cubic feet. If SpaceX wins a NASA contract to haul astronaut crews to and from space, the company says it can fit seven people inside the capsule.

NASA officials also attempted to curb expectations for Wednesday's flight.

"This is a test flight," said Phil McAlister, acting director of NASA's commercial spaceflight development office. "Spaceflight is very, very difficult and if history is any guide, there's undoubtledly going to be some anomalies as we go through the test program. That is what the test program is designed for -- to learn. If we get through this demonstration flight and we have a clear path to this second demonstration flight, we will have achieved a great success."

Engineers programmed a series of regimented maneuvers to exercise the Dragon's propulsion, navigation and control systems while it is in orbit. Some of the tests will simulate a rendezvous with the International Space Station, something the next Dragon mission plans to do for real in mid-2011, assuming this flight is a success.

The Dragon will be flying 18 Draco maneuvering thrusters, each of which produces up to 90 pounds of thrust. Firing in unison, several Draco engines will be used for the ship's de-orbit burn at the end of the mission.

"They're going to be doing a series of maneuvers, essentially pretending as if it were berthing with the real space station, and then come back a couple of orbits later," Musk said.

SpaceX spent several weeks this fall double-checking the Dragon's software and computer systems, according to Musk.

"We feel at this point very confident in the software and the avionics system. It is quite a complex system that's meant to be very robust in space," Musk said.

"It's just testing all the brains and logic of the system, which is a lot more complex than what's in the rocket," Musk said. "The rocket has a brain, but the rocket's function is quite straightforward. It only lasts maybe 10 minutes or so, but this system is going to be on-orbit for weeks on end, in the case of some of our customers."

The demos will begin less than 10 minutes after liftoff, when the 11,500-pound capsule pulls free from the Falcon 9's upper stage and starts up its Draco propulsion system.

"Then it's going to basically turn on all its systems and engines and start flying around the sky," Musk said. "Then something could go wrong in the propulsion system, electronics, software, pressure vessel integrity, we could lose telemetry. I'm taking this pessimistic view and naming everything that could go wrong. It's important to let the public know there's so much that can go wrong. This really is rocket science."

Temperatures outside the Dragon spacecraft will reach more than 3,000 degrees Fahrenheit during re-entry. Credit: SpaceX

But most of Musk's concern centered on the Dragon's high-speed re-entry from orbit, a fiery regime only conquered by five nations -- plus the European Space Agency -- and never attempted by a commercial firm.

"Probably the most dangerous portion of the whole thing is that re-entry," Musk said. "Is our primary heat shield constructed properly? Can it keep everything inside nice and cool? Will the parachutes deploy when they're supposed to? Those are the two biggest risks, I think, of the whole thing."

The Dragon is protected by an ablative carbon heat shield known as PICA-X, an improved version of the thermal protection system used on robotic probes returning to Earth after missions in deep space.

SpaceX the Dragon is built to survive falls back to Earth from distant destinations like the moon and Mars, missions that would demand a tougher heat shield because of the higher velocities, and temperatures, during re-entry.

"It is not, in any way, an indictment, for or against the overall program, if you have anomalies," McAlister said. "We expect anomalies. Again, the purpose of the test flight is to learn. So as long as we're learning and we have a clear path to demonstration flight two, we would consider that successful."