Cape Canaveral, Fla — Pending a final software review next week, Space Exploration Technologies was cleared April 16 for a test flight to the international space station, a critical milestone for NASA’s lower-cost and controversial plan to purchase flight services for cargo and eventually for crew, rather than developing and operating its own spaceships.

SpaceX’s Falcon 9 rocket and Dragon capsule are scheduled to lift off from the company’s launch pad here at 12:22 p.m. EDT April 30. If successful, the capsule would take a day to put itself into the station’s orbit and then spend another day demonstrating maneuverability, abort commands, communications and other systems before flying to within reach of the station’s robot arm so it can be grappled and berthed to the outpost May 3.

“This is really a tough flight. What we’re asking them to go do on this demonstration flight is amazing,” Bill Gerstenmaier, NASA associate administrator for human exploration and operations, said at an April 16 press conference following the mission’s Flight Readiness Review. “When you look at all those things that have to work, and all the software has to interact, and the six computers and the 18 thrusters, and all this has to work as a nice combined set to get into this precise box to get picked up by the [space station’s arm], that is no easy task.

“We need to look at it as a test flight, that’s what it should be. We’ll see how well the test works out, but they’ve really done a good job getting ready.”

SpaceX founder and Chief Executive Elon Musk declined to put odds on the chance of total mission success, including the capsule’s retrieval from the Pacific Ocean about three weeks after launch.

“I think we’ve got a pretty good shot, but it is worth emphasizing that there’s a lot that can go wrong in a mission like this,” Musk said.

Although SpaceX’s Falcon 9 rocket has successfully flown twice, “it’s still relatively new,” Musk said. On its last launch in December 2010, the rocket put a test Dragon capsule into orbit.

For its second and possibly last test run, Dragon’s solar arrays will be deployed for the first time and the capsule will need to demonstrate its maneuverability in orbit and its berthing systems.

“There’s no space station on the ground, so our work to date has been done by simulation and by approximating the circumstances that it will find in orbit and approaching the space station. This is pretty tricky,” Musk said.

NASA already has hired SpaceX, as well as a second company, Orbital Sciences Corp., to deliver cargo to the space station under contracts worth a combined $3.5 billion. Both companies also received development funds. So far, SpaceX has received $381 million for a program that Musk estimates cost about $1 billion to date.

The company also is vying for related NASA funding to upgrade the Dragon capsule to carry astronauts.

“There’s no question that there are going to be some people who put too much weight on this flight because it is explicitly a test flight and indeed we may not succeed on getting all the way to the space station,” Musk said. “I think it would be a mistake to put too much weight on this flight because there are hopefully going to be two more flights later this year to the space station, which will be almost identical configuration. So if this one doesn’t succeed in getting to space station, I’m confident that one of the other two will. There should be no doubt about our resolve. We will get to the space station.”

“I don’t think that if this mission doesn’t get all the way there that it should be taken as a verdict on commercial crew transport. That just wouldn’t be right, although there will be some people who try to do that.”

With the retirement of the space shuttle last year, NASA is dependent on SpaceX and Orbital Sciences to fly cargo to the outpost, a $100 billion project of the United States, Russia, Europe, Japan and Canada.

“This is absolutely critical to space station. We really need this cargo capability, and the return capability that Dragon provides is truly unique,” Gerstenmaier said. “We’re really rooting for the teams to come through.”

Dragon will be carrying 521 kilograms of cargo to the station and is expected to return with 660 kilograms — nothing critical in case the test run does not go as planned, said Mike Suffredini, NASA space station program manager.

Following liftoff, Dragon will spend one to three days phasing itself into the station’s orbit and another day demonstrating maneuvers, including a flyby 2.5 kilometers beneath the station to test communications directly with the outpost for the first time.

“This is an absolute requirement for proximity operations,” said NASA Flight Director Holly Ridings.

After the flyby, Dragon will head back down to about 10 kilometers beneath the station and then lap around the outpost at a distance of more than 200 kilometers.

If all goes as planned, the next day Dragon will return to a point about 2.5 kilometers beneath the station to begin its approach for berthing. The capsule will head to a point 1.4 kilometers from the station and make any adjustments to its approach, aiming for a point 350 meters below the outpost. It will pause automatically at 250 meters for an additional spate of tests involving the station crew, which will command Dragon to retreat.

SpaceX, operating from its Hawthorne, Calif., control center, will command Dragon to resume its course toward the station. At about 220 meters the station crew is expected to order Dragon to halt.

“That will be the last of our go/no-go objectives in terms of the demonstration objectives,” Ridings said.

Another round of equipment, communications and systems checks will follow before SpaceX is cleared to navigate Dragon into what is called “the keep-out sphere” — a safety zone 200 meters around the outpost.

At 30 meters, Dragon will automatically pause again for more equipment checks before proceeding to a point about 10 meters away from the station and within reach of the station’s robotic crane.

After final clearance from NASA, the station crew will take over operations to pluck Dragon from orbit and berth it to the station’s Earth-facing port on the Harmony connecting node. Hatch opening will be scheduled for the following day.

The capsule will remain attached to the station until about May 21 and splash down in the Pacific Ocean for recovery by SpaceX.

If weather or technical issues prevent a launch April 30, SpaceX has a backup launch opportunity May 3.