WASHINGTON On the eve of an historic launch of a commercial spacecraft to the International Space Station scheduled for Saturday (May 19), NASA and SpaceX officials again emphasized that the ambitious mission is ultimately a test flight and a learning opportunity.

The long delayed flight of an unmanned SpaceX Dragon spacecraft aboard a Falcon 9 rocket to the space station has a tight launch window early Saturday morning in order to catch up and dock with the space station after about 75 hours of orbital flight.

This is a test flight [and] we want to make sure we learn something as well as wring out the spacecraft, SpaceX President Gwynne Shotwell said during a briefing at the Kennedy Space Station.

The test flight is intended to show that SpaceX can launch its Dragon cargo ship into orbit and guide it to a rendezvous and docking with the space station. Only four nations have linked up with the orbiting laboratory, and SpaceX is the first commercial supplier to attempt a docking.

This mission is extremely complicated, stressed NASAs Alan Lindenmoyer, manager of its commercial crew and cargo program. Given the difficulty of SpaceX achieving all of its goals on only the second test flight of the Falcon 9 and Dragon spacecraft, Lindenmoyer said unmet objectives would roll over to the next test flight scheduled for later this year.

Beyond that, NASA and SpaceX officials declined to define what would represent a successful mission.

SpaceX successfully launched the Dragon spacecraft to orbit and returned it to Earth in December 2010, marking the first time a commercial space company had done so.

But this weekends cargo mission is far more complex. The Dragon spacecraft will carry with it a trunk containing rendezvous and proximity sensors needed to catch up with and dock with the space station. Shortly after launch, solar panels will be deployed to generate power for onboard systems. Dragon also will carry redundant flight computers needed to guide the spacecraft to the space stations docking hatch along with UHF communications so that space station astronauts can control Dragons approach and docking.

The spacecraft also will be controlled by what Shotwell called dramatically more complex software needed for navigation, rendezvous and docking. NASA managers and SpaceX engineers spent months examining every line of spacecraft code as part of a software assurance program. NASA officials were reportedly concerned about Dragon systems interfering with space station equipment.

Shotwell said the SpaceX team validated every code and configuration change in the flight software before NASA would sign off on the systems. The software assurance drill has delayed the Dragon launch several times.

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NASA, SpaceX lower expectations for historic flight to space station