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SpaceX's commercial liftoff to space station put on hold again


The SpaceX Dragon capsule is prominent in this photo of the Falcon 9 rocket in its lowered position at Cape Canaveral Air Force Station's launch complex in Florida.

The first private-sector spaceship destined to hook up with the International Space Station will have to wait a few days longer than planned for its Florida launch.

SpaceX's Falcon 9 rocket had been scheduled to lift off from Cape Canaveral Air Force Station's Launch Complex 40 on May 7, on a test flight that could climax with a space station berthing of its unmanned Dragon cargo capsule several days later. A launch-pad engine test went off successfully on Monday, but more time is needed to analyze changes in SpaceX's flight software and make sure all systems are go.

"At this time, a May 7 launch appears unlikely," SpaceX communications director Kirstin Brost Grantham said in an email. "SpaceX is continuing to work through the software assurance process with NASA.  We will issue a statement as soon as a new launch target is set."

Due to the orbital mechanics involved in a space station rendezvous, the Falcon 9 can be launched only at a precise time during the day, on specific dates. The next opportunity for launch comes on May 10, but it's not yet clear whether liftoff will be reset for that date. In a Twitter update, Space News' Brian Berger cited an internal NASA manifest that showed the launch slipping to no earlier than May 10. After that date, SpaceX would have to stand down to let the Russians launch a three-person crew in a Soyuz craft to the space station on May 14.

SpaceX conducts a test firing of its Falcon 9 rocket's engines on April 30 at Cape Canaveral.

California-based SpaceX has received hundreds of millions of dollars from NASA to support the development of the Falcon 9 and the gumdrop-shaped Dragon capsule for resupplying the space station. The Falcon 9 has been sent into orbit only twice before — once in June 2010 with a test capsule, and again in December 2010 with a functional Dragon spacecraft that returned to Earth after two orbits.

The upcoming demonstration launch has been rescheduled repeatedly, from February to April to May, due to the need for intensive software reviews. The flight plan calls for Dragon to execute a series of maneuvers near the space station. If the spaceship's sensors and flight systems work as designed, Dragon will then fly a rendezvous and approach. If Dragon reaches the station safely, the station's astronauts will use a robotic arm to bring the commercial spaceship in for berthing, and then unload the non-essential cargo that's aboard.

A couple of weeks later, Dragon would be sent back down to a Pacific Ocean splashdown.

A success during this first berthing attempt would open the way for SpaceX to start regular robotic resupply missions to the space station under the terms of a $1.6 billion NASA contract. It also could help pave the way for Dragon to ferry Americans to and from the space station in three to five years, depending on further NASA funding. 

Since the last space shuttle left the station last July, Americans can travel into orbit only as passengers aboard Russian spacecraft, at a cost of about $60 million a seat.

More about SpaceX and the commercial space race:

Last updated 3:25 p.m. ET. Tip o' the log to NBC News' Cape Canaveral correspondent, Jay Barbree.

 Alan Boyle is msnbc.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or adding Cosmic Log's Google+ page to your circle. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for other worlds.