While engineers analyze and tweak software coding, SpaceX will continue making physical preparations to the Dragon spacecraft and Falcon 9 booster for a commercial launch to the International Space Station as soon as May 7, officials said Tuesday.

Artist's concept of the Dragon spacecraft in orbit. Credit: SpaceX
 
Managers on Tuesday officially reset the flight's target launch date to May 7. The precise launch opportunity will be at 9:38 a.m. EDT (1338 GMT). SpaceX could make a second launch attempt May 10 if officials are comfortable with flying the Dragon mission before the arrival of three astronauts aboard a Russian Soyuz spacecraft.

SpaceX decided Monday to delay the launch from April 30 to allow for more hardware-in-the-loop testing and proper data reviews. The company has conducted extensive software testing since last year to meet NASA's stringent safety requirements for approaching the space station.

The hardware-in-the-loop testing employs test units at SpaceX's headquarters in Hawthorne, Calif., and the flight spacecraft in the hangar at Cape Canaveral's launch pad 40.

"It's where you get the hardware and the software together and you make sure that they operate as you expect," said Mike Suffredini, NASA's space station program manager, in a media briefing earlier this month.

SpaceX's software testing must prove the Dragon spacecraft is safe enough to fly in close proximity to the space station and its crew. NASA is responsible for ensuring the complex and its residents are not threatened by visiting vehicles.

The Dragon capsule features redundant systems, but its software must be able to recognize failures and respond properly, switching to backup strings if necessary.

"We appreciate that SpaceX is taking the necessary time to help ensure the success of this historic flight," said Bill Gerstenmaier, NASA's associate administrator for human exploration and operations. "We will continue to work with SpaceX in preparing for the May 7 launch to the International Space Station."

Launch preparations continued Tuesday while SpaceX and NASA worked on the Dragon's software reviews.

Filling of the Dragon capsule's hypergolic propellant tanks was supposed to be completed Tuesday, according to Kirstin Brost Grantham, a SpaceX spokesperson.

The spacecraft was scheduled to be rotated from a vertical position to a horizontal orientation Thursday and attached to the upper stage of the Falcon 9 launcher inside SpaceX's hangar.

The aerodynamic nose cone is set to be integrated to the forward end of the Dragon capsule Friday.

A brief firing of the Falcon rocket's nine Merlin first stage engines is scheduled for April 30. SpaceX conducts a hotfire test of the Falcon 9's engines before each launch to check their health.

A technical review meeting is planned for Friday, and a SpaceX launch readiness review is set for May 5, according to an official familiar with the Dragon mission.

As currently envisioned, the schedule would make possible a launch attempt May 7, assuming NASA and SpaceX finish software testing.

SpaceX's launch on May 7 is booked on the U.S. Air Force range after the May 3 blastoff of a United Launch Alliance Atlas 5 rocket with a military communications satellite.

If the Dragon spacecraft takes off May 7, it will catch up with the space station in orbit for a flyby of the outpost May 9. The initial rendezvous will demonstrate the craft's ability to navigate and communicate with the space station.

If the flyby goes as planned, NASA and SpaceX will give the go-ahead for the capsule to approach the space station May 10. The Dragon will meticulously close in on the station, stopping several times at predetermined points to ensure all systems are functioning as expected.

The automated spaceship will reach a location 30 feet underneath the space station during the May 10 rendezvous, close enough for the lab's manually-operated robot arm to grab the capsule and install it on a berthing port.

NASA has signed up for 12 operational resupply missions with SpaceX's Falcon 9 rocket and Dragon spacecraft. The $1.6 billion contract covers cargo delivery and return services.

The space agency has also provided $381 million to help pay for development of the SpaceX vehicles under a public-private partnership.