NASA says additional software tests are needed before Space Exploration Technologies Corp. (SpaceX) can be cleared to launch its unmanned Dragon capsule to the international space station this spring.

“SpaceX notified us last week that they would like to make additional software modifications,” Josh Byerly, a NASA spokesman based at the Johnson Space Center in Houston, told Space News Jan. 21. “Those modifications will require an integrated software retest. We are planning on supporting a software regression test with the SpaceX updated software in mid to late February.”

Kirstin Grantham, a SpaceX spokeswoman in Washington, confirmed that Dragon would launch no earlier than March 20. The mission is a demonstration flight meant to clear SpaceX to begin delivering cargo to the space station under a $1.6 billion contract with NASA.

Grantham declined to cite a specific technical reason for the delay or elaborate on a Jan. 16 SpaceX press release announcing Dragon would not launch in early February as previously planned.

“We believe there are a few areas that will benefit from additional work and will optimize the safety and success of this mission,” SpaceX said in the release.

NASA announced the Feb. 7 target date for Dragon’s launch from Cape Canaveral Air Force Station, Fla., atop a SpaceX Falcon 9 rocket during the second week of December. NASA said at the same time that it agreed to allow SpaceX to send Dragon to rendezvous and berth with the space station on the upcoming mission, an objective SpaceX originally proposed to attempt only after completing two Dragon demo flights not bound for the space station.

By the time SpaceX conducted its first Dragon demo launch in December 2010, the company was lobbying to combine the objectives of the second and third demo flights into a single mission.

Byerly said NASA still backs SpaceX’s plan to send Dragon to the space station during the upcoming mission, provided the autonomous spacecraft passes a series of on-orbit tests to ensure it can do so safely. “Any objectives not met on this flight will be addressed during the next SpaceX flight to the [international space station],” he said.

NASA and SpaceX have not set a new target launch date for the mission. Byerly said activity at the international space station will play a role in setting a new date.

A European Autonomous Transfer Vehicle is slated to deliver a fresh load of supplies to the station in mid-March. That mission, the craft’s third, will be followed in late March by the launch of a Russian Soyuz capsule carrying a new three-person crew to the station. At the end of April, Russia is due to launch an unmanned Progress ship on the second of three space station supply runs the vehicle is expected to make this year.