(Adds statement from SpaceX on next launch target)
Dec 20 (Reuters) - Thunderstorms forced Elon Musk's SpaceX to postpone Thursday's launch of a navigation satellite for the U.S. military, which was poised to be the rocket company's first national security space mission for the United States.
SpaceX said its Falcon 9 rocket and payload, a roughly $500 million GPS satellite built by Lockheed Martin Corp, were "in good health" and that it was now targeting a Saturday morning launch from Florida's Cape Canaveral.
The cancellation, which followed two previous launch attempts this week that were scrubbed for technical reasons, came as thunderstorms and wind gusts swirled around the launch site.
Patrick Burke, a meteorologist with the National Weather Service's Weather Prediction Center in College Park, Maryland, said the weather might not clear up until Saturday.
A successful launch would be a significant victory for Musk, a billionaire and Tesla Inc chief executive, who spent years trying to break into the lucrative market for military space launches, long dominated by Lockheed and Boeing Co.
It would have marked SpaceX's first so-called National Security Space mission, as defined by the U.S. military, SpaceX said.
SpaceX sued the U.S. Air Force in 2014 in protest over the military's award of a multibillion-dollar, non-compete contract for 36 rocket launches to United Launch Alliance, a partnership of Boeing and Lockheed. It dropped the lawsuit in 2015 after the Air Force agreed to open up competition.
The next year, SpaceX won an $83 million Air Force contract in 2016 to launch the GPS III satellite, which will have a lifespan of 15 years.
The launch would be the first of 32 satellites in production by Lockheed under contracts worth a combined $12.6 billion for the Air Force GPS III program, Lockheed spokesman Chip Eschenfelder said.
Air Force spokesman William Russell said: "Once fully operational, this latest generation of GPS satellites will bring new capabilities to users, including three times greater accuracy and up to eight times the anti-jamming capabilities."
The launch was originally scheduled for 2014 but has been hobbled by production delays, the Air Force said.
The next GPS III satellite is due to launch in mid-2019, Eschenfelder said, while subsequent satellites undergo testing in the company's Colorado processing facility. (Reporting by Rich McKay in Atlanta, Peter Szekely in New York, and Eric M. Johnson in Seattle; Additional reporting by Joey Roulette in Seattle Editing by Jonathan Oatis and Steve Orlofsky)