WASHINGTON (Reuters) -Boeing and Lockheed Martin are in talks to sell their rocket-launching joint venture United Launch Alliance to Sierra Space, two people familiar with the discussions said. A deal could value ULA at around $2 billion to $3 billion, the sources said. A deal to sell ULA, a major provider of launch services to the U.S. government and a top rival to Elon Musk's SpaceX, would mark a significant shift in the U.S. space launch industry as ULA separates from two of the largest defense contractors to a smaller, privately held firm.