WASHINGTON — NASA has narrowed the field of competitors still in the running for $50 million in economic stimulus money the U.S. space agency intends to award this year to seed development of commercial crew transportation systems, according to industry sources.

NASA contacted at least six companies Nov. 6, inviting Denver-based United Launch Alliance, Chicago-based Boeing, Hawthorne,Calif.-based Space Exploration Technologies (SpaceX), Sparks, Boulder, Colo.-based Ball Aerospace & Technologies Corp., Nev.-based SierraNevada Corp. and Tucson, Ariz.-based Paragon Space Development Corp. to  discuss the Commercial Crew Development, or CCDev, proposals each submitted in September.

Industry sources say that while no single company is expected to receive the entire pot of money, it remains unclear how NASA will divide the $50 million for commercial crew initiatives the agency received in February as part of the American Recovery and Reinvestment Act.

Boeing announced in September it would use CCDev money to accelerate development of a crew capsule that could be launched atop a human-rated variant of the U.S. Air Force’s Atlas 5 or other expendable rocket. The project, which proposed a teaming arrangement with North Las Vegas-based Bigelow Aerospace, is one of at least three Boeing-backed proposals that industry sources say made the cut.

Multiple industry sources said NASA’s CCDev down-select appears to have favored proposals geared toward development of crew capsules designed to launch atop ULA’s Atlas 5.

NASA spokeswoman Stephanie Schierholz said the agency plans to announce CCDev awards this month. The announcement is expected to come as the White House works toward a decision on the future of NASA’s manned spaceflight programs.

For weeks senior officials in the administration of U.S.President Barack Obama have been mulling over the findings of a blue-ribbon panel tasked with reassessing NASA’s manned spaceflight goals.

The panel, led by former Lockheed Martin chief Norm Augustine, said the agency’s current plan to replace the space shuttle with rockets and spacecraft optimized for the Moon is incompatible with the agency’s budget. The panel’s report, issued Oct. 22, urged the administration to consider outsourcing operations in low Earth orbit to the private sector, freeing NASA to pursue more ambitious exploration efforts in deep space.