WASHINGTON — NASA expects to order launches for three Earth science missions by the end of the summer, and United Launch Alliance (ULA) looks like the strongest contender for the job with its medium-lift Delta 2 rocket, an agency official said.

NASA sent a request for proposals to its current stable of approved launch services providers — ULA, Space Exploration Technologies Corp. (SpaceX), Orbital Sciences Corp. and Lockheed Martin Corp. — in early February seeking bids for three missions: Soil Moisture Active-Passive, Orbiting Carbon Observatory-2 and the Joint Polar Satellite System-1. Proposals are due April 8, with the launches taking place from 2014-2017.

Steve Volz, associate director of flight programs in NASA’s Earth Science Division, said only two of the approved NASA Launch Services 2 vendors, ULA and SpaceX, currently have rockets that meet the agency’s criteria. But he said the limited flight heritage of SpaceX’s Falcon 9 rocket — two successes in two launches, with a third slated for April 30 — puts it at a disadvantage.

“Right now, the two possible proposals … are the Delta 2 from ULA and the SpaceX Falcon 9,” Volz told the NASA Advisory Council during a March 21 meeting here. “Delta 2 can bid, and they’re certified; it’s easy. Falcon 9, they may bid, but they haven’t been certified, so there’s a risk on those.”

The Delta 2, which for years was the most reliable vehicle in the U.S. fleet, is out of production, but ULA has five of the vehicles remaining for sale. ULA spokeswoman Jessica Rye confirmed March 22 that the company will be bidding the Delta 2 for the NASA contract.

SpaceX spokeswoman Kirstin Grantham said March 22 that her company will bid for at least a share of the work. “We are submitting a certification plan with our proposal,” she said.

While Volz was skeptical that Falcon 9 could achieve NASA certification in time to launch any of the three upcoming missions, the vehicle is “likely to be a viable contender” for Earth science missions “that launch in 2018, 2019, 2020.”

Jim Norman, director of the NASA Launch Services Program, said in a March 22 email that the launch solicitation is open to rockets that “will meet (at minimum) Category 2 certification” requirements. Those requirements call for one to three successful flights and a raft of NASA reviews.

NASA previously had given the Orbiting Carbon Observatory-2 launch contract to Orbital, but rescinded the award after an Orbital-built Taurus XL failure destroyed NASA’s Glory climate-monitoring satellite last March. An earlier Taurus XL failure destroyed the original Orbiting Carbon Observatory craft.

“The Taurus XL isn’t available until it’s recertified,” Volz said. “We’re not going to be the next ones on that launch vehicle.”

NASA has looked at using the U.S. Air Force Minotaur 4 rocket, assembled by Orbital using excess missile stages, but Volz said the agency is unlikely to go that route.

“If we get only proposals that are extremely expensive or extremely high risk, we have the avenue to continue to pursue the Minotaur 4,” Volz said. “The likelihood is small. … I don’t expect it to happen.”

NASA announced in 2007 that it would phase out the Delta 2 by the end of the decade because the rocket would be unaffordable in the absence of Air Force support. The Air Force had been the primary customer for the Delta 2 but stopped using the vehicle in 2009.

The Delta 2 last launched in October, when it delivered the Suomi NPP climate and weather satellite to orbit.

Dan Leone is a SpaceNews staff writer, covering NASA, NOAA and a growing number of entrepreneurial space companies. He earned a bachelor’s degree in public communications from the American University in Washington.