WASHINGTON — NASA and Denver-based United Launch Alliance (ULA) are negotiating to add the Delta 2 medium-class rocket to the agency’s list of available launch vehicles capable of lofting small- to intermediate-sized science payloads to orbit.

“ULA has indicated to NASA it is interested in on-ramping the Delta 2,” NASA spokeswoman Stephanie Schierholz said in a May 12 email response to questions.

Schierholz said the agency could add the Delta 2 this summer to NASA’s recently negotiated launch services contract, a roster that includes eight U.S.-made launch vehicles capable of delivering between 175 and 14,280 kilograms to orbit. However, “NASA may accelerate the start of the on-ramp period to better distribute launch services program resources during the summer/fall timeframe,” Schierholz said.

Since 1998, NASA has relied on the Delta 2 rocket to carry almost 60 percent of its scientific satellites into orbit, according to a November 2010 report by the U.S. Government Accountability Office (GAO), “NASA: Medium Launch Transition Strategy Leverages Ongoing Investments but is Not Without Risk.”

Delta 2, however, is no longer in production, and just five of the rocket’s heavy variant remain in ULA’s inventory, while “no other vehicle in the relative cost and performance range is currently certified for NASA use,” the GAO report states.

NASA’s plan to phase out Delta 2 stemmed from a decision by the U.S. Air Force to end its use of the rocket in 2009, leaving NASA to foot the bill for maintaining the launch vehicle’s infrastructure. Instead, NASA opted to bet on development of new commercial launchers to fill the void, including the Falcon 9 rocket built by Space Exploration Technologies Corp. of Hawthorne, Calif., and Dulles, Va.-based Orbital Science Corp.’s new Taurus 2 rocket, two launch vehicles with lift capabilities similar to the Delta 2. However, it takes approximately three years for a new launch vehicle to be certified by NASA’s Launch Services Program, a process that generally does not begin until the vehicle has been selected by the space agency for a particular mission, according to the GAO.

In the meantime, NASA has few reliable options for launching small-to-medium-sized science payloads in the coming years. Orbital’s Taurus XL rocket, a ground-launched version of its highly reliable air-launched Pegasus vehicle, has sent two of NASA’s Earth science payloads into the Pacific Ocean over the past two years due to a faulty payload fairing. The most recent mishap, which resulted in the loss of NASA’s climate-monitoring Glory spacecraft during a March 4 launch attempt from Vandenberg Air Force Base, Calif., is currently under review by an accident investigation board that is not required to complete its work before Sept. 6, NASA spokesman Stephen Cole said May 11.

A similar Taurus XL mishap occurred in February 2009 when the rocket failed to launch NASA’s Orbiting Carbon Observatory (OCO), a satellite built by Orbital Sciences to measure the Earth’s greenhouse gas emissions. NASA spent four months investigating the mishap and another year-and-a-half implementing corrective actions prior to the Glory launch, Schierholz said.

“What will need to be done this time will depend in large part on the findings and recommendations of the Mishap Investigation Board,” she said.

Ed Weiler, head of NASA’s Science Mission Directorate, said he will not fly another payload atop a Taurus XL rocket until NASA recertifies the vehicle. In April the agency stopped payments on a Taurus XL launch vehicle contract with Orbital Sciences for a February 2013 re-flight of the OCO satellite, dubbed OCO 2.

“We can’t fly on Taurus XL now, it’s got to be recertified,” Weiler told a May 11 meeting of the NASA Advisory Council’s Earth science subcommittee.

“We’ve got a broader issue in [the Science Mission Directorate] in that we don’t have a reliable launch vehicle right now that we can count on below an Atlas 5,” Weiler said during the May 11 meeting, referring to ULA’s intermediate-class Atlas 5 rocket. Atlas 5 costs more than doubled between NASA’s previous launch services contract and the updated agreement NASA inked with ULA last year.

“Atlas 5s are being priced out of our range,” Weiler said. “This is now starting to affect all of our missions.”

Michael Freilich, director of NASA’s Earth Sciences Division, said NASA has lost nearly $700 million in Earth science payloads to Taurus XL over the past two years. In a May 11 interview Freilich said he would like to see the rocket fly successfully before putting another Earth-monitoring probe atop it.

“I would go more than recertified, personally,” he told Space News. “I would go demonstrated.”

Absent a demo flight, “I don’t know what anybody would say about how [we] were good stewards of the taxpayer money if we had a third consecutive launch vehicle failure; $693 million-worth of payload has gone down between the two,” Freilich said.

In addition to OCO 2, Freilich said NASA has several upcoming Earth science missions that currently lack a medium-class launch vehicle. These include the Soil Moisture Active/Passive spacecraft, slated to launch in 2014; ICESat 2, slated to launch in 2016; and Jason 3, an ocean altimetry satellite is being built by Thales Alenia Space as part of a joint French-U.S. program, which is slated to launch in late 2013 or early 2014.

“We have an existential problem here,” Freilich said. “We are the nation’s space agency and it’s not obvious that we have predictable and reliable access to space. It makes it a little hard to do our job.”

Adding ULA’s remaining five Delta 2 rockets to NASA’s launch contract could allow the agency to keep OCO 2 on track.

“We will fly OCO 2 on the most cost effective, reliable rocket that’s available at the time, and if there isn’t one ready, we won’t fly it,” Weiler said. “Hopefully with all these options there’ll be something ready.”