SAN FRANCISCO — NASA science missions face potential delays in the years ahead as the space agency makes the transition from its workhorse Delta 2 medium-class launch vehicle to new rockets, according to a recent report by the U.S. Government Accountability Office (GAO).

Since 1998, NASA has relied on Delta 2 rockets built by Denver-based United Launch Alliance to carry almost 60 percent of its scientific satellites into orbit, according to the report, “NASA, Medium Launch Transition Strategy Leverages Ongoing Investments but is Not Without Risk.”

“Delta 2, however, is no longer in production, and no other vehicle in the relative cost and performance range is currently certified for NASA use,” according to the report released Nov. 22. “Thus, NASA faces a potential gap in the availability of medium class launch vehicles that could cause design challenges, delays, or funding issues.”

To fill that gap, NASA is counting on the Falcon 9 rocket built by Space Exploration Technology Corp. (SpaceX) of Hawthorne, Calif., and Orbital Science Corp.’s Taurus 2 rocket, two launch vehicles with lift capabilities similar to the Delta 2. Since 2006, NASA has helped fund development of the Falcon 9 and Taurus 2 rockets as part of the Commercial Orbital Transportation Services program. In late 2008, the space agency announced plans to pay a total of $3.5 billion to Orbital Sciences and SpaceX for 20 cargo flights to and from the international space station as part of the Commercial Resupply Services program.

Still, it will take years before either rocket is ready to take NASA science missions into orbit. It takes approximately three years for a new launch vehicle to be certified by NASA’s Launch Services Program and that process generally does not begin until the vehicle has been selected by the space agency for a particular mission, according to the GAO report. What’s more, a vehicle cannot be selected for a mission until it has completed at least one successful flight.

In June, the Falcon 9 accomplished that goal. The Taurus 2 rocket is scheduled for its first flight between July and September of 2011, according to Barron Beneski, spokesman for Dulles-Va.-based Orbital Sciences.

Even if NASA moves promptly to award a contract to SpaceX to buy a Falcon 9 rocket for a science mission, the rocket is not likely to be certified for launch until the middle of 2013, according to the GAO report. Further certification will be required before the rocket is cleared to launch high-value payloads. That certification, which requires the completion of multiple successful launches, is unlikely to be completed until late 2013 or early 2014, the report added.

“NASA’s plan has inherent risks that need to be mitigated,” according to the GAO report. “NASA has not developed detailed estimates of the time and money required to resolve technical issues likely to arise during the launch vehicle certification process. As these costs are currently unknown, according to Science Mission Directorate officials, NASA has not yet budgeted for them.”

NASA’s plan to phase out its reliance on Delta 2 stems from studies conducted since 2006 to examine alternative launch vehicles. Those studies determined that once the U.S. Air Force concluded its use of the Delta 2 in 2009, it would be too expensive for NASA alone to maintain the Delta 2 launch vehicle infrastructure. Instead, the space agency should encourage the development of new medium-class rockets and rely on larger launch vehicles, including the United Launch Alliance Atlas 5 and Delta 4, as an interim solution until the new rockets are certified, according to the GAO report.

NASA is scheduled to launch only three more science spacecraft on Delta 2s: the Aquarius sea-surface salinity mission in June 2011, the Gravity Recovery and Interior Laboratory mission in September 2011, and the National Polar-orbiting Operational Satellite System Preparatory Project in October 2011.

In NASA’s response to the GAO report, Edward Weiler, NASA’s associate administrator for science, said the space agency is aware of the risks associated with concluding the Delta 2 launch vehicle line and relying on new rockets. “NASA has been actively engaged in managing risk associated with launch vehicle availability and cost, including those risks related to the close out and/or start up of launch vehicle lines,” he said in a Nov. 9 letter to GAO.

Weiler pointed out that while NASA already has awarded a launch services contract to SpaceX for the Falcon 9, it has not yet done so for the Taurus 2. The Taurus 2 will be included in a future NASA Launch Services contract, Weiler wrote. “While NASA intends to follow a similar process to certify both vehicles at the appropriate time, currently the Agency’s activities in support of launch services for science missions are focused on the Falcon 9,” he added.

In concluding its report, GAO recommended that NASA perform detailed analyses to determine costs related to certifying new medium-class launch vehicles as well as ensuring that those costs are included in future budgets. Cost estimates should include an examination of the expenses associated with delaying science missions or sending them into orbit on larger spacecraft if necessary, according to the GAO report. NASA concurred with those recommendations, Weiler wrote.

Debra Werner is a correspondent for SpaceNews based in San Francisco. Debra earned a bachelor’s degree in communications from the University of California, Berkeley, and a master’s degree in Journalism from Northwestern University. She...