The next generation of NASA remote sensing satellites and space science probes could be burdened by rising launch costs and delays as the agency incorporates new medium-lift rockets, according to a Government Accountability Office report released Monday.

The uncertainty surrounds 12 to 14 science missions through 2020 that have not yet received launch vehicle assignments, the government watchdog report said.

NASA is ending its use of the Delta 2 rocket, a workhorse launcher that has delivered nearly 60 percent of the agency's scientific satellites to space since 1998.

NASA is shifting future medium-class missions to SpaceX's Falcon 9 rocket and the Taurus 2 launcher being developed by Orbital Sciences Corp. The GAO found both vehicles serve the same market as the Delta 2 and have similar costs. [Gallery: SpaceX's First Falcon 9 Rocket Launch]

But the Falcon 9 and Taurus 2 are not certified to launch NASA's most expensive and important science missions.

United Launch Alliance has three more NASA missions on its Delta 2 manifest. Parts for producing five more Delta 2 rockets are also available, but there are high costs of modifying and maintaining launch pads to host any extra flights, according to the GAO.

The report addressed NASA's efforts to support the remaining Delta 2 flights and the agency's medium-class launch strategy.

"NASA is taking an appropriate approach to help ensure the success of the remaining Delta 2 missions by adequately addressing workforce, support, and launch infrastructure risks," the report said. "Nevertheless, an affordable and reliable medium launch capability is critical to NASA meeting its scientific goals."

Most of the medium-class missions in NASA's portfolio are Earth observation satellites, which require polar orbit launches from West Coast sites at Vandenberg Air Force Base, Calif., or Kodiak Launch Complex in Alaska.

Neither company has an operational West Coast launch site. SpaceX plans to construct a Falcon 9 launch pad at Vandenberg, and Orbital is still evaluating potential sites for polar Taurus 2 missions.

The first NASA decision point is expected in the next few months. NASA must decide on a launch vehicle for three Earth observation satellites in 2011. The missions are due for liftoff in 2014 and 2015.

NASA is expected to settle on a rocket for the Soil Moisture Active and Passive, or SMAP, satellite by March. Other civil space missions scheduled for launch by 2015 and still lacking a launch vehicle assignment include the ice-mapping ICESat 2 spacecraft and the first member of NOAA's revamped polar-orbiting weather satellite fleet.