The National Aeronautics and Space Administration (NASA) has long relied on the Delta II medium class launch vehicle to launch science missions. Delta II, however, is no longer in production, and no other vehicle in the relative cost and performance range is currently certified for NASA use. Thus, NASA faces a potential gap in the availability of medium class launch vehicles that could cause design challenges, delays, or funding issues. GAO was asked to assess (1) NASA's and the Delta II contractor's, steps to ensure resources (budget, workforce, and facilities) are available to support safe Delta II operations through the last planned NASA flight in 2011; (2) NASA's plans and contingencies for ensuring a smooth transition from current small and medium class launch vehicles to other launch vehicles for future science missions; (3) the risks associated with NASA's planned approach to fill the medium launch capability gap; and (4) technical and programmatic implications to science missions if NASA commits to new launch vehicles before they are certified and proven. GAO identified and assessed transition plans and mitigation activities and interviewed responsible NASA and government officials. GAO recommends that NASA perform a detailed cost estimate based on knowledge gained during launch vehicle certification and adequately budget for potential additional costs. NASA concurred.