This document presents a case study running high-performance computing (HPC) applications in public clouds. It evaluates the performance and economic feasibility of running benchmarks and a full-size NASA application across Amazon EC2, GoGrid, and IBM cloud infrastructures. The study finds that while HPC applications can run in the clouds, performance often lags dedicated HPC systems and scales poorly with problem size. Embarrassingly parallel applications achieve the best results. Network performance and economics remain challenges for using public clouds for HPC.