Scientists at Johns Hopkins UniversityApplied Physics Laboratory in Laurel, Md., and at the National Institute of Standards and Technology (NIST) in Gaithersburg, Md., have proposed an optical system that would enable spacecraft 1000 astronomical units away to communicate with Earth. They estimate that technological advancements within the next five to 10 years could make such a system a reality.The distances and velocities associated with an interstellar space probe suggest that an optical downlink is the most feasible approach for the presumed mass and power budget. Higher-power laser diodes and/or diode-pumped fiber lasers with spatially tight and temporally stable outputs will have to be developed to construct such a system, which would employ a microelectromechanical beam-steering mechanism developed at NIST to offer a pointing accuracy of 400 nrad rms.