The first round-trip space mission to Mars would to bring back rock and soil samples for laboratory study on Earth. A critical aspect of such a mission will be the automated separation of a surface lander from an orbiting mother ship, and its docking after a visit to the planet's surface. Unlike the Apollo missions to the moon of 30 years ago, no astronauts will be present to guide the two craft together. The ability to safely dock unmanned spacecraft is also likely to be important in building and servicing the international space station. To this end, NASA's Marshall Space Flight Center in Huntsville, Ala., is developing technology to automate rendezvous and capture. An essential component of this technology is the video guidance sensor, which combines diode laser illumination, optics and machine vision to gauge the range, direction and relative attitude of a passive target vehicle in real time. This technology works well in Earthly applications such as industrial robotics and ship docking, and recent Space Shuttle tests indicate that it is precise and reliable enough for space applications as well.