repair is difficult or impossible. The ultimate remote application is for spacecraft, such as Cassini or Galileo, which have traveled so far from the sun that solar energy is insufficient to power the control, data collection and communication systems. In these spacecraft electrical power is provided by radioisotope thermoelectric generators (RTGs), which convert the heat generated by a radioactively decaying material into electrical energy.1 Similar devices have been used in remote terrestrial locations, such as light houses or beacons, but are being decommissioned due environmental concerns.2