HIGH-ALTITUDE BALLOON launched near McMurdo Base in Antarctica carries cosmic-ray detectors above most of the atmosphere. Winds 40 kilometers above the ice cap blow in a circle around the Pole, returning the balloon to the vicinity of its starting point after about 10 days. Balloon detectors are not as sensitive as those placed on board satellites, but they can be made much larger and lofted much more cheaply.

           Credit: George Kelvin