BESS is a particle physics experiment carried by a balloon. BESS stands for Balloon-borne Experiment with Superconducting Spectrometer. It is in fact a series of experiments that started in 1993, and its current incarnation, BESS-Polar, was circling the Antarctic from December 13 to December 21, 2004, for a total of 8 days 17 hours and 2 minutes. This joint Japanese and American project is supported by the Laboratory for High Energy Astrophysics (LHEA) at NASA's GSFC and the KEK.
Read more about Bess: Overview