What triggers the sudden, magnificent movements of the aurorae, the beautiful "Northern Lights" that appear around the far poles of the earth? It's been a nagging question of space science for decades. It's known they're caused by space "substorms" (releases of accumulated energy in the magnetic field around the earth). But there's a debate about where they're set off - close to the earth and moving away from it, or away from the earth and moving toward it. UC Berkeley's Space Sciences Laboratory is heading a mission to find out. Taking its name from the Greek goddess of justice, THEMIS is the final judge. It's the first mission to launch five satellites together, lining them in space like buoys in an ocean. When a substorm occurs, the satellites measure the energy passing from one to the other, to determine where it actually began, providing important information about the relationship of the earth to the sun. NASA chose Berkeley's proposal from 70 bids for its Explorer program. Berkeley built the satellites and guides them, and is collaborating with UCLA, Canada, France, Germany, and Austria