Number 131 (Story #1), June 3, 1993 by Phillip F. Schewe and Ben Stein RADIO SIGNALS FROM THE HELIOPAUSE have been detected by the two Voyager spacecraft. The heliopause, where the solar wind particles streaming out from the sun meet the directional flow of interstellar-medium particles, essentially constitutes the edge of the solar system. Speaking at last week's meeting of the American Geophysical Union, Donald Gurnett of the University of Iowa provided this explanation: a powerful solar flare event in May-June 1991 caused a surge of solar-wind particles which subsequently interacted with the heliopause, setting up huge radio bursts (at more than 10**13 Watts, the most powerful radio source in the solar system) detected by the Voyagers beginning in July 1992. High in power but low in frequency (2-3 kHz), the radio signals could not be detected in the inner solar system. However, Voyager 1, at a distance of 52 AU (an astronomical unit is the distance between the Earth and sun), and Voyager 2, at 40 AU, were well placed to made a measurement. Ralph McNutt of Johns Hopkins said that from the timing of the signals the distance to the heliopause could be estimated to be between 80 and 130 AU.