On 26 September 2005, Cassini conducted its only close targeted flyby of Saturn’s small, irregularly
shaped moon Hyperion. Approximately 6 min before the closest approach, the electron spectrometer (ELS),
part of the Cassini Plasma Spectrometer (CAPS) detected a field-aligned electron population originating from
the direction of the moon’s surface. Plasma wave activity detected by the Radio and Plasma Wave instrument
suggests electron beam activity. A dropout in energetic electrons was observed by both CAPS-ELS and the
Magnetospheric Imaging Instrument Low-Energy Magnetospheric Measurement System, indicating that the
moon and the spacecraft were magnetically connected when the field-aligned electron population was
observed. We show that this constitutes a remote detection of a strongly negative (~ 200 V) surface potential
on Hyperion, consistent with the predicted surface potential in regions near the solar terminator.