PURPOSE: To immediately control the electric potential of a space vehicle by combining accelerating means and a light source with a photocathode electrically connected to a portion of a satellite, the accelerating means being adapted to accelerate electrons emitted from the photocathode, and the light source capable of exciting the photocathode to cause it to emit electrons.

CONSTITUTION: A photocathode 1 of an appropriate shape and material is electrically connected to the structure of a satellite S. A light source 3 for irradiating the photocathode 1 is disposed in an appropriate place in front of the photocathode 1. The light source 3 is either powered from a simple battery 5 or connected to a main power supply system of the satellite S, e.g. a solar battery. The photocathode 1 is irradiated and activated by the light source 3 to emit electrons toward an outer surface. The electrons are accelerated by an electron grid 11 which is positively polarized by an appropriate power supply 13 with respect to the electric potential of the satellite S.