  ASTROD I is planned as a single spacecraft mission. It will use
interferometric and pulse ranging techniques between the spacecraft and ground
stations, to make high precision measurements of the parameters that describe
the solar system, and to test relativistic gravity with improved accuracy. At
the heart of the spacecraft is a test mass, which the spacecraft will follow
using a drag-free control system. The mission critically depends on maintaining
the geodesic motion of the test mass. Charging of the test mass due to cosmic
rays and solar particles will disturb its geodesic motion. We have modelled the
charging process using the GEANT4 toolkit and a simplified, geometrical model
and estimate that the ASTROD I test mass will charge positively, at a rate of
24 +/-7 e+/s, due to cosmic ray protons and alpha particles (3He and 4He) at
solar minimum. We have used the results of this simulation to estimate the
magnitude of disturbances associated with test mass charging, for the
worst-case scenario, taking into account uncertainties in the model and
potential charging contributions from minor cosmic-ray components.
