A problem with accurate measurements of charged particles and electric fields in space is electrical charging of the spacecraft. This charging is created by ultraviolet radiation from the sun that knocks off electrons from the surface of the spacecraft. The negatively charged electrons remain in the neighborhood of the spacecraft, which is now positively charged up to one hundred volts. This creates an electric field that strongly influences the orbits and energies of oncoming charged particles.

A solution to this problem is the active emission of positive charges through a beam of high energy ions. The objective of the ASPOC (Active Spacecraft Potential Control) instrument is to investigate this ion beam and its interaction with the surrounding particles, connected with the reduction efficiency of the spacecraft charge to acceptable levels.

This instrument, which is deployed in similar form in the ESA Cluster mission, is being developed by an international workgroup, which the Space Research Institute of the Austrian Academy of Sciences chairs as Principle Investigator. This institute is also responsible for the complete digital electronics of the instrument. An important part is made up by the ion emitters, which were developed by the Austrian Research Centers Seibersdorf. The liquid metal ion emitter that will be operated is especially suited for outer space because of the low melting point of Indium. The electronics for the current supply and high voltage generation is developed by Forsvarets Forskningsinstitut (FFI) in Kjeller, Norway. The instrument casing, the lid and shutter mechanism for the ion emitter module and test support are the responsibility of the Space Science Department of ESA at ESTEC. The mass of the instrument for Double Start is 2<nobr>.7 kg,</nobr> the power usage up to <nobr>2.7 Watt.</nobr>

• Space Research Institute (IWF) of the Austrian Academy of Sciences (AAS)
• Institute of Physics, Austrian Research Centers Seibersdorf
• Space Science Department, ESA/ESTEC
• Norwegian Defence Research Establishment, Kjeller, Norway
• University of New Hampshire, Durham, NH, USA
• University of Washington, Seattle, WA, USA
• Department of Physics, Mullard Space Science Laboratory, University College London, Great Britain
• Center for Space and Applied Research, Chinese Academy of Sciences, Beijing, China

• Klaus Torkar, Principal Investigator, Instrument Development and Data Analysis
• Wolfgang Baumjohann, Co-Investigator, Data Analysis
• Gerhard Fremuth, Hardware and Software
• Franz Giner, Ground Support Software
• Christoph Kürbisch, Hardware
• Gunter Laky, Commando
• Sonja Neukirchner, Hardware
• Robert Wallner, Hardware