Each of the four spacecraft carries an identical set of 11 instruments to investigate charged particles, electrical and magnetic fields. These were built by European and American instrument teams led by Principal Investigators.
ASPOC - Active Spacecraft Potential Control experiment
Principal Investigator: Klaus Torkar, Austria
Electrical charging of a Cluster spacecraft in orbit can have a severe impact on the performance of the scientific instruments. ASPOC emits charged particles that cancel out the electrical charge that the satellite acquires. The number of particles needed to do this is determined by other onboard instruments (EFW and PEACE), which record the spacecraft's electrical potential.
CIS - Cluster Ion Spectrometry experiment
Principal Investigator: Iannis Dandouras, France/Greece
Analyses the composition, mass and distribution of particles captured by the spacecraft. CIS consists of two different instruments, a Hot Ion Analyser (HIA) and a time-of-flight COmposition and DIstribution Function analyser (CODIF), plus a sophisticated instrument control and data processing system, which permits extensive onboard data processing.
EDI - Electron Drift Instrument
Principal Investigator: Roy B. Torbert, USA
EDI fires two beams of electrons into space around each spacecraft. The electrons travel in orbits of 10 km or more before returning to receivers on the opposite side of the spacecraft. From the firing directions and the round-trip travel time, scientists can determine the strength of the electric and magnetic fields around each Cluster spacecraft.
FGM - Fluxgate Magnetometer
Principal Investigator: Elizabeth Lucek, UK
FGM is composed of two magnetometers that measure the magnetic fields encountered by Cluster. Located on a five-metre-long boom to avoid interference from the spacecraft, FGM can record measurements up to 67 times per second.
PEACE - Plasma Electron And Current Experiment
Principal Investigator: Andrew Fazakerley, UK
Like CIS, this experiment also sifts incoming particles. PEACE looks at electrons in the plasma that have low to medium energies, counts them and measures their direction of travel and speed.
RAPID - Research with Adaptive Particle Imaging Detectors
Principal Investigator: Patrick Daly, Germany
An advanced particle detector that records the highest energy electrons and charged particles entering it from space. The particles pass through pinholes to reach the detector, enabling it to build up a picture of where the particles came from.