Information on the small-size meteoroid and space debris environment can only by gained by the analysis of retrieved spacecraft or spacecraft parts (only possible for relatively low orbits) or by in-situ monitors in orbit.

Instruments to detect impacts from natural meteoroids and man made space debris particles have been flown in Low Earth Orbits (LEO) (e.g. on LDEF, EURECA, MIR, BREMSAT) and on interplanetary missions (e.g. Giotto, Vega, Ulysses, Galileo, Hiten).

However, very little information on the particulate environment for Earth orbits above about 600 km altitude is available. Especially the space debris environment in the important geostationary ring is largely unknown. Ground based detection in GEO is limited to objects larger than about 0.5 m.

To obtain information on the submicron to millimetre size particle population in GEO the GORID (Geostationary Orbit Impact Detector) experiment was initiated. GORID is a joint project between ESA, the Max-Planck Institute (MPI) für Kernphysik in Heidelberg, the Scientific Production Association of Applied Mechanics (NPO-PM) from Krasnoyarsk and the Novosibirsk State University (NSU). Main objectives of the experiment are to:

• monitor the space debris environment in the geostationary orbit and its long term variation.
• monitor the meteoroid flux at 1 AU heliocentric distance , its dependence on the season and its long term variation.
• Investigate the small particle mass region of meteor streams and its relation to the position of the parent bodies.
• Act as a third point for simultaneous measurements with the corresponding Ulysses (out of the ecliptic) and Galileo (at Jupiter) instruments.

On 26 September 1996 the Russian Express-2 telecommunications spacecraft was launched into geostationary orbit (GEO). The GORID Cosmic Dust/Space Debris detector was included as a piggyback instrument. The instrument consists of a plasma type detector and associated electronics and is essentially identical to the Dust detectors flying on the Ulysses and Galileo spacecraft.

The aperture size is 0.1 m² and the instrument is capable of detecting particles with a mass down to 10^-14 g (velocity dependent). The detector is stationed at 80^o Eastern longitude. It has a fixed viewing direction which is 65^o away from the flight direction towards North.

The diameter of the detector opening is 43 cm. The extracted parameters include particle mass, velocity and crude impact direction. To some extend orbital debris and natural meteoroids can be separated by the impact velocity which at the GEO altitude is typically below 5 km/s for debris and higher for meteoroids.

During its first year of operation GORID has detected between 0 and 12 events per day which have all the signatures of hypervelocity impacts. It appears that many of the impacting particles, especially those arriving during the local night times, are highly negatively charged when they enter the detector. An interesting observation which, if confirmed, could have important implications for the solid particle environment in GEO.