Figure 1. The meteoroid and debris impact detector GORID.
Résumé
Le 26 septembre dernier a été mis en orbite le
satellite de télécommunications géostationnaire
russe Express-2, avec à son bord un détecteur de
poussières cosmiques et de débris spatiaux.
L'instrument, comprenant une sonde à plasma et son
électronique, est identique aux détecteurs de
poussières d'Ulysses et Galileo. Son ouverture d'un
dizième de mètre carré lui permet de
déceler d'infimes grains de matière (<=10-14
gramme selon la vitesse) et de
déterminer leur masse, leur vitesse et la direction
approximative d'où ils proviennent. Les débris
d'engins spatiaux se distinguent des météorites par
leur vitesse d'impact, généralement inférieure
à5 km/s pour les débris que l'on rencontre sur
l'orbite des satellites géostationnaires.
Contractors:
NPO-PM, (Rus)
NSU, (Rus)
Funding:
Fixed Support Costs
Instruments for detecting the impacts due to natural meteoroids and particles of man-made space debris have been flown in low Earth orbits (e.g. spacecraft such as LDEF, Eureca, Mir, Bremsat) and on interplanetary missions (e.g. Giotto, Vega, Ulysses, Galileo, Hiten). However, very little information on the particulate environment is available for Earth orbits above about 600 km altitude. In particular, the space debris environment in the important geostationary ring is largely unknown. Ground based detection in geostationary orbits is limited to objects larger than about 0.5 m. To obtain information on the submicron to millimeter size particle population in geostationary orbit, the GORID (geostationary orbit impact detector) experiment was initiated.
GORID is a joint project between ESA, the Max-Planck Institute (MPI) für Kernphysik, the Scientific Production
Association of Applied Mechanics (NPO-PM) and the Novosibirsk State University (NSU). Main objectives of the experiment are:
The impact detector launched is the flight-quality engineering model of the Ulysses detector (which is essentially identical to the Galileo instrument). It was made available by MPI. Figures 1 and 2 show views of the detector. GORID is an impact ionisation detector.A particle impacting at hypervelocity speed creates a plasma of electrons and ions. The electrons and ions generated during the impact are measured separately. The velocity and mass of the impactor can be deduced from the rise times and total intensities of the measured plasma signals by use of empirical calibration curves respectively.
Figure 1. The meteoroid and debris impact detector GORID.
Figure 2. A view into the opened detector (In space the cover
is opened 180°).
The detecting surface is roughly hemispherical in shape and has an area of 0.1 m². The detector opening has a diameter of 43 cm. The gold coated sensor surface also collects the electrons of the impact plasma. The ions are accelerated towards the centre of the hemisphere where they are collected. A few ions are further intensified and measured by a channeltron behind the main ion collector grid.
Agreement was reached with NPO-PM to place the detector on one of their new Express telecommunication satellites. As prime contractor, MPO-PM was responsible for the requirements specification, the manufacturing of a cover for GORID to protect it from contamination on ground and during launch, for other required modifications of the detector and the spacecraft, for tests and analyses and for the launch and operation.
NSU has been assigned responsibility for building the electronic interface block and for pre-processing and transfer of data to ESTEC. Before the detector was shipped to Russia, additional calibration tests were carried out at the hypervelocity impact facility of the MPI. Figure 3 shows the GORID detector model mounted on the (still incomplete) Express satellite.
Figure 3. GORID mock-up mounted on the Express satellite
GORID/Express-2 was launched successfully from Baikonur on 26 September, 1996 with a Proton rocket. The spacecraft then took up its geostationary position at 80° Eastern longitude. Figure 4 shows a drawing of the GORID/Express system. The main viewing direction of GORID is pointing 65° away from the flight direction towards North. The viewing cone has a full angle of 140°.
Figure 4. Geometry of GORID and Express in orbit (drawn to scale).
The cover of GORID was opened on 18 October. All initial tests were successful. The first impact was measured on 29 October. Its relatively low velocity of below 5 km/s indicates that it was most likely a particle of space debris. By mid-November all planned tests had been completed and GORID was fully operational. During its first month of operation GORID has detected on average one to three clearly identifiable impacts per day.
In normal operation the data are taken from the instrument every 12 hours. The read-out frequency can be increased to once per hour if desired (e.g. to monitor a meteoroid stream). Every ten days the data are down-linked to Earth and sent to NSU where they are preprocessed, and then forwarded to ESTEC by electronic mail.
The initial agreement with NPO-PM covers a period of two years after launch for instrument operation and data transfer. The lifetime of the GORID experiment is expected to be dependent on the Express satellite which has a design lifetime of five to seven years.
The meteoroid and debris impact detector GORID was successfully launched on board the Russian Express-2 telecommunications satellite. Its main objectives are to monitor the presently unknown population of small particles of space debris in the geostationary orbit and to gain new information on the mass distribution and the directional and temporal dependence of meteoroids and meteoroid streams. GORID is fully operational and should remain functional during the expected five to seven years lifetime of the Express satellite. The results of this experiment are of scientific value and will also allow more reliable impact risk assessments and shielding guidelines for geostationary satellites.
Preparing for the Future Vol. 7 No. 1