The increasing amount of space debris poses a great threat for active satellites in space.
Besides currently approx. 1000 active and more than 1000 inactive satellites, roughly 40000 space debris parts (measured with radar) and more than 500000 space debris particles with diameters of less than 1 cm are in orbit around Earth.
These space debris parts are mainly upper stages of old rockets, or parts from explosions (mainly due to aging accumulators or remaining fuel), or from satellite collisions. Depending on their orbital height, these debris parts can remain in orbit for quite a long time. While an old rocket body in 1000 km altitude will reenter and burn up in the atmosphere within a few 1000 years, an upper stage orbiting in 6000 km will orbit around Earth for the next few million years.
The Graz SLR station has an international leading position concerning space debris research. Currently, it concentrates on the following research topics.
In multi-static space debris laser ranging Graz sends photons to a space debris target, using a 20 Watt laser. The light is diffusely reflected on the laser's surface and the Graz photons are spread over Central Europe. These reflected photons are then detected by other stations across the continent. In a unique experiment, Graz sent photons with a green laser and Wettzell sent photons with an infrared laser, simultaneously. The Graz photons were detected by Graz and Wettzell, the Wettzell photons by Wettzell, Graz, and Stuttgart. Data analysis proved a significant increase in orbital prediction accuracy of space debris targets.
In the Stare & Chase method a low cost camera system with a field of view of approx. 10° "stares" in arbitrary direction into the sky and records the stellar background up to magnitude 9. From the position of stars in the background the accurate pointing direction of the camera is calculated. As soon as a sunlit space debris particle passes through this field of view, its celestial coordinates as referenced to the background stars are determined and stored. Using only the pointing information – without a-priori orbital information – an orbit is calculated and immediately used to track ("chase") the target with laser-based distance measurements. The whole process from the first detection of the target in the camera's field of view to successful space debris laser ranging can be completed within a few minutes.
For the spin period and attitude determination of space debris laser measurements are combined with light curves. Knowing the retro-reflector geometry on the satellite, it is possible to determine spin period and spin attitude parameters of the target. The environmental satellite Envisat has eight retroreflectors arranged in a pyramid. Due to the rotation of the satellite the distance to the single retro-reflectors varies periodically a few millimeters. From SLR measurements it is hence possible to accurately determine spin period and attitude parameters of such targets. Simultaneously to SLR, sunlight reflected from the satellite is used to record light curves. They display the reflected intensity in dependence of one full rotation of the satellite (phase). From light curves one can easily recognize reflection patterns from various parts of the satellite such as the solar panel or the central body.