As of June 1994 some 7,255 objects larger than about 10cm were tracked and catalogued by the USSpaceCom Space Surveillance System. This Catalogue population is believed to represent at most one tenth of the unshieldable and also untrackable objects larger than 1cm on Earth orbits. Using the Catalogue population as reference, the population growth and changes in spatial distributions over the past decade are analysed, corresponding spatial densities are determined, and resulting collision fluxes are derived for typical LEO and GEO target orbits. The underlying computational techniques use 3D volume discretisation and quasi-deterministic orbit propagation to establish collision fluxes with high spatial resolution. Collision risks for small-size, untrackable debris (down to 0.1 mm) are determined from modeled on-orbit fragmentations. Results of supporting ground-based explosion tests are summarised. Finally, the implementation of the described methods in ESA's Meteoroid and Space Debris Terrestrial Environment Reference (MASTER) is outlined. The corresponding data, executables, and documentation will be available on CD ROMs in early 1995.