The threat posed by human-made space debris to space-based activities has given a heightened sense of awareness to the two decade old quest for a quantitative description of orbital debris properties. Traditionally, a global radar network has been employed as the primary tool for studying the population. These instruments are tasked with maintaining the 'master' catalogue of orbital characteristics and sizes for all objects larger than their approximately 10 cm detection capability. Because of the rapid diminution of return power with distance, radar based systems are range-limited and thus best suited to LEO and MEO (Low/Middle Earth Orbit) work. The restriction to one part of the EM spectrum also places limits on their usefulness as a probe of the intrinsic properties of debris, such as surface texture and albedo.
Optical telescopes, which have typically served a secondary role, are becoming viable and cost effective debris survey instruments. The sparse network of 1.0 meter class and smaller telescopes in worldwide operation have demonstrated their utility to probe not only LEO and MEO ranges, but GEO (Geosynchronous Orbit) as well. This region of finite and invaluable 'real estate' is the subject of much concern due to uncertainty about both the quantity of resident debris and its rate of generation. Optical telescopes provide the only practical means of sampling this region of space. They also provide a complimentary and independent set of statistics which can be correlated with the radar observations to yield a more complete picture of the debris population. (Many objects have only an optical or radar signature, but not both.)
Born from the desire to study GEO and from a need to aquire information on LEO/MEO debris as small as 1 cm diameter, NASA conceived of constructing a large aperture optical telescope. In this era of fiscal restraint, the expense of a traditional glass mirror was supplanted in favor of a then unproven technique, that of using a spinning dish of liquid mercury as the primary mirror. The underlying principle is the parabolic equilibrium configuration of a fluid rotating in a uniform gravitational field. Although presently restricted to a zenith-staring configuration, this type of instrument, because of its low cost and relative simplicity, makes an ideal survey tool for debris or astronomy, and in the wake of recent events, near-earth object (NEO) searches.
Developed at NASA's Johnson Space Center, Houston, TX, at a cost of roughly 1/20th that of a traditional telescope, the NASA 3.0 meter LMT (Liquid Mirror Telescope) at its Cloudcroft, NM home, is capable of surveying LEO/MEO debris down to a nominal diameter of 1 cm. Similar performance is expected at GEO when this instrument, or one like it, is moved to an equatorial site. The detection limits demonstrated are approximately 5 to10 times smaller than that of any dedicated optical instrument and represents performance essential to better determining the number/size distribution function for orbital debris. The distribution function provides the fundamental basis for determining the debris associated risk to space-station, shuttle, or any space-faring activities.