What are radar observations telling us about the low-Earth orbital debris environment?

NASA has been observing the low-Earth orbital debris environment with high power radars for more than a decade, and the observations continue to reveal valuable information about the character of the debris particles. Instruments such as the Haystack, Haystack Auxiliary (HAX), and Goldstone radars have been used to observe debris particles smaller than are normally visible using only the US Space Command resources. Haystack can routinely detect objects less than 1 cm in size in LEO. HAX can routinely detect debris objects less than about 3 cm in LEO. Goldstone, while limited in capability compared to HAX and Haystack, can detect objects only a few millimeters in size in LEO. By making statistical observations of the debris, however, we have given up the possibility of tracking these objects. So, in order to enhance our information on the orbital distribution of debris in LEO, several unusual radar geometries have been employed. These have been used to identify families of orbital debris with similar orbital inclinations and altitude distributions. In addition, polarization measurements have allowed us to probe the characteristics of the debris - primarily the shape of the particles. New statistical data analysis techniques reveal information on the elliptical orbit distributions, and allow us to make the first direct estimates of the contributions of elliptical orbits of centimeter objects to the flux in LEO and to characterize the sources of these debris. This paper updates the results of measurements made by the Haystack, Haystack Auxiliary, and Goldstone radars and how NASA is finding new ways to use these instruments to answer questions about the orbital debris environment.