Carsten Wiedemann (Institute of Aerospace Systems, Technische Universität Braunschweig), Michael Oswald (Institute of Aerospace Systems, Technische Universität Braunschweig), Sebastian Stabroth (Institute of Aerospace Systems, Technische Universität Braunschweig), Peter Vörsmann (Institute of Aerospace Systems, Technische Universität Braunschweig)
Heiner Klinkrad (Space Debris Office, ESA/ESOC)
Keywords: Orbital Debris
Abstract:
An important contribution to the space debris population near 900 km orbital altitude are the NaK droplets. Sixteen nuclear powered satellites of the type RORSAT launched between 1980 and 1988 activated a reactor core ejection system close to this altitude. The core ejection causes an opening of the primary coolant circuit. The liquid coolant has been released into space during these core ejections, forming droplets up to a diameter of 5.5 cm. These droplets consist of an alloy of two alkali metals, sodium and potassium (NaK). In this paper the monochromatic and the total reflectivity of NaK is calculated using theoretical models. The reflectivity depends on the alloy composition and temperature of a droplet. The alloy composition may change due to evaporation, resulting in an enrichment of sodium especially at the droplet surface. According to the literature, there is only a limited number of available measurement data concerning the optical properties of NaK alloys. Furthermore the published data for pure sodium and potassium are controversial. Thus it is necessary to investigate the optical properties of alkali metals and their alloys. Mainly two types of optical absorption, the intraband and the interband absorption, are considered. The intraband absorption is calculated using the Drude-model which uses electrical properties to derive the optical constants of pure metals or alloys. Drude assumes that the valence electrons can be treated as free electrons. The electrons behave like an ideal gas of uncharged particles. The theory of free electrons is a very simple model for the description of the valence electrons in metals. This assumption is sufficient for alkali metals, because they show a nearly free electron behavior. For the interband absorption the classical Butcher-model is used. Furthermore an absorption anomaly which has been observed in some alkali metals is discussed. Especially for potassium, some measurements revealed an unexpected absorption in the visible and the near infrared. This absorption can be determined using a model according to Overhauser who assumes charge-density waves (CDW). But the existence of the anomalous absorption is controversial. The influence of the different models on the reflectivity is discussed. The reflectivity is calculated depending on the alloy composition and the temperature at the surface of a droplet. The results are compared to measurement data from the literature. It is shown that NaK droplets have a very high total reflectivity.
Date of Conference: September 10-14, 2006
Track: Orbital Debris