NaK Droplet Source Modeling

Abstract

As part of the NASA orbital debris modeling effort, a quantitative model is developed for the large population of spherical electrically conducting objects in 65° inclination circular orbits at the disposal altitude of the Russian Radar Ocean Reconnaissance Satellite (RORSAT) spacecraft. These are believed to be droplets of liquid sodium-potassium (NaK) reactor coolant released from the RORSAT satellites. Observations of the droplets from 1990 through 2002 have shown slow orbital decay. Data from the Lincoln Laboratory Haystack and HAX radars have been examined for the time period from 1994 to 2002. Electrically conducting spheres produce a distinctive polarization signature allowing their identification, with high probability, among the other objects detected by the radars. The droplets are comparable in diameter to the radar wavelengths. This produces a size ambiguity for a given radar cross-section because of diffraction effects. Previous work, used the NASA orbital debris size estimation model that is based upon radar measurements of randomly shaped objects with the radar wavelength comparable to the object size. Here, the size distribution is estimated using the radar cross section of a conducting sphere, permitting an accurate population size distribution determination. BACKGROUND Prior to 1990 knowledge of the orbital debris environment came only from United States Department of Defense (DOD) radars and from surfaces returned from space. There was no knowledge of the orbital debris population of objects between 100 μ and 10 cm in size and objects of a few mm in size and larger can destroy a space vehicle. In 1990 an agreement was reached between NASA and the DOD whereby NASA would be provided with significant observation time with the LRIR (Haystack) radar and the under construction HAX radar each year until 2004. NASA provided funding construction of near-by HAX Radar as part of the agreement. The first Haystack debris data showed unexpected ~100% polarized return signals (indicating spherical conductors) from debris band between 800 and 1000 km. These objects were soon identified as NaK coolant from ejected cores of Russian Radar Ocean Reconnaissance Satellites (RORSATs). The primary objective of this work is the modeling of the observed NaK population. This report examines Haystack and HAX data from the 1994 –2002 time period. Haystack data from the time period 1990 to 1993 lacks accurate orbital inclination information and has much higher background contamination. CHARACTERISTIC OF HAYSTACK AND HAX RADARS The Haystack radar is operated in a continuous wave monopulse stare mode for the orbital debris measurements. The monopulse range is determined from 8 to 16 overlapping range 54th International Astronautical Congress of the International Astronautical Federation, the International Academy of Astronautics, and the International Institute of Space Law 29 September 3 October 2003, Bremen, Germany IAC-03-IAA.5.2.02 This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. 2 windows. The path through the beam is determined from voltage rations between measurements on opposite sides of the return beam axis. The Haystack dish is 36 meters in diameter. It operates at 10 GHz with 400 kW peak power. The Haystack beam is right circularly polarized and is 0.058° in diameter. Range rate, principal and orthogonal polarization are measured. The HAX radar has a 12-meter disk and operates at 16.67 GHz with 60 kW peak power with a beam diameter of 0.1°. It is identical to Haystack in all other aspects when operated in the orbital debris measurement mode. Figure 1 shows the sensitivity of the Haystack and HAX radars as a function of range. Haystack can reliably detect 5 cm objects at 950 km while HAX can reliably detect objects about 2.5 cm in diameter. The knee in the Haystack curve corresponds to the transition from Rayleigh to Mie / optical regions. 200 400 600 80