Upper Atmospheric Densities Derived From Starshine Spacecraft Orbits

Between June 1999 and January 2003 three Starshine spacecraft were launched into low-earth orbits. Their lifetimes cover an extended period near the maximum of solar cycle 23. Two additional Starshine spacecraft are ready for launch near the minimum of solar cycle 23. The Starshine missions are especially suitable for estimating average upper atmospheric densities, since the orbits are approximately circular and the spacecraft are mirrored spheres for which ballistic coefficients are essentially independent of orientation with respect to the direction of motion. We have derived total neutral atmospheric mass densities along the Starshine spacecraft trajectories using quantities from their Two-Line Element sets (TLEs). We compare these densities with corresponding determinations by a semi-empirical model of upper atmospheric neutral densities developed at the Naval Research Laboratory. This model, NRLMSIS, has been formulated for both research and operational use from a database that now includes total mass densities from satellite accelerometers and orbit determinations, more recent temperatures from incoherent scatter radar, and molecular oxygen number densities from solar UV occultation. The Starshine orbits were not included in the NRLMSIS model formulation and thus provide independent validation of the modeled variations on time scales of days to months during a period of relatively high solar activity. The Starshine Spacecraft The Starshine spacecraft are small spherical satellites whose mass, radii and launch dates are summarized in Table 1. Figure 1 shows the Starshine 2 spacecraft prior to launch. Its outer surface is covered with 845 aluminum mirrors, polished by 30,000 students from around the world. Starshine Orbits and Two-Line Element Sets The Starshine spacecraft were launched into almost circular, moderate inclination, low-earth orbits, of about one year duration. The North American Aerospace Defense Command (NORAD) tracked the orbits of each Starshine spacecraft and produced Two-Line Element sets (TLEs) that provide orbital parameters derived from General Perturbation theory. The parameters are values of the six Kozai mean elements of inclination, right ascension of the ascending node, argument of perigee, eccentricity, mean motion and mean anomaly at epoch. Additional elements relate to drag, including ndot/2 (mean motion time derivative). The TLEs of the Starshine orbits were typically updated a few times per day, at selected times called epochs. They are available courtesy of Dr. T. Kelso at http://www.celestrak.com. Figure 1. The Starshine 2 spacecraft prior to launch. Figure 2 illustrates the variations in selected orbital parameters archived in the TLEs for each Starshine spacecraft throughout its mission.