Orbit perturbation analysis of West Ford needles clusters

The West Ford package placed in orbit in 1963 contained almost 500 million copper dipoles. Due to a delay in releasing the needles in orbit a number of clusters of needles formed. Analysis of 34 years of mean orbital elements of 20 different clusters has revealed perturbations of 10 km or more in the semi- major axis due to direct solar radiation pressure, atmospheric drag, and Earth albedo pressure. The orbits have perigee heights ranging from 1500 km to 3600 km. A simplified Earth albedo model has been adopted to account for the long periodic perturbations observed in the semi-major axis. The periodic terms are a function of the sun and cluster nodal longitude angles. The atmospheric drag perturbation was modeled by extending the CIRA72 model atmosphere to heights above 2000 km. Cluster area-to-mass ratios were determined from fitting direct solar radiation pressure coefficients over 25 to 30 year data spans. Twenty to thirty year semi-major axis fits produced mean atmospheric density factors for hydrogen and helium for each cluster. These factors show a very high correlation with average perigee height. The results show that average hydrogen concentrations are from 3 to 5 times greater than given by the CIRA72 model. Helium concentrations also are from 1.5 to 4 times greater than predicted for these heights.