Experimental bound on the orbital effects of charge drag

Abstract

In May 1963 a package containing 4.8×10s copper dipoles, each 1.78 cm in length and 0.00178 cm in diameter, was ejected from a parent satellite into a nearly circular, nearly polar orbit at a mean altitude of about 3650 km. These Project West Ford dipoles were subsequently dispensed individually such that each tumbled rapidly about its center of mass [Waldron et al., 1964; Shapiro et al., 1964]. Because of their extreme thinness, the dipoles are more susceptible to charge (i.e., Coulomb) drag perturbations than are any other satellites; they thus enable a stringent upper bound to be placed on the orbital effects of the drag [Shapiro, 1963a]. As is well known, the main effect of a drag force is to decrease the period and the mean altitude of a satellite. Since orbital period can be measured more accurately, its variations are commonly used to study drag phenomena. But West Ford dipoles can neither be distinguished nor detected individually; it is therefore impossible to monitor the orbital period variations of any given dipole. Fortunately, the dipoles were dispensed in a manner such that their density along the orbit exhibited a logarithmic singularity [Shapiro et al., 1964]. For the first 80 days (i.e., for the first 700 orbital revolutions) the motion of this peak in the density represented essentially the group velocity of the dipole ensemble, and the changes in its orbital period contained the average effects of charge drag. After 80 days, the peak became asymmetrically ‘contaminated’ by dipoles that had lapped and been lapped by it; in addition the peak became far less pronounced and was therefore no longer useful for studying charge drag.