An analytical model is described to calculate the low-frequency acceleration, which is intended to replace the rather crude estimations applied to analyze previous Spacelab missions. The three principal low-frequency perturbations of the Space Shuttle Orbiter are taken into account: atmospheric drag, gravity gradient (tidal effect), and spacecraft rotation. These effects have been modeled on the basis of rigid-body dynamics, which requires a sufficient margin to the system's fundamental frequency .#• (1 Hz for the Orbiter-Spacelab combination). The calculation of the atmospheric drag is based on a dynamic model of the atmosphere that takes into account the diurnal (day-night) density variation. Gravity-gradient and rotational accelerations are calculated from the spacecraft attitude data. The main characteristics of the low-frequency accelerations are discussed on the basis of predicted data for Spacelab mission D-2. a Cd d Fd FIQ 7 / // go h m v0 (*, y (*', a A/ , z) ', z') Nomenclature = spacecraft cross section projected normal to VQ = acceleration vector = spacecraft drag coefficient = local vector between c.m. and P (Fig. 7) = atmospheric drag force = solar flux at 10.7-cm wavelength, W-s/m2 = frequency = spacecraft fundamental frequency = Earth gravitational acceleration, 9.81 m/s2 = orbit altitude = spacecraft mass = orbital velocity vector = spacecraft-fix ed coordinate system. = coordinate system centered on c.m. (Fig. 7) = right ascension = difference between // and upper limit of rigid behavior (Fig. 2) = declination