The discovery of high area-to-mass ratio (HAMR) debris in near geosynchronous orbit (GEO) raises concern for the sustainability of this unique resource. It is thought that HAMR objects are sheets of multilayer insulation detaching from satellites in GEO disposal orbits due to material degradation. Such objects are subject to gravitational perturbations due to the Earth's oblateness and the Moon and Sun, as well as the effects of solar radiation pressure (SRP).We have developed a first-order averaged model, explicitly given in terms of the Milankovitch elements, which provides a very accurate description of the long-term orbit behavior and allows for a qualitative understanding. We present this model and discuss its fundamental predictions, including the Saros resonance and the systematic structure of the inclination-node phase space. We also extend the model's domain of validity by incorporating the parallactic term into the third-body disturbing function expansion and the Earth's shadow for SRP.