  In this paper, we study the long-term dynamical evolution of
highly-elliptical orbits (HEOs) in the medium-Earth orbit (MEO) region around
the Earth. The real population consists primarily of Geosynchronous Transfer
Orbits (GTOs), launched at specific inclinations, Molniya-type satellites and
related debris. We performed a suite of long-term numerical integrations (up to
200 years) within a realistic dynamical model, aimed primarily at recording the
dynamical lifetime of such orbits (defined as the time needed for atmospheric
reentry) and understanding its dependence on initial conditions and other
parameters, such as the area-to-mass ratio (A/m). Our results are presented in
the form of 2-D lifetime maps, for different values of inclination, A/m, and
drag coefficient. We find that the majority of small debris (> 70%, depending
on the inclination) can naturally reenter within 25-90 years, but these numbers
are significantly less optimistic for large debris (e.g., upper stages), with
the notable exception of those launched from high latitude (Baikonur). We
estimate the reentry probability and mean dynamical lifetime for different
classes of GTOs and we find that both quantities depend primarily and strongly
on initial perigee altitude. Atmospheric drag and higher A/m values extend the
reentry zones, especially at low inclinations. For high inclinations, this
dependence is weakened, as the primary mechanisms leading to reentry are
overlapping lunisolar resonances. This study forms part of the EC-funded
(H2020) "ReDSHIFT" project.
