The Medium Earth Orbit (MEO) region hosts satellites for navigation,
communication, and geodetic/space environmental science, among which are the
Global Navigation Satellites Systems (GNSS). Safe and efficient removal of
debris from MEO is problematic due to the high cost for maneuvers needed to
directly reach the Earth (reentry orbits) and the relatively crowded GNSS
neighborhood (graveyard orbits). Recent studies have highlighted the
complicated secular dynamics in the MEO region, but also the possibility of
exploiting these dynamics, for designing removal strategies. In this paper, we
present our numerical exploration of the long-term dynamics in MEO, performed
with the purpose of unveiling the set of reentry and graveyard solutions that
could be reached with maneuvers of reasonable DV cost. We simulated the
dynamics over 120-200 years for an extended grid of millions of fictitious MEO
satellites that covered all inclinations from 0 to 90deg, using non-averaged
equations of motion and a suitable dynamical model that accounted for the
principal geopotential terms, 3rd-body perturbations and solar radiation
pressure (SRP). We found a sizeable set of usable solutions with reentry times
that exceed ~40years, mainly around three specific inclination values: 46deg,
56deg, and 68deg; a result compatible with our understanding of MEO secular
dynamics. For DV <= 300 m/s (i.e., achieved if you start from a typical GNSS
orbit and target a disposal orbit with e<0.3), reentry times from GNSS
altitudes exceed ~70 years, while low-cost (DV ~= 5-35 m/s) graveyard orbits,
stable for at lest 200 years, are found for eccentricities up to e~0.018. This
investigation was carried out in the framework of the EC-funded "ReDSHIFT"
project.Comment: 39 pages, 23 figure