  We identify a new mechanism that can lead to the destruction of small,
close-in planetary satellites. If a small moon close to the planet has a
sizable eccentricity and inclination, its ejecta that escape to planetocentric
orbit would often re-impact with much higher velocity due to the satellite's
and the fragment's orbits precessing out of alignment. If the impacts of
returning ejecta result in net erosion, a runaway process can occur which may
end in disruption of the satellite, and we term this process ``sesquinary
catastrophe''. We expect the moon to re-accrete, but on an orbit with
significantly lower eccentricity and inclination. We find that the large
majority of small close-in moons in the Solar System, have orbits that are
immune to sesquinary catastrophe. The exceptions include a number of resonant
moonlets of Saturn for which resonances may affect the velocities of re-impact
of their own debris. Additionally, we find that Neptune's moon Naiad (and to a
lesser degree, Jupiter's Thebe) must have substantial internal strength, in
line with prior estimates based on Roche limit stability. We also find that
sesquinary instability puts important constraints on the plausible past orbits
of Phobos and Deimos or their progenitors.
