  Stellar astrospheres--the plasma cocoons carved out of the interstellar
medium by stellar winds--are continually influenced by their passage through
the fluctuating interstellar medium (ISM). Inside dense interstellar regions,
an astrosphere may be compressed to a size smaller than the liquid-water
habitable zone distance. Habitable planets then enjoy no astrospheric buffering
from the full flux of Galactic cosmic rays and interstellar dust and gas, a
situation we call ``descreening.'' Recent papers (Yeghikyan and Fahr, Pavlov et
al.) have suggested such global consequences as severe ozone depletion and
glaciation. Using a ram-pressure balance model that includes gravitational
focusing of the interstellar flow, we compute the size of the astrosphere in
the apex direction as a function of parent star mass. We derive a dependence on
the parent-star mass M due to gravitational focusing for densities larger than
about 100 (M/M_\odot)^{-2} cm^{-3}. We calculate the interstellar densities
required to descreen planets in the habitable zone of solar- and subsolar-mass
stars and find a critical descreening density of roughly 600 (M/M_\odot)^{-2}
cm^{-3} for the Sun's velocity relative to the local ISM. Finally, we estimate
from ISM observations the frequency of descreening encounters as 1--10 Gyr^{-1}
for solar-type stars and 10^2--10^9 times smaller for M stars. Given this
disparity, we conclude that M star habitable-zone planets are virtually never
exposed to the severe effects discussed by Yeghikyan and Fahr and Pavlov et al.
