  We study the classical stability of a class of S-brane geometries having
cosmological horizons. By considering the perturbations of the metric in these
geometries we establish that their horizons are unstable in the sense that an
observer trying to cross the horizon experiences an infinite flux of radiation
at the instant of crossing. The backreaction of this radiation is likely to
convert the horizons into curvature singularities, similar to the instability
of the internal Cauchy horizon of the Reissner-Nordstrom black hole. We also
compute the particle production by the time-dependent fields in the future
regions of these geometries, and find that the spectrum of produced particles
is thermal, with temperature coinciding with the Hawking temperature computed
by euclideanizing the metric in the static region. Possible implications of
these results are discussed.
