  We investigate the claim that the ratio {\beta} of radiation pressure force
to gravitational force on a dust grain in our solar system can substantially
exceed unity for some grain sizes, provided that grain porosity is high enough.
For model grains consisting of random aggregates of silicate spherules, we find
that the maximum value of {\beta} is almost independent of grain porosity, but
for small (<0.3 {\mu}m) grains, {\beta} actually decreases with increasing
porosity. We also investigate the effect of metallic iron and amorphous carbon
inclusions in the dust grains and find that while these inclusions do increase
the radiation pressure cross-section, {\beta} remains below unity for grains
with 3 pg of silicate material. These results affect the interpretation of the
grain trajectories estimated from the {\it Stardust} mission, which were
modeled assuming {\beta} values exceeding one. We find that radiation pressure
effects are not large enough for particles Orion and Hylabrook captured by {\it
Stardust} to be of interstellar origin given their reported impact velocities.
We also consider the effects of solar radiation on transverse velocities and
grain spin, and show that radiation pressure introduces both transverse
velocities and equatorial spin velocities of several hundred meters per second
for incoming interstellar grains at 2 AU. These transverse velocities are not
important for modeling trajectories, but such spin rates may result in
centrifugal disruption of aggregates.
