  The deflection of interstellar dust grains in the magnetic field near the
heliopause has been investigated based on the assumption that interstellar
grains are homogeneous spheres. However, remote observations have shown that
interstellar grains are more likely to be composites of a large number of
subunits. This has profound significance when interpreting data obtained
through in-situ measurements, for the deflection of interstellar grains depends
on their charge-to-mass ratio, and aggregates acquire different surface charges
from spheres due to their complex structure. In this paper, the charging of
aggregates near the heliopause is examined including both plasma charging and
secondary electron emission. The results show that aggregates generally have a
higher charge-to-mass ratio than spheres, and the small particle effect from
secondary electron emission is evident for aggregates consisting of nano-sized
particles. A new approach to estimate the aggregate charge with the aid of its
structural characteristics is presented. The charge-to-mass ratio is used to
derive the mass distribution of interstellar dust near the heliopause, and the
result shows an overall agreement with Ulysses data.
