  The origin of fast radio bursts (FRBs) is still a mystery. One model proposed
to interpret the only known repeating object, FRB 121102, is that the radio
emission is generated from asteroids colliding with a highly magnetized neutron
star (NS). With N-body simulations, we model a debris disc around a central
star with an eccentric orbit intruding NS. As the NS approaches the first
periastron passage, most of the comets are scattered away rather than being
accreted by the NS. To match the observed FRB rate, the debris belt would have
to be at least three orders of magnitude more dense than the Kuiper belt. We
also consider the rate of collisions on to the central object but find that the
density of the debris belt must be at least four orders of magnitude more dense
than the Kuiper belt. These discrepancies in the density arise even if (1) one
introduces a Kuiper-belt like comet belt rather than an asteroid belt and
assume that comet impacts can also make FRBs; (2) the NS moves 2 orders of
magnitude slower than their normal proper-motion velocity due to supernova
kicks; and (3) the NS orbit is coplanar to the debris belt, which provides the
highest rate of collisions.
