  Impacts from icy and rocky bodies have helped shape the composition of solar
system objects, for example the Earth-Moon system, or the recent impact of
comet Shoemaker-Levy 9 with Jupiter. It is likely that such impacts also shape
the composition of exoplanetary systems. Here we investigate how cometary
impacts might affect the atmospheric composition/chemistry of hot Jupiters,
which are prime targets for characterisation. We introduce a parametrised
cometary impact model that includes thermal ablation and pressure driven
breakup, which we couple with the 1D `radiative-convective' atmospheric model
ATMO, including disequilibrium chemistry. We use this model to investigate a
wide range of impactor masses and compositions, including those based on
observations of Solar System comets, and interstellar ices (with JWST). We find
that even a small impactor (R = 2.5 km) can lead to significant short-term
changes in the atmospheric chemistry, including a factor $>10$ enhancement in
H$_2$O, CO, CO$_2$ abundances, and atmospheric opacity more generally, and the
near complete removal of observable hydrocarbons, such as CH$_4$, from the
upper atmosphere. These effects scale with the change in atmospheric C/O ratio
and metallicity. Potentially observable changes are possible for a body that
has undergone significant/continuous bombardment, such that the global
atmospheric chemistry has been impacted. Our works reveals that cometary
impacts can significantly alter or pollute the atmospheric
composition/chemistry of hot Jupiters. These changes have the potential to
mute/break the proposed link between atmospheric C/O ratio and planet formation
location relative to key snowlines in the natal protoplanetary disc.
