  Binary neutron star mergers (BNS) produce high-energy emissions from several
physically different sources, including a gamma-ray burst (GRB) and its
afterglow, a kilonova, and, at late times, a remnant many parsecs in size.
Ionizing radiation from these sources can be dangerous for life on Earth-like
planets when located too close. Work to date has explored the substantial
danger posed by the GRB to on-axis observers: here we focus instead on the
potential threats posed to nearby off-axis observers. Our analysis is based
largely on observations of the GW 170817/GRB 170817A multi-messenger event, as
well as theoretical predictions. For baseline kilonova parameters, we find that
the X-ray emission from the afterglow may be lethal out to $\sim 5$ pc and the
off-axis gamma-ray emission may threaten a range out to $\sim 4$ pc, whereas
the greatest threat comes years after the explosion, from the cosmic rays
accelerated by the kilonova blast, which can be lethal out to distances up to
$\sim 11$ pc. The distances quoted here are typical, but the values have
significant uncertainties and depend on the viewing angle, ejected mass, and
explosion energy in ways we quantify. Assessing the overall threat to
Earth-like planets, have a similar kill distance to supernovae, but are far
less common. However, our results rely on the scant available kilonova data,
and multi-messenger observations will clarify the danger posed by such events.
