  Mirror sectors have been proposed to address the problems of dark matter,
baryogenesis, and the neutron lifetime anomaly. In this work we study a new,
powerful probe of mirror neutrons: neutron star temperatures. When neutrons in
the neutron star core convert to mirror neutrons during collisions, the
vacancies left behind in the nucleon Fermi seas are refilled by more energetic
nucleons, releasing immense amounts of heat in the process. We derive a new
constraint on the allowed strength of neutron--mirror-neutron mixing from
observations of the coldest (sub-40,000 Kelvin) neutron star, PSR 2144$-$3933.
Our limits compete with laboratory searches for neutron--mirror-neutron
transitions but apply to a range of mass splittings between the neutron and
mirror neutron that is 19 orders of magnitude larger. This heating mechanism,
also pertinent to other neutron disappearance channels such as exotic neutron
decay, provides a compelling physics target for upcoming ultraviolet, optical
and infrared telescopes to study thermal emissions of cold neutron stars.
