  Core-collapse supernovae emit of order $10^{58}$ neutrinos and antineutrinos
of all flavors over several seconds, with average energies of 10--25 MeV. In
the Sudbury Neutrino Observatory (SNO), which begins operations this year,
neutrinos and antineutrinos of all flavors can be detected by reactions which
break up the deuteron. For a future Galactic supernova at a distance of 10 kpc,
several hundred events will be observed in SNO. The $\nu_\mu$ and $\nu_\tau$
neutrinos and antineutrinos are of particular interest, as a test of the
supernova mechanism. In addition, it is possible to measure or limit their
masses by their delay (determined from neutral-current events) relative to the
$\bar{\nu}_e$ neutrinos (determined from charged-current events). Numerical
results are presented for such a future supernova as seen in SNO. Under
reasonable assumptions, and in the presence of the expected counting
statistics, a $\nu_\mu$ or $\nu_\tau$ mass down to about 30 eV can be simply
and robustly determined. If zero delay is measured, then the mass limit is
independent of the distance $D$. At present, this seems to be the best
possibility for direct determination of a $\nu_\mu$ or $\nu_\tau$ mass within
the cosmologically interesting range. We also show how to separately study the
supernova and neutrino physics, and how changes in the assumed supernova
parameters would affect the mass sensitivity.
