  Radiation pressure on dust grains may be an important physical mechanism
driving galaxy-wide superwinds in rapidly star-forming galaxies. We calculate
the combined dust and gas Eddington ratio (Gamma) for the archetypal superwind
of M82. By combining archival GALEX data, a standard dust model, Monte Carlo
dust scattering calculations, and the Herschel map of the dust surface density
distribution, the observed FUV/NUV surface brightness in the outflow constrains
both the total UV luminosity escaping from the starburst along its minor axis
(L_*, UV) and the flux-mean opacity, thus allowing a calculation of Gamma. We
find that L_(*, UV) ~ 1-6*10^42 ergs s^-1, ~2-12 times greater than the UV
luminosity observed from our line of sight. On a scale of 1-3 kpc above the
plane of M82, we find that Gamma ~ 0.01 - 0.06. On smaller scales (~0.25-0.5
kpc), where the enclosed mass decreases, our calculation of L_(*, UV) implies
that Gamma ~ 0.1 with factor of few uncertainties. Within the starburst itself,
we estimate the single-scattering Eddington ratio to be of order unity. Thus,
although radiation pressure is weak compared to gravity on kpc scales above the
plane of M82, it may yet be important in launching the observed outflow. We
discuss the primary uncertainties in our calculation, the sensitivity of Gamma
to the dust grain size distribution, and the time evolution of the wind
following M82's recent starburst episodes.
