  Earth-space interferometry with RadioAstron provides the highest direct
angular resolution ever achieved in astronomy at any wavelength. RadioAstron
detections of the classic quasar 3C273 on interferometric baselines up to
171,000 km suggest brightness temperatures exceeding expected limits from the
"inverse-Compton catastrophe" by two orders of magnitude. We show that at 18
cm, these estimates most probably arise from refractive substructure introduced
by scattering in the interstellar medium. We use the scattering properties to
estimate an intrinsic brightness temperature of 7*10^12 K, which is consistent
with expected theoretical limits, but which is ~15 times lower than estimates
that neglect substructure. At 6.2 cm, the substructure influences the measured
values appreciably but gives an estimated brightness temperature that is
comparable to models that do not account for the substructure. At 1.35 cm, the
substructure does not affect the extremely high inferred brightness
temperatures, in excess of 10^13 K. We also demonstrate that for a source
having a Gaussian surface brightness profile, a single long-baseline estimate
of refractive substructure determines an absolute minimum brightness
temperature, if the scattering properties along a given line of sight are
known, and that this minimum accurately approximates the apparent brightness
temperature over a wide range of total flux densities.
