  We report on observations of near-Earth asteroid 2011 MD with the Spitzer
Space Telescope. We have spent 19.9 h of observing time with channel 2 (4.5
{\mu}m) of the Infrared Array Camera and detected the target within the
2{\sigma} positional uncertainty ellipse. Using an asteroid thermophysical
model and a model of nongravitational forces acting upon the object we
constrain the physical properties of 2011 MD, based on the measured flux
density and available astrometry data. We estimate 2011 MD to be 6 (+4/-2) m in
diameter with a geometric albedo of 0.3 (+0.4/-0.2) (uncertainties are
1{\sigma}). We find the asteroid's most probable bulk density to be 1.1
(+0.7/-0.5) g cm^{-3}, which implies a total mass of (50-350) t and a
macroporosity of >=65%, assuming a material bulk density typical of
non-primitive meteorite materials. A high degree of macroporosity suggests 2011
MD to be a rubble-pile asteroid, the rotation of which is more likely to be
retrograde than prograde.
