PHYSICAL PROPERTIES OF NEAR-EARTH ASTEROID 2011 MD

We report on observations of near-Earth asteroid 2011 MD with the Spitzer Space Telescope. We have spent 19.9 hr 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(-2)(+4) ) m in diameter with a geometric albedo of 0.3(-0.2)(+0.4) (uncertainties are 1 sigma). We find the asteroid's most probable bulk density to be (1.1(-0.5)(+0.7) ) 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 that 2011 MD is a rubble-pile asteroid, the rotation of which is more likely to be retrograde than prograde.