  We report on Spitzer Space Telescope IRAC observations of near-Earth object
(NEO) 2009 BD that were carried out in support of the NASA Asteroid Robotic
Retrieval Mission (ARRM) concept. We did not detect 2009 BD in 25 hrs of
integration at 4.5 micron. Based on an upper-limit flux density determination
from our data, we present a probabilistic derivation of the physical properties
of this object. The analysis is based on the combination of a thermophysical
model with an orbital model accounting for the non-gravitational forces acting
upon the body. We find two physically possible solutions. The first solution
shows 2009 BD as a 2.9+/-0.3 m diameter rocky body (rho = 2.9+/-0.5 g cm-3)
with an extremely high albedo of 0.85(+0.20/-0.10) that is covered with
regolith-like material, causing it to exhibit a low thermal inertia (Gamma =
30(+20/-10) SI units). The second solution suggests 2009 BD to be a 4+/-1 m
diameter asteroid with pV = 0.45(+0.35/-0.15) that consists of a collection of
individual bare rock slabs (Gamma = 2000+/-1000 SI units, rho = 1.7(+0.7/-0.4)
g cm-3). We are unable to rule out either solution based on physical reasoning.
2009 BD is the smallest asteroid for which physical properties have been
constrained, in this case using an indirect method and based on a detection
limit, providing unique information on the physical properties of objects in
the size range smaller than 10 m.
