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 \mu m. 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 \pm 0.3 m diameter rocky body ( \rho = 2.9 \pm 0.5 g cm ^ { -3 } ) with an extremely high albedo of 0.85 _ { -0.10 } ^ { +0.20 } that is covered with regolith-like material , causing it to exhibit a low thermal inertia ( \Gamma = 30 _ { -10 } ^ { +20 } SI units ) .
The second solution suggests 2009 BD to be a 4 \pm 1 m diameter asteroid with p _ { V } = 0.45 _ { -0.15 } ^ { +0.35 } that consists of a collection of individual bare rock slabs ( \Gamma = 2000 \pm 1000 SI units , \rho = 1.7 _ { -0.4 } ^ { +0.7 } 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 .