Context : Near-Earth asteroid 162173 ( 1999 JU3 ) is the primary target of the Japanese Aerospace Exploration Agency ( JAXA ) Hayabusa-2 sample return mission , and is also on the list of potential targets for the European Space Agency ( ESA ) Marco Polo sample return mission .
Earth-based studies of this object are fundamental to these missions .

Aims : Our aim is to provide new constraints on the surface properties of this asteroid .

Methods : We present a mid-infrared spectrum ( 5-38 \mu m ) obtained with NASA ’ s Spitzer Space Telescope in May 2008 and results from the application of thermal models .

Results : These observations place new constraints on the surface properties of this asteroid .
To fit our spectrum we used the near-Earth asteroid thermal model ( NEATM ) and the more complex thermophysical model ( TPM ) .
However , the position of the spin-pole , which is uncertain , is a crucial input parameter for constraining the thermal inertia with the TPM ; hence , we consider two pole orientations .
First is the extreme case of an equatorial retrograde geometry from which we derive a rigorous lower limit to the thermal inertia of 150 Jm ^ { -2 } s ^ { -0.5 } K ^ { -1 } .
Second , when we adopt the pole orientation of Abe et al .
( 2008a ) our best-fit thermal model yields a value for the thermal inertia of 700 \pm 200 Jm ^ { -2 } s ^ { -0.5 } K ^ { -1 } and even higher values are allowed by the uncertainty in the spectral shape due to the absolute flux calibration .
Our best estimates of the diameter ( 0.90 \pm 0.14 km ) and geometric albedo ( 0.07 \pm 0.01 ) of asteroid 162173 are consistent with values based on previous mid-infrared observations .

Conclusions : We establish a rigorous lower limit to the thermal inertia , which is unlikely but possible , and would be consistent with a fine regolith similar to wthat is found for asteroid 433 Eros .
However , the thermal inertia is expected to be higher , possibly similar to or greater than that on asteroid 25143 Itokawa .
An Accurately determining the spin-pole of asteroid 162173 will narrow the range of possible values for its thermal inertia .