We present mid-infrared ( 8– \unit 33 \micro \metre ) observations of the binary L5-Trojan system ( 617 ) Patroclus-Menoetius before , during , and after two shadowing events , using the Infrared Spectrograph ( IRS ) on board the Spitzer Space Telescope .
For the first time , we effectively observe changes in asteroid surface temperature in real time , allowing the thermal inertia to be determined very directly .
A new detailed binary thermophysical model is presented which accounts for the system ’ s known mutual orbit , arbitrary component shapes , and thermal conduction in the presence of eclipses .

We obtain two local thermal-inertia values , representative of the respective shadowed areas : 21 \pm \unit { 14 } { \joule \usk \power { \second } { -1 / 2 } \power { \kelvin } { -1 } \power { \metre% } { -2 } } and 6.4 \pm \unit { 1.6 } { \joule \usk \power { \second } { -1 / 2 } \power { \kelvin } { -1 } \power { % \metre } { -2 } } .
The average thermal inertia is estimated to be 20 \pm \unit { 15 } { \joule \usk \power { \second } { -1 / 2 } \power { \kelvin } { -1 } \power { \metre% } { -2 } } , potentially with significant surface heterogeneity .
This first thermal-inertia measurement for a Trojan asteroid indicates a surface covered in fine regolith .
Independently , we establish the presence of fine-grained ( < a few \micro \metre ) silicates on the surface , based on emissivity features near 10 and \unit 20 \micro \metre similar to those previously found on other Trojans .

We also report V -band observations and report a lightcurve with complete rotational coverage .
The lightcurve has a low amplitude of 0.070 \pm \unit { 0.005 } { \text { mag } } peak-to-peak , implying a roughly spherical shape for both components , and is single-periodic with a period ( 103.02 \pm \unit { 0.40 } { \hour } ) equal to the period of the mutual orbit , indicating that the system is fully synchronized .

The diameters of Patroclus and Menoetius are 106 \pm 11 and 98 \pm \unit { 10 } { \kilo \metre } , respectively , in agreement with previous findings .
Taken together with the system ’ s known total mass , this implies a bulk mass density of 1.08 \pm \unit { 0.33 } { \gram \usk \power { \centi \metre } { -3 } } , significantly below the mass density of L4-Trojan asteroid ( 624 ) Hektor and suggesting a bulk composition dominated by water ice .

All known physical properties of Patroclus , arguably the best studied Trojan asteroid , are consistent with those expected in icy objects with devolatilized surface ( extinct comets ) , consistent with what might be implied by recent dynamical modeling in the framework of the Nice Model .