We conducted a polarimetric observation of the fast–rotating near–Earth asteroid ( 1566 ) Icarus at large phase ( Sun–asteroid–observer ’ s ) angles \alpha = 57°–141° around the 2015 summer solstice .
We found that the maximum values of the linear polarization degree are P _ { \mathrm { max } } =7.32 \pm 0.25 % at phase angles of \alpha _ { \mathrm { max } } =124° \pm 8° in the V -band and P _ { \mathrm { max } } =7.04 \pm 0.21 % at \alpha _ { \mathrm { max } } =124° \pm 6° in the R _ { \mathrm { C } } –band .
Applying the polarimetric slope–albedo empirical law , we derived a geometric albedo of p _ { \mathrm { V } } =0.25 \pm 0.02 , which is in agreement with that of Q-type taxonomic asteroids . \alpha _ { \mathrm { max } } is unambiguously larger than that of Mercury , the Moon , and another near–Earth S–type asteroid ( 4179 ) Toutatis but consistent with laboratory samples with hundreds of microns in size .
The combination of the maximum polarization degree and the geometric albedo is in accordance with terrestrial rocks with a diameter of several hundreds of micrometers .
The photometric function indicates a large macroscopic roughness .
We hypothesize that the unique environment ( i.e. , the small perihelion distance q =0.187 au and a short rotational period of T _ { \mathrm { rot } } =2.27 hours ) may be attributed to the paucity of small grains on the surface , as indicated on ( 3200 ) Phaethon .