We measured the system-integrated thermal emission of the binary Kuiper Belt Object ( 47171 ) 1999 TC _ { 36 } at wavelengths near 24 and 70 \mu m using the Spitzer space telescope .
We fit these data and the visual magnitude using both the Standard Thermal Model and thermophysical models .
We find that the effective diameter of the binary is 405 km , with a range of 350 – 470 km , and the effective visible geometric albedo for the system is 0.079 with a range of 0.055 – 0.11 .
The binary orbit , magnitude contrast between the components , and system mass have been determined from HST data ( Margot et al. , 2004 ; 2005a ; 2005b ) .
Our effective diameter , combined with that system mass , indicate an average density for the objects of 0.5 g/cm ^ { 3 } , with a range 0.3 – 0.8 g/cm ^ { 3 } .
This density is low compared to that of materials expected to be abundant in solid bodies in the trans-Neptunian region , requiring 50 – 75 % of the interior of ( 47171 ) 1999 TC _ { 36 } be taken up by void space .
This conclusion is not greatly affected if ( 47171 ) 1999 TC _ { 36 } is “ differentiated ” ( in the sense of having either a rocky or just a non-porous core ) .
If the primary is itself a binary , the average density of that ( hypothetical ) triple system would be in the range 0.4 – 1.1 g/cm ^ { 3 } , with a porosity in the range 15 – 70 % .