In understanding the composition and internal structure of asteroids , their density is perhaps the most diagnostic quantity .
We aim here to characterize the surface composition , mutual orbit , size , mass , and density of the small main-belt binary asteroid ( 939 ) Isberga .
For that , we conduct a suite of multi-technique observations , including optical lightcurves over many epochs , near-infrared spectroscopy , and interferometry in the thermal infrared .
We develop a simple geometric model of binary systems to analyze the interferometric data in combination with the results of the lightcurve modeling .
From spectroscopy , we classify Ibserga as a Sq-type asteroid , consistent with the albedo of 0.14 ^ { +0.09 } _ { -0.06 } ( all uncertainties are reported as 3- \sigma range ) we determine ( average albedo of S-types is 0.197 \pm 0.153 , see Pravec et al. , 2012 , Icarus 221 , 365-387 ) .
Lightcurve analysis reveals that the mutual orbit has a period of 26.6304 \pm 0.0001 h , is close to circular ( eccentricity lower than 0.1 ) , and has pole coordinates within 7 ^ { \circ } of ( 225 ^ { \circ } , +86 ^ { \circ } ) in Ecliptic J2000 , implying a low obliquity of 1.5 ^ { +6.0 } _ { -1.5 } degree .
The combined analysis of lightcurves and interferometric data allows us to determine the dimension of the system and we find volume-equivalent diameters of 12.4 ^ { +2.5 } _ { -1.2 } km and 3.6 ^ { +0.7 } _ { -0.3 } km for Isberga and its satellite , circling each other on a 33 km wide orbit .
Their density is assumed equal and found to be 2.91 ^ { +1.72 } _ { -2.01 } g.cm ^ { -3 } , lower than that of the associated ordinary chondrite meteorites , suggesting the presence of some macroporosity , but typical of S-types of the same size range ( Carry , 2012 , P & SS 73 , 98–118 ) .
The present study is the first direct measurement of the size of a small main-belt binary .
Although the interferometric observations of Isberga are at the edge of MIDI capabilities , the method described here is applicable to others suites of instruments ( e.g. , LBT , ALMA ) .