Discovery of the binary nature of the Mars – crosser ( 1139 ) Atami ?

Context. Asteroids with satellites are useful to probe the dynamical evolution of the solar system. Aims. The minor planet (1139) Atami exhibited mutual eclipses on its light curve during the 2005 opposition. We used these events to study the binary nature of this Mars–crossing asteroid. Methods. We used geometric methods applied to photometric data and Doppler-resolved radar echoes to deduce orbital and physical characteristics of the two bodies. Results. We assume that (1139) Atami is a double body system composed by two prolate ellipsoids (of size 2ra × 2rb × 2rb) with the same axial ratio rb/ra. From photometric light curves, the major–axes should point each other implying that rotation and revolution are synchronous with a period T = 1.14361 ± 0.00023 day. According to the model taken in this study, photometric and radar data analysis reveal that rb/ra=0.65 ±0.02 and a circular orbit with a center-to-center separation of s = 17.56 ± 1.48 km. From the duration of the eclipse features of the light curve, we derive the bulk density ρ = 2.6 ± 0.5 g/cm3, and the pole orientation: λp = 264 ± 5◦, βp = +5 ± 5◦. The intensity of radar echoes implies that the body dimension ratio is 0.82 ±0.08. The bulk density is very sensitive to the eclipse duration. As we observed a fast evolution of the eclipse features, this value must be refined during the next oppositions (next observable eclipses are predicted in 2009). Spectrometry and BVRI photometry performed during eclipses reveal that the two bodies have the same reflectance properties and are probably members of the S spectral class. Radar albedo and archived spectroscopic observations show that the surface nature is probably moderatly rough at decimetre scales. Conclusions.