Small binary asteroids and prospects for their observations with Gaia

Abstract

We studied a capability of Gaia to detect binary systems among small asteroids (with diameters ≲10km) by observing their photocenter oscillation. The closest binary systems with orbital periods about 1 day or shorter show mostly a too low amplitude of the photocenter oscillation and Gaia will not be able to detect most of them with its expected astrometric performance. Wider binaries with orbital periods on an order of several days or longer shall be detectable with their amplitudes of the photocenter oscillation on an order of 10-times greater than the expected standard uncertainty of their Gaia astrometric measurements. A confusion of binaries with slowly rotating asteroids that show a rotational photocenter variation of a similar magnitude will not be significant unless the satellite is small or very large; in the range of medium-distance binaries (with the orbital periods on an order of several days), we shall be able to uniquely distinguish binaries with the diameter ratio D2/D1 between ∼0.1 and ∼0.95. Gaia will be the first survey to sample the largely unknown population of medium-distance binary systems among small main-belt asteroids where the present detection techniques (photometric and AO observations) are inefficient. A combination of Gaia binary observations with measurements with other techniques will be needed to eliminate existing degeneracy of the astrometric binary detection and to provide unique estimates of parameters of the binary systems.