Masses, bulk densities, and macroporosities of asteroids (15) Eunomia, (29) Amphitrite, (52) Europa, and (445) Edna based on Gaia astrometry

Abstract

Context. Gaia Data Release 2 (DR2) includes milliarcsecond-accuracy astrometry for 14 099 asteroids. One of the main expected scientific applications of these data is asteroid mass estimation via the modeling of perturbations during asteroid-asteroid encounters. Aims. We explore the practical impact of the Gaia astrometry of asteroids for the purpose of asteroid mass and orbit estimation by estimating the masses individually for four large asteroids. We use various combinations of Gaia astrometry and/or Earth-based astrometry so as to determine the impact of Gaia on the estimated masses. By utilizing published information about estimated volumes and meteorite analogs, we also derive estimates for bulk densities and macroporosities. Methods. We apply a Markov chain Monte Carlo (MCMC) algorithm for asteroid mass and orbit estimation by modeling asteroid-asteroid close encounters to four separate large asteroids in an attempt to estimate their masses based on multiple simultaneously studied close encounters with multiple test asteroids. In order to validate our algorithm and data treatment, we apply the MCMC algorithm to pure orbit determination for the main-belt asteroid (367) Amicitia and compare the residuals to previously published ones. In addition, we attempt to estimate a mass for (445) Edna with Gaia astrometry alone based on its close encounter with (1764) Cogshall. Results. In the case of the orbit of (367) Amicitia, we find a solution that improves on the previously published solution. The study of (445) Edna reveals that mass estimation with DR2 astrometry alone is unfeasible and that it must be combined with astrometry from other sources to achieve meaningful results. We find that a combination of DR2 and Earth-based astrometry results in dramatically reduced uncertainties and, by extension, significantly improved results in comparison to those computed based on less accurate Earth-based astrometry alone. Conclusions. Our mass estimation algorithm works well with a combination of Gaia DR2 and Earth-based astrometry and provides very impressive results with significantly reduced uncertainties. We note that the DR2 has a caveat in that many asteroids suitable for mass-estimation purposes are not included in the relatively small sample. This limits the number of asteroids to which mass estimation can be applied. However, this issue will largely be corrected with the forthcoming third Gaia data release, which is expected to allow for a wave of numerous accurate mass estimates for a wide range of asteroids.