Aims.The possibility of determining the mass of large asteroids (other than Ceres, Pallas, and Vesta) from recent close encounters with smaller bodies has been investigated, and a new and more accurate determination of the mass of the asteroid (15) Eunomia is presented.
Methods: .The circumstances of the close approaches between large asteroids and the ca. 129 000 numbered ones were found by a numerical integration over the past 20 years. The cases for which an appreciable perturbation could be expected were investigated by fitting the mass of the perturbing body to the available observations.
Results: .A very close approach took place on March 4, 2002, between Eunomia and (50278) 2000CZ12, at a nominal distance of 55 200 km. The resulting perturbation in the mean motion of the smaller body was substantially larger than for the other detected events, amounting to ca. 0.9 arcsecs/y. The orbital elements of (50278) 2000CZ12 were fitted together with the mass of Eunomia on 106 of 110 observations spanning 1950 to 2006, resulting in a mass of 3.25×1019 kg (1.64× 10-11 M_Sun) with a formal 1σ relative uncertainty of 3.7%. The optimized mass is 30-60% higher and the formal error 4-7 times smaller than previous estimates.