Mass and density of asteroids (16) Psyche and (121) Hermione

Abstract

The first determination of the mass of asteroids (16) Psyche and (121) Hermione is reported. From an encounter with asteroid (94) Aurora, the value (8.7 ± 2.6) 10-12 M⊙ (solar mass) was derived for the mass of (16) Psyche. A mean density of (1.8±0.6) g/cm3 was deduced for this asteroid, which is much smaller than expected, since Psyche is believed to be a M-type asteroid (i.e. an asteroid of metallic composition). Some reasons that can explain this result are given, as well as some directions for further studies that could help improving the accuracy of the mass and the density of Psyche. On the other hand, the value (4.7 ± 0.8) 10-12 M⊙ found for the mass of (121) Hermione from an encounter with (278) Paulina, and which gives a mean density of (1.8 ± 0.4) g/cm3 for Hermione, corresponds to the expected value.