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

The first determination of the mass of asteroids (16) Psyche and (121) Herm ione is reported. From an encounter with asteroid (94) Aurora, the value (8 .7 +/- 2.6) 10(-12) M-circle dot (solar mass) was derived for the mass of ( 16) Psyche. A mean density of(1.8 +/- 0.6) g/cm(3) was deduced for this ast eroid, 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 furt her studies that could help improving the accuracy of the mass and the dens ity of Psyche. On the other hand, the value (4.7 +/- 0.8) 10(-12) M-circle dot found for the mass of (121) Hermione from an encounter with (278) Pauli na, and which gives a mean density of (1.8 +/- 0.4) g/cm(3) for Hermione, c orresponds to the expected value.