The issue of New Astronomy for December (vol. 121) includes:
Borodin, V. I. Bubenchikov, M. A. Bubenchikov, A. M. et al. Complex rotations of the asteroid 45 Eugenia Art. 102458 .2025.102458
Binaries: once again, handy to have. The parent asteroid (45) Eugenia happens to have two satellites (a triple system), and the motions of the three tell us (per Kepler’s Laws) much about these celestial bodies. For one, Eugenia has an oddly low density. That low density- lighter than most rocks- could be due to ice content, or porosity (voids), or both. Lots of asteroids are “rubble piles” (unconsolidated debris) with nontrivial porosity, but Eugenia is a rather large body (210 by roughly 150 by 100 kilometers). As bodies get larger, so does their gravity, and significant porosity gets less and less plausible.
What about ice, then? Eugenia is in the middle of the Main Belt, nothing exceptional. Therefore, though exposed ice is not stable on the surface (too much solar UV), interior ice (where it’s optically and thermally protected) is neither assumed nor dismissed. Sorry if you were expecting unambiguous results- nature is not here for our convenience.
What is unambiguous is our tracking of Eugenia’s motions. Eugenia tugs on its satellites (“perturbation”), and they tug right back. Knowing the range to the satellites, and guessing realistic masses for them (nothing too huge), we can calculate their effects on the primary. Eugenia is, again, a lightweight body for being hundreds of km. The tugs of the satellites, and therefore the libration (‘rocking’) of Eugenia, have too strong of an effect for Eugenia to be a solid rock body. Again, (45) Eugenia has significant ice content, porosity, or likely some combination.