More or less hollow. It’s orbit doesn’t work for a solid body the same size.
How so?
Fascinating!
The observations which led Shklovsly to theorize that it was jollow proved to be inaccurate; the altitude loss was less than initially calculated, and the new loss rate is entirely consistent with a solid body.
But Phobos isn’t solid! 1/3 its volume is empty space! It’s actually a porous conglomerate.