Shklovsky, Iosif Samuilovich; The Universe, Life, and Mind, Academy of Sciences USSR, Moscow, 1962 Shklovsky based his analysis on estimates of the upper Martian atmosphere's density, and deduced that for the weak braking effect to be able to account for the secular acceleration, Phobos had to be very light—one calculation yielded a hollow iron sphere 16 kilometers across but less than 6 cm thick.