Difference in mean anomaly (referred to in B. P. Sharpless 1945 as longitude) between solid and hollow Phobos simulations over time. In blue (dashed line), we use a hollow Phobos reduced to 2.1 × 10⁻⁵ times the real mass of Phobos to reproduce Sharpless’s calculated inspiral rate of 5.7 cm yr⁻¹. In orange (solid line), we used 6.8 × 10⁻⁵M_Phobos to reproduce Phobos’s actual inspiral rate of 1.8 cm yr⁻¹. The black xs are Sharpless’s data, with his estimated error bars.