SETI at Home: Detecting Non-Gravitational Accelerations in the Solar System through Astrometry

In 1959, Soviet astronomer Iosif Shklovsky calculated that the semimajor axis of Phobos’s orbit around Mars was decreasing by about 5cm/year, based on previous observations from American astronomer Bevan Sharpless and others. Searching for explanations of the orbital decay, Shklovsky ruled out the effects of atmospheric drag, tidal friction, electromagnetic braking, solar radiation pressure, and classical mechanical perturbations as too weak to produce a decay as quick as 5cm/year. However, he suggested that the rate of decay could be explained if Phobos were much less dense than expected, like a hollow structure left behind by a technologically advanced civilization. Later calculations revealed systematic errors in the previous observations, and that Phobos’s rate of orbital decay is only about 1.8cm/year, explainable by tidal friction. However, no studies to date have calculated the minimum density Phobos could have without a detectable change in its semimajor axis relative to the currently observed rate of decay. In this work, we use a REBOUND simulation of Phobos orbiting Mars to calculate this minimum density. Additionally, we simulate a hollow Phobos with a similar density to human-made artificial satellites to determine how quickly human technology could detect the difference between a natural Phobos and an artificial Phobos.