Phobos phase angles explained

ESA’s Mars Express recently tracked the small martian moon Phobos as it flew past the spacecraft, capturing detailed views of the pockmarked, irregularly shaped body at different angles and stages of the flyby. This image sequence nicely demonstrates the concept of ‘phase angle’: the angle between a light source (in this case, the Sun) and the observer (Mars Express’ HRSC), as viewed from the target object itself (Phobos). The initial phase angle is 17 degrees, drops to almost 0 degrees mid-way through (w