Bounding the Photon Mass with Solar Probes

Proposed close-encounter solar probes (Vulcan and Solar Probe) are planned to have highly eccentric orbits, with a perihelion of about $4R_S$ and an inclination close to $90^\circ$ out of the plane of the ecliptic. We show this could allow at least an order-of-magnitude improvement in the present directly measured limit on the photon mass.