Limit on the Photon Mass Deduced from Pioneer-10 Observations of Jupiter's Magnetic Field

We report an analysis of the Pioneer-10 data on Jupiter's magnetic field, in which the mass $\ensuremath{\mu}$ of the photon was treated as a free parameter. We set a limit of $\ensuremath{\mu}<~2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11} {\mathrm{cm}}^{\ensuremath{-}1}\ensuremath{\equiv}6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16} \mathrm{eV}\ensuremath{\equiv}8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}49} \mathrm{g}$. This is the smallest limit so far obtained from direct measurements. We compare our result with other published limits.