Diameters of the Galilean satellites from pioneer data

Pioneers 10 and 11 have transmitted seven images of the Galilean satellites with surface resolutions on the order of several hundred kilometers. Because the point-spread function is well determined, it has been possible to measure the radius of each of the four satellites to a precision of typically ±30 km. The method used fits a semicircle to the illuminated limb by varying the center coordinates and radius until the best-fit criteria are satisfied. Careful attention is given to locating the true edge position within the blurred image. The radius determinations and corresponding densities for the satellites are: Io (1840 ± 30, 3.41 ± 0.19), Europa (1552 ± 20, 3.06 ± 0.15), Ganymede (2650 ± 25, 1.90 ± 0.06), and Callisto (2420 ± 20, 1.81 ± 0.05), where the units are in kilometers and grams per cubic centimeters, respectively. Since three images of Callisto were received, it has been possible to substantially decrease the uncertainties of the radius and density.