Letter from Thomas Gold to Professor J A Simpson, The Enrico Fermi Institute for Nuclear Studies, University of Chicago, Chicago, Illinois Has 'read with great interest the Pioneer 6 releases including yours'. Says that 'I would of course be very interested to know what consideration allows one to tell whether the periodic anisotropy is due to injection at the sun or to rattling around in a pre-existing magnetic bottle configuration. Evidently you have some other consideration in your mind which is not referred to in the release, and I would be very glad to know. The phenomenon that you observe is of course a very interesting one and it will be important to understand it'. 17 May 1966