PS = The Planetary Society (TPS To Avoid Confusion)
It is the largest SA group. Prospace is the fanatic wing of a fanatic group IMO. I find their dedication admirable, but their goals rather poorly thought out. I'm a member of TPS and NSS as well as MS. I joined the MS out of frustration with how little the other two have helped accomplish, but I do like their magazines.
Regards,
Mark
