These First Day Covers are an absolute minefield (although why you thought Babs would sign an FDC related to scientists back in the day is probably pertinent).
I have never actually bought one and probably never will but I have seen one thousand that I wanted to pull the trigger on. There is no information source (that I know of) which reliably identifies which covers have been hand-signed by the subject(s) and it is usually very difficult to assess online whether the signatures are printed or not. I have most recently almost been caught out by the Monty Python one in the UK (printed, with different coloured inks used) .
Best avoided, I feel, unless you are an experienced philatelist as well.