Re:
Time for the Stars ---
Although he is not an "Old" master writer from the '50's and 60's, Spider Robinson wrote
Variable Star, which was based on an incomplete outline from Heinlein's papers, which was for a sequel or expansion of
Time for the Stars.
I found
Variable Star very entertaining, but I am a great fan of the Spider, and so the Spider prose with strong undertones of Heinlein was grand. Lots of humor --- and why, I wonder, do I see so little discussion of the HUMOR to be found in Heinlein's writing? (of course, it could very well be that I just haven't run across such discussions.)
PS -- I don't really think we will be able to map habitable worlds only by long-distance --- that might be like trying to locate a specific ant-hill with Google Maps--
There would be a lot more data needed than just what would be available from spectral analysis, etc. But maybe things actually will be so advanced soon (... some interesting ideas about unmanned exploration and artificial intelligence in Greg Bear's
Queen of Angels from 1990.)