I like the idea of using the name of someone you admire, but why not make it more subtle? Use Charles instead of Darwin, for instance. Then you can choose to tell the story of who the name was in honor of or not.
And don’t use Barack…he seems like an admirable person, but his ultimate legacy hasn’t been written yet.
The reason I haven’t wanted to use the normal first names of people I admire is that they don’t evoke anything for me. i think they’re boring. I want to hang at least one creative name on a kid, but I don’t want it to be too creative. I know I’m trying to thread the needle here (and you’re right about Barack – too early).
Incidentally, my wife just floated Marley…as a girl’s name.
Last name, and it’s ridiculous. Did she really, or are you trying to get a rise out of us? 
I like “Martin,” suggested upthread, for a boy. It’s classic, and it’s not very common. For a girl, I suggest Rosalind. If you like science, surely you can appreciate Rosalind.
I get what you’re saying, I’m just imagining the kid constantly having to affirm that, yes, his/her dad’s a big fan of Jimi Hendrix. Perhaps with an eye roll to accompany it. There’s always a chance the child won’t turn out to admire Hendrix the way you do!
There’s a little 1-year-old Marley at my son’s daycare. I suspect she was named after Bob, but I have not confirmed it with the parents.
I had this situation; my brothers got more traditional ethnic names, and I got a first name of a different ethnicity. I don’t really identify with the ethnicity that the first name represents, even though it’s a part of my background and kind of wish I would have been given an ethnic name that would have better represented my ethnic background. The only real problem with all of this is that there aren’t a lot of girls’ names that would have been universally pronounceable with English speakers.
As for Sagan and Ripley, I think they sound a little ridiculous and agree that you should probably go with some of the naming patterns that you’ve already established. Strangely enough, I don’t have a problem with Coltrane as a middle name; I think that last names as middle names are a lot more acceptable than last names as first names, though.
'Course there’s no rule that says you only have to use one of their names: Carl Sagan <Smith> or Ellen Ripley <Smith> both capture the name you want and give the kid a choice.
To the OP question of opinion: Sagan and Ripley are both (IMO) perfectly serviceable names. (But then my son has a playmate named “Wilco”)
Charisma ? 
(Charisma Carpenter, known as “Chrissy”, was named after an Avon perfume).
Sounds like you’re expecting a negative reaction from me, but I say go to it and good luck. I think it’s a great name from an inspiring musician, and it’s not trendy. And it’s usually a girl’s name - I actually consider that a positive for me; it helped me feel unique as a kid. That’s the same reason I voted yes for Sagan: screw teasing, I think it’s a great gift to give a kid a name that’s uncommon. (As long as the name isn’t stupid or just creative spelling of an uninteresting name.)
So that idea gets a big thumbs-up from me. A minor warning, though: ever since a certain freaking book got published, it seems like everybody has a friend with a dog named Marley, and they all think I’ll be just fascinated and tickled to hear about it. And it may get worse, since it sounds like there will be a movie out by the end of the year. If you named your daughter Marley, by the time she’s old enough to talk the book and the dog naming will have faded from the popular consciousness somewhat, but you might hear this from a couple of schmucks. One more reason to keep baby name ideas a secret. 
I know that nobody wants to think of their own daughter in this context, but…
…I can’t imagine moaning “Oh, Sagan!” while in bed with a girl.
There’s nothing sexy about the name.
I’m a Mike. I’ve never been in a class where I didn’t have at least one other person responding to that name, and often several of us. Hell’s bell, my college roommate was a Mike. “Hello. Thank you for calling Mike and Mike’s room. Mike speaking. Mike’s not in right now. If you care to leave a message with Mike for Mike, Mike will make sure Mike gets it when Mike gets in.” I’d always been a bit jealous of those people who two last names, because of such experiences.
Sagan sounds great to me. I’m not even that much of a fan of the late astronomer, but I still like the name. I’d object to talking about pushing the child into Astronomy or the like, but that’s not what you’re talking about, and I doubt you’d do that anyways.
However, in a further caveat, my sister has threatened to kidnap any children I might have to make sure that I can’t name them. She thinks I’ll give them odd names for some reason.
How about “Gregory House”? Or if it’s a girl, “Cutthroat Bitch”?
I like Sagan, for a boy. What the the Middle name choices? Archimedes?
You did realise you were going to step on some toes with this post yes?
Anyways
I personally would join the ‘No’ crowd, but wouldn’t condemn the name. I’m not getting the “Sag” angle either as the name is pronounced Say-gan. I would also say the name would fit a girl better than a boy.
Good luck!
Carl
I don’t really like it- “Sag” just isn’t a great syllable- but I don’t hate it as much as a could.
I hate Ripley. It sounds like, errr, that person has ripples. Gross.
Sagan, I don’t care for and Ripley is a cool name in and of itself, but I wouldn’t name a kid after a movie character.
Marley I really like, for a boy or a girl.
I don’t approve of people reproducing, but if you’re going to have a kid, don’t name it Sagan.
Go with Franklin for a boy, or Cassandra for a girl. I’ve spoken.
It sounds too much like “Fagin” (the evil Jew from Oliver Twist) if it’s pronounced correctly, and too much like “Saggin’” if it’s not - neither of which are particularly euphonious or sensual words. Like I said before, it’s not a sexy name at all, and I think the average guy would have a hard time screaming it out during sex, unless he was obsessed with Carl Sagan and getting some kind of fetishistic kick out of it. (Sorry to be so perverted, especially in reference to an unborn child, but I believe in thinking ahead.)
I’ve always associated names ending in “an” with men rather than women. Logan, Harlan, Ryan, et cetera. I don’t think it works for a girl.
For a guy, it smacks of pretension, though as others have said, of the nerd variety rather than the yuppie variety. But to me, that is much less of an issue than the fact that the sound of the name itself is just sort of an ugly sound. It’s not bad as a last name, but I really think it’s a horrible first name. And I think that the constant “are you named for Carl Sagan?” questions are really going to get old after a while.
Having a weird name is a pain in the ass. My parents decided to give me a hyphenated last name, which sounds like British royalty or something, and one of the names in it is misspelled by maybe 50 percent or more of the people who I say it to (when registering for stuff, for instance, or anything else where I need to tell someone my last name and have them write it down,) despite the fact that it’s very prominently featured in the most popular book of all time (the Bible.) I’m constantly needing to spell my surname out for people, and sometimes I even need to explain what a hyphen is to the morons who don’t understand the concept (you’d be surprised at how many of them there are.)
I often wish my parents had just picked one of the damn names. I frequently find myself just giving out one of the names and not both of them as my last name, just so I don’t have to waste time explaining it.
Don’t do this, Dio. Please!!
This reminds me of an unfortunately named child I once met, of obviously hippie parents: her name was Autumn Wind. The poor child resembled autumn wind in no way that I could see; she was overweight, had snarly hair and glasses. But of course she had inner beauty, I am sure.
I am the mother of an unusually named child, whose name is the same as his father’s. He is in fact the fifth generation to have this very unique first name. We also have a very unusual and confusing to pronounce (if you see the spelling) last name [it is a very common sounding word, phonetically]. My husband loves his unique name and I love our last name, too. He says he always enjoyed having a name that was all his own and wasn’t teased at all.
So there’s a vote for a meaningful, unique name.
They’re both dreadful. Sorry. And they don’t go with your daughters’ names at all. I do think that if you really want to use them, they’d be fine as middle names. A Diana Ripley or a Charles Sagan would still have a story to tell.
Hate it.
Too close to the sound of “Satan” and “pagan,” neither of which have positive connotations in mainstream America.
I also don’t think Carl Sagan was famous enough to still be well-known in 20+ years.