You're asking the wrong questions. The real important statement is "billions of dollars spent on rocks." This is not true. We did not put billions of dollars in a box, mail it to Mars, and wait for someone to mail us back a stack of rock pictures.

We spent those billions of dollars here on Earth, on engineers, designs, new technologies, universities, technological industries, materials, aerospace companies, and many other places. We got a lot more than pictures of rocks, we got advancements in many fields and we got to maintain facilities and teams of brilliant people who may be called on some day to solve something a lot more important than rock compositions.

Granted, the private sector accomplishes this as well, or better (witness the microchip revolution), and I am philosophically reluctant to support government spending on the sciences, (even as I am currently eating by courtesy of a NASA contract!) but the billions could not be honestly be considered a waste even by the most raving libertarian.

At some point the private sector will find a reason to go out there in a big way. Then watch and see what happens!