[trainingaccount]

Actually for Apollo 17 it was found to be easier to train a geologist to be an astronaut than the other way around.

[rory096]

The Apollo astronauts were also trained quite extensively in geology. Highly recommend the Apollo 12 episode of From the Earth to the Moon for a great look at that.

[psergeant]

I guess as long as a high enough percentage of the crew are flight people, you can teach someone else to not get in the way too much?

[manicdee]

Or it turns out that 20 years if field experience is harder to teach than 5 years experience in wearing pressure suits and zero/low gravity.