Adult book from the 1960s or 1970s, in English. A group is sent out on a mission to something in space (don’t remember what), but at some point one of their colleagues can’t see them and appears to be looking for them. He also walks right threw them, like a ghost. Except eventually they figure out that (1) they have been uploaded into the ship’s computer — they’re the “ghosts,” so to speak, but there was a problem with their colleague, and (2) they all had some incurable condition that should have killed them as kids, and they were specifically saved to go on this mission.
I think there might have been something about lacing minerals in their food to make the computer scan work, but I may be misremembering that. I think it might have had the word “Mission” in the title. I thought the author was Douglas R. Mason, but I can’t seem to match it.