I do not make such charges lightly. As a university instructor, I know what plagiarism is and what it isn't.

I am not talking about similarities of plots or incidents, as you're right, some of that is to be expected. I'm talking about identical language. It certainly is natural that this book would describe say, what an astronaut experiences during a launch; it is not natural that Baxter would use exactly the same descriptive language as another author.

Here's the email I sent to a friend about it a couple of years ago, with page numbers for comparison. Unfortunately I don't have a copy of Voyage anymore so I can't fill in the actual quotes:

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Quite frankly, there are a ton of scenes in the novel that are almost verbatim lifted from Chaiken, though with the names changed to those of Baxter's fictional characters. Just from a quick scan in the first 200 pages I so, I found: Saturn V launch: Baxter, pp 8-12, Chaiken, pp 86-87; space sickness scene: Baxter p. 57, Chaikin p. 97; Trans-Mars(Lunar) Injection: Baxter p. 112, Chaiken p. 90 ; taking a leak in space: Baxter p 159, Chaiken p. 93; entering lunar orbit: Baxter 208-211, Chaiken 107-109.

Even more commonly, Baxter lifts phrases that figure prominently in Chaiken: "full-up mission", "crown of an astronauts career", "it won't fail because of me". Other scenes are obviously inspired by Chaiken: an Al Sheppard-like astronaut doing field geology training with a Caltech geologist, for example, reads like a condensed version of Chaiken's description of such a trip (and Chaiken's specifically, rather than say, Wilhelm's "To A Rocky Moon"). I notice that some of his reviewers at Amazon mentioning that he draws heavily from some other non-fiction, such as Murray & Cox's "Apollo", which I have not read. This stuff doesn't rise to actual copying but I certainly expect better from a professional novelist; describing a realistic Mars mission doesn't mean you simply take Apollo histories and apply a search-and-replace, not to make an interesting book at least.

I've read a lot of space program histories, and I've read a lot of space program fiction. Baxter is not simply reciting stories that everyone knows in the words that everyone uses, and I've never had this sense about a novel before. Some of those passages are barely paraphrased copies; I've flunked students for less.
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Why wasn't Baxter sued? Good question. Some well-known historians have recently been caught on plagiarism that had gone undetected for years, so who knows. Perhaps the amount of lifted material just wasn't large enough to bother; I am talking a paragraph here and there, not wholesale chapters.