By now Chris is approaching the impossibly ancient age of forty and so the time has come for him to get pensioned off. The space exploration series ends with Passage to Pluto (pub 1973). Chris doesn't go on this mission as he's now been made head of the united world space program where he gets to spend more time with the oddly-shaped bloke Gail.
His three chums go off without him to find out why Pluto has such a weird orbit. The answer turns out to be a rogue tenth planet that's on a mission to wreck havoc through the solar system. With Chris's help from back home they send the blighter packing.
I can’t remember much about this one. It read it only the once, as I was then old enough to be moved into the adult section of the library. From then on, I had to make do with science fiction for adults featuring plots that were less plausible and less entertaining.
The Pluto novel didn't quite mark the end for the series. Another six books appeared throughout the rest of the 1970s, usually with Tony Hale as the lead, but he didn't have an exploratory agenda. Instead the stories where either earth-based mystery yarns or rehashs of the First Contact tale in which nice aliens arrive on earth wanting to be nice to us and we try to kill them.
I didn't read the later books, but I gather these additions were weak, probably because Walters forgot that nasty radiation wielding aliens who want to conquer the earth are more entertaining than chummy aliens who want to be liked. So the later books didn't get much of an audience. The cover above suggests that the contents didn't match the greatness of the earlier titles. This helped to ensure that the books were never reprinted and so they didn't get a chance to excite new generations of teens.
The low-key fizzling out of the series is sad, as this was a great juvenile science fiction saga. As I said in my first review, I may have poked fun at some of the antiquated science and attitudes displayed, but I adored these books. These reviews have been my belated thank you to the author for the hours of fun he provided me with his tales of Brits in space.