Interplanetary politics

1. From what I've read, Perelandra is the most popular entry in the Space Triology, although a few connoisseurs (e.g. Rowan Williams) prefer That Hideous Strength. Perlandra's my personal favorite in the Space Trilogy. 

Perelandra was initially Lewis's favorite until he wrote Til We Have Faces. Some literary critics agree that that's his best novel, but that may be because they think they're supposed to admire it and rank it higher than the others. For a couple of reasons, I think it's possible that Lewis himself overrated Til We have Faces. The myth of Cupid and Psyche had captivated him since he read it for the first time in 1916, when he was still a teenager. But there were many false starts. He tried to do a poetic version. He struggled with how to retell the myth for almost 40 years. His own worldview as well as the interpretation evolved over time. Cf. Peter Schakel's chapter (20) in The Cambridge Companion to C. S. Lewis.

In addition, the breakthrough moment came when he talked it over with his future wife:

Jack has started a new fantasy–for grownups. His methods of work amaze me. One night he was lamenting that he couldn't get a good idea for a book. We kicked a few ideas around till one came to life. Then we had another whiskey each and bounced it back and forth between us. The next day, without further planning, he wrote the first chapter! I read it and made some criticisms…he did it over and went on with the next. D. King, ed. Out of My Bone: The Letters of Joy Davidman (Eerdmans 2009), 242.

So he may have associated the book with his wife, which lends it a special poignancy for him.

2. Because for most readers, myself included, That Hideous Strength is a letdown after Perelandra, it raises the question of how or whether it could be better written. Another question is whether he should have ended with Perelandra. What do you do for an encore? If the final installment can't equal, must less surpass, the second entry, would it be wiser for Lewis to quit while he was ahead rather than end with an anticlimactic climax?

In fairness, it's not a bad book. It has some memorable scenes. Strokes of genius. It's prophetic. But it's not all of a piece. 

3. As every commentator explains, That Hideous Strength marks an abrupt stylistic shift from the first two installments, due in part to the newer influence of Charles Williams and spent impact of Tolkien. Tolkien was no longer a creative stimulus for Lewis, in part because Lewis had outgrown Tolkien, who was a smaller talent, and due to irreconcilable artistic visions.    

But over and above that, a change was inevitable. At a scenic level, That Hideous Strength can't compete with the extraterrestrial landscapes and seascapes of Venus and Mars, or their species. That's exacerbated by the fact that Lewis makes no pretense of astronomical accuracy. They exist in his cosmological mythos. That frees him to indulge in surreal flights of fancy unconstrained by what's physically possible. By contrast, That Hideous Strength must have a more realistic setting. After all, his readers are earthlings. 

4. One of the tensions in Out of the Silent Planet is Lewis attacking secular science and its counterpart in hard science fiction. In particular, the materialist notion that outer space is mostly deserted and dead. But as a matter of fact, that's the case. And even though Lewis didn't have the benefit contemporary astronomy, c. 2020, he must have known back in the 1930s that there was no presumption of other life in our solar system. 

Of course, this is soft SF, not meant to be accurate, but then, what does his critique amount to in that respect? It works at the level of his fictional cosmology, but it's not a refutation of the hard SF view of the universe as mostly deserted and dead. 

However, that's offset by the fact that soft SF is never obsolete, whereas the danger of futuristic hard SF is to become dated when overtaken by real events. It works if you have a dualistic view of reality, where there are spiritual agents behind the physical realm, who participate in the physical realm. Interaction between two different domains. Sacramental universe. 

5. Despite the comedown, there is some justification in the third and final installment. All three share the common theme of a primordial angelic rebellion. Against his will, Ransom is drawn into the internal affairs of Mars. Then he is summoned to Venus. The Martian guardian angel visits earth to facilitate the trip. 

In terms of dramatic logic and closure, it makes sense that events come to a head on earth. Having decisively intervened on Venus, it's only fair that heavenly angels lend Ransom a hand for a critical battle with the dark side on earth. Especially since earth is the epicenter of the cosmic rebellion. That rounds out the dramatic arc of a story that began with Mars and proceeded through Venus. 

6. In addition, it gives Lewis a pretext to reinterpret the King Arthur mythos–a theme many English poets and novelists find irresistible. Merlin fits into Lewis's philosophy of myth and magic. However, making Ransom a descendent of King Arthur is ad hoc. King Arthur has no useful role to play in a 20C setting. He's timebound in a way that Merlin is not.