What is the title of C.S. Lewis's science-fiction trilogy about Dr. Elwin Ransom?

C.S. Lewis's science-fiction trilogy about Dr. Elwin Ransom is called The Space Trilogy.

The trilogy consists of Out of the Silent Planet, Perelandra, and That Hideous Strength. The first book, published in 1938, sends philologist Elwin Ransom to Mars, which the novel calls Malacandra. The second places him on Venus, or Perelandra, while the third returns to Earth for a conflict involving science, power, and the misuse of knowledge.

The books are sometimes called the Ransom Trilogy because Ransom is their connecting protagonist. “Space Trilogy” is the commonly used title for the sequence, even though the third novel is set mainly on Earth rather than in outer space. Lewis wrote the books as imaginative fiction with philosophical and theological concerns, rather than as straightforward technical space adventure.