The end of verse 9 of Psalm 47 is, "...for God, which is very high exalted, doth defend the earth, as it were with a shield."
It made me think of the barrier around the earth at the moon's border which acted as a sort of quarantine to keep Thulcandra bound within the circle of the earth's "sphere". Of course there the "shield" is more to protect the Field of Arbol from his destructive influence than to protect Earth (although being part of the Great Dance and great plan of God, perhaps it was instrumental in bringing about the Incarnation of Maleldil the Younger, and therefore could be seen as a sort of, or part of, the "protection" of Earth?)
Anyway, I doubt Lewis would have had that specifically in mind when he wrote about that barrier in the Space Trilogy, but it was fun to think about.
trivialities,
--Stanley