I have read extensively concerning the rules governing that vs. which and felt I understood them until the following example. I am operating under the assumption that "that" is used for restrictive clauses and "which" is used for non-restrictive clauses. Despite this I am torn on the following example:
“Astronaut suits contain dozens of materials THAT separate an astronaut from space.”
It seems to me the phrase "separate an astronaut from space" is important to the meaning of the sentence, and therefore "that" should be used. Is this correct? How does one come to the correct conclusion in a short sentence like this?