I have learned to use the word impossible with the greatest caution.

The remark reflects von Braun’s engineer’s skepticism toward absolute limits. In rocketry and spaceflight, many “impossible” feats (reaching orbit, landing on the Moon, returning safely) moved from speculation to practice through incremental advances, testing, and institutional commitment. The quote cautions against treating current technical constraints, incomplete knowledge, or prevailing opinion as permanent barriers. It also implies a disciplined optimism: not that anything is easy, but that declaring something impossible is often a failure of imagination, resources, or method rather than a proven law of nature. Used broadly, it encourages ambition tempered by rigor—questioning assumptions while respecting evidence.