system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do space vehicles need so much heat shielding for reentry but not so much for launch?
gpt
When going up, they are accelerating at a fairly slow rate, they only get to very significant speed once outside the atmosphere. When they return however, they are immediately coming in very fast, and the slowing by air resistance is very significant, and causes heat.