Astronomy
Explanation
The Joke
The comic is split into two halves. The top half, labeled "Stated Reason," shows a scientist giving an inspirational answer to why she studies space propulsion: "Humans have always been explorers. We once looked over mountains, then seas, then clouds. One day we will look over the walls of Heaven." The bottom half, labeled "Actual Reason," shows a rocket blasting off from Earth with someone inside yelling "Fk you, Earth! By God, fk you!"
The Humor
The joke contrasts the noble, poetic justification scientists give publicly for space exploration with a much more visceral and petty "real" motivation -- simply wanting to escape Earth and tell it off. The stated reason is a beautiful, humanistic narrative about exploration and transcendence. The actual reason is pure misanthropic rage.
The comedy comes from the recognition that both motivations might genuinely coexist. Many space enthusiasts are driven by both a sincere love of exploration and a deep frustration with earthly problems. The comic suggests that behind every soaring speech about humanity's destiny in the stars, there might be someone who just really, really wants to leave this planet.
It also plays on the trope of scientists presenting sanitized, grant-proposal-friendly versions of their motivations when the real driving emotion is something far less dignified.