“Going to the moon is not a matter of physics but of economics.”

Source: The Glass Menagerie

His listeners nodded unhappily.
“So everything is expanding. But it can’t happen in contradiction to the law of conservation of matter-energy. No matter how efficient your throughput is, you can’t get an output larger than the input.”
Source: Green Mars (1993), Chapter 2, “The Ambassador” (pp. 76-77)

Source: River of Gods (2006), Ch. 9 (p. 93).

Source: The Boy Crisis (2018), pp. 106

Source: Dear John

Source: The Kite Runner

Sam, in Bad Day on the Moon
Sam and Max comics

1963, UN speech
Context: Finally, in a field where the United States and the Soviet Union have a special capacity — in the field of space — there is room for new cooperation, for further joint efforts in the regulation and exploration of space. I include among these possibilities a joint expedition to the moon. Space offers no problems of sovereignty; by resolution of this Assembly, the members of the United Nations have foresworn any claim to territorial rights in outer space or on celestial bodies, and declared that international law and the United Nations Charter will apply. Why, therefore, should man's first flight to the moon be a matter of national competition? Why should the United States and the Soviet Union, in preparing for such expeditions, become involved in immense duplications of research, construction, and expenditure? Surely we should explore whether the scientists and astronauts of our two countries — indeed of all the world — cannot work together in the conquest of space, sending someday in this decade to the moon not the representatives of a single nation, but the representatives of all of our countries.

“Coventry”, pp. 500-501; originally published in Astounding Science Fiction (July 1940)
Short fiction, The Past Through Tomorrow (1967)