Terraforming

"The Snows of Ganymede," IV.

Terraforming Mars had involved, among many other things, diverting moon-sized meteors of ice from the Saturnian rings to fall onto the planet in just the right places.

This reminds us of Larry Niven's Known Space future history where human beings prefer to colonize the Moon and the Belt rather than Mars. The Martian surface is down at the bottom of a gravity well. However, Jack Brennan, who has become the first protector of the human species, diverts an ice asteroid onto a collision course with Mars. Brennan's purpose is not to begin terraforming but to exterminate the native Martians, here called "martians," because they had killed a handful of human explorers. To a protector, that is a capital offence. Protectors are genetically programmed the way Asimov's robots are positronically programmed. If a protector sees you light a cigarette, he will grab it away from you. If he protects only his own relatives, not the entire species, then he will kill other human beings as casually as he exterminated the martians - who fortunately still survive in the Map of Mars on the Ringworld.

OK. That was a sideways tour into another future history series.