Asimov had a short story once in which spacecraft propulsion was dependent on water as fuel, and a colony on Mars (in response to an anti-space-exploration political movement on Earth) started harvesting huge chunks of ice from the outer solar system, wrangling them into Martian orbit with webs of high-strength cabling (and using some of the ice in the process as fuel to move it all around) and letting them crash down in uninhabited areas (where, under Mars' lower gravity and frigid temperatures, they'd hold together enough to be mined for water).