In “Slingshot,” a space travel thriller directed by Mikael Håfström, the title refers to a risky flight maneuver involving orbital mechanics. Astronauts journeying a billion and a half miles to one of Saturn’s moons, Titan, will need a gravity assist — the “slingshot” in question — from Jupiter’s orbital velocity in order to make it all the way. Why are they going to Titan? It’s the only other planet that has liquid on its surface, methane that they intend to harvest for clean energy to combat climate change on Earth.