Orbholt

Orbholt is a gravity sandbox. Drag back from any point and let go to fling a moon; a dotted line shows the path it will take around every star and planet on the field. In free play, place up to eight stars and planets, erase them, speed time up and watch the trails draw orbits, slingshots and crashes. Fourteen challenges ask for real orbital feats, from easy to hard: two laps around a star, three moons up at once, a slingshot exit through a gate, a delivery that lands on a planet behind a star, a pass through the gap between two stars, a figure eight, a comet that skims its star, a parking ring, a moon of a moon that circles a planet instead of the star, stardust sparks on a stretched orbit, a way round an asteroid wall, twin dust around two stars, three moons around two suns and an empty sky where you place the planet yourself. Each challenge awards up to three stars for using fewer moons. It plays in the browser with a mouse, touch or keyboard.