Why do comets never seem to run out of stuff? Like Halley’s Comet, how does it just keep coming back? Turns out, comets are massive. Some are tens of kilometres across, so there's a lot of material to vaporise each time they pass the Sun. But they do run out eventually! Roughly 6% of near-Earth asteroids might actually be extinct comets. These are comets that lost their tail and fizzled out. Only 10% of long-period comets survive more than 50 orbits. That’s about 10,000 years of cosmic flybys...