COMETS FADE NEAR SATURN'S ORBIT

It's not just about the Sun's heat

- Elizabeth Howell

When comets first appear near Earth, their bright tails of ionised gas stun observers, but on every return they become dimmer. Astronomers believed that these objects become dimmer on return because they released too much ice and gas during earlier visits to the inner Solar System. The comets melted and shrank because of the Sun's warmth, so when they return there's less material left to release, and therefore a fainter coma. But a new study by scientists from the University of Oklahoma found that even comets that only skirt the inner Solar System and remain beyond the orbit of Saturn fade over time. That makes no sense as in those far reaches of the Solar System, the Sun's light is so weak that it shouldn't be able to melt a comet's ice.