The science of celestial visitors
What are comets, why are they bright and why do some visit us more regularly than others?
Comets are essentially icy leftovers from the birth of the Solar System, around five billion years ago. They orbit the Sun like the planets do, held captive by its gravity. However, unlike planetary orbits, which are roughly circular, comet orbits are much more eccentric, long loops rather than circles. This means comets spend most of their time far, far away from the Sun, out in the dark depths of space, and are only illuminated and warmed by the Sun for brief periods when they venture close and swing around it, before returning to the darkness. Some comets, known as short-period comets, have orbits that carry them around the Sun every few years, while others, known as long-period comets, take many thousands of years to go around it once. As a comet nears the Sun, it ‘wakes up’, releasing gas and dust from beneath its icy crust. This material forms a misty cloud or ‘coma’ around the nucleus. Some – but not all – comets then go on to form glowing tails, as gas and dust is pushed away from them, trailing behind. How bright and impressive a comet becomes in our sky depends on how close it gets to the Sun and to Earth, how much material is released from it, how long a tail it grows and the angle we see that tail from. Each comet is different, and that’s what makes them so fascinating.