There's a link between comets, which we see only occasionally, and shooting stars or meteors, which can often be seen in our skies.
Press play to learn about Comets
Transcript of MP3 recording
Despite their sometimes spectacular appearance, comets are all show. The tail of a really bright comet can stretch halfway across the inner Solar System, when the Sun's heat can get to it and turn the ice into gas. But out here, comets are in deep freeze and hardly give off any gas at all.
Right now, Halley's Comet, the most famous comet of all, is out beyond the orbit of Neptune, and reaches its farthest point from the Sun in 2025. It's actually on the opposite side of the Sun from Neptune, by the way, so there's no risk of a collision even in the future. Then it begins its slow progress inwards again until it comes quite close to Earth in July 2061, and should be a spectacular sight.
But you don't need to wait until then to see a bit of Halley's Comet. Spread around a wide band surrounding its orbit are tiny particles of dust that have been released from the nucleus of the comet over time. The Earth ploughs through these particles around every 21 October, so they enter our atmosphere at speeds of around 65 km/s, which causes them to burn up. We see this as a shooting star, which looks just as if a star has fallen from its perch in the sky and dropped to Earth.
There's no danger at all from shooting stars, as they're so small that they never actually reach the ground. Astronomers call them meteors, and there are other similar streams of particles from other comets that give rise to meteor showers. Starting in January, there are the Quadrantid meteors around the 3rd or 4th January. Then there's a gap until August, when we get the Perseids around the 12th or 13th. These are the most popular because it's much warmer to sit out at night in August than in January. Then there are the Orionids from Halley's Comet in October, and the Geminids around 13th December. The actual dates vary slightly from year to year. And on any clear night you might see a shooting star that's unconnected with any shower.
Very occasionally there is a sudden burst of activity from a meteor shower. There's a fairly minor shower called the Leonids, which occur around 17th November each year. Way back in 1833, these gave rise to a really spectacular display, with literally thousands of meteors pouring down from the direction of the constellation of Leo, the Lion, which is why they're called Leonids. There was another big display in 1866, and another in 1966. There's a dense clump of dust that comes around about every 33 years, and in between the numbers are quite low. Back around 1999 to 2002 there were high rates, though nothing like as high as in the past. The next burst of activity will be in the 2030s - something to look forward to.
Photo: Comet Ikeya-Seki, observed in 1966 from the USA.
Credit: Roger Lynds/AURA/NOAO/NSF