Every 5.4 years, the comet 46P/Wirtanen orbits the Sun, swinging through Earth’s skies on its way. This year, its visit is in December, and we’re going to be able to see it – in fact, it’s visible in the sky now if you have binoculars or a telescope.
Usually, 46P/Wirtanen is too far away for us to see, so this is a real treat – the comet’s closest approach in 70 years.
It’s going to be practically skimming Earth at a distance of just 11.6 million kilometres (7.2 million miles), around 30 times the average distance between Earth and the Moon.
Just shy of being a Christmas Eve treat, the comet’s closest approach to the Sun is set to occur on December 12, and its closest approach to Earth on December 17, so you have plenty of time to get out there and start comet-gazing.
Comet 46P/Wirtanen was discovered in January 1948 by American astronomer Carl Wirtanen, and it’s one of just a handful of comets that’s (sometimes) visible to the naked eye – it gets about as bright as a faint star.
We don’t yet know whether it’s going to be bright enough to be visible by the naked eye this time around. But it will certainly be bright enough to see with binoculars – although keep in mind that, as with all stargazing, you’ll get a better view if you can get away from light pollution.
The comet’s tail points away from Earth, so most of the time that plume won’t be visible; however, it may make an appearance on December 13-14, according to Australian comet-watching website Southern Comets: “There is a possibility of observing a dust tail or trail around December 14 when the Earth crosses the orbital plane of the comet and we see the enhanced dust tail edge-on.”
And, like many other comets, such as Lovejoy and Machholz, it’s going to be glowing a brilliant green. This is because its coma – the particle cloud around the nucleus – contains cyanogen and diatomic carbon, both of which glow green when ionised by sunlight.
Comets move, so you can expect its position is going to change in the night sky. Time and Date has a handy interactive night-sky map on its website so you can trace where it is, whether you’re in the Northern or Southern hemisphere (you’ll need your location turned on in your browser).