Heads up: a comet potentially visible to the naked eye is on its way past Earth.
Comet 46P/Wirtanen, first discovered in 1948, makes its way around the sun every 5.4 years. And this time it's making its closest approach yet to Earth.
It's not a particularly big comet — about 1.2 kilometres in diameter — but it has some interesting qualities that could make for a good chance to see it.
For one, 46P/Wirtanen is approaching perihelion, the closest an object is to the sun in its orbit. For comets, this means that solar radiation increases sublimation of its icy and dusty core, a process where a solid (ice, in this case) turns directly to a gas, skipping the liquid phase. And it's that process that creates the tails we associate with comets.
This process will occur just four days before the comet makes its closest pass by Earth, called a perigee.
As well, 46P/Wirtanen belongs to small group of comets called "hyperactive comets." These objects seem to emit more vapour than is typical, particularly based on the size of their nucleus, the solid central part of a comet.
Right now, 46P/Wirtanen can be found in the night sky in the constellation Cetus, the whale. It's not exactly in an ideal viewing position: it's somewhat low on the horizon and only visible during the early morning. It's also too dim, at a magnitude of about 10, which is pushing even the limits of binocular viewing.
The apparent brightness of celestial objects is on a scale that goes from the very brightest — the sun — to the dimmest. And it doesn't go in the direction you'd think: the lower the number (negative values) the brighter the object.
But 46P/Wirtanen is going to slowly brighten over the next two months. It's also going to get higher in the sky away from the horizon where it's more difficult to see. By mid-November, it should be around magnitude 6, bright enough to spot from dark skies with the naked eye. However, it's probably best to use a pair of binoculars for an advantage.
On Dec. 12, the comet will be at its closest to the sun; on Dec. 16, it will be closest to Earth at a distance of 11.5 million kilometres. Astronomers are anticipating that by then — as it will only have been a few days after its closest approach to the sun — 46P/Wirtanen will brighten to naked-eye visibility.
Current predictions are that it will be near a magnitude of three to six. If it's a magnitude three, it should be visible even in light-polluted cities.
If it's closer to magnitude six, you'll likely need to go to a dark-sky location or use binoculars.
46P/Wirtanen isn't the lone comet in the night sky at the moment (though it's faint). People have been enjoying photographing comet 21P/Giacobini-Zinner with some beautiful results.
So, mark your calendars, dust off those binoculars and get ready for an almost-Christmas comet. Comets are known to be somewhat unpredictable, so surprises may await.