The dual tail of comet NEOWISE

I’m sure you heard about NEOWISE, the comet that dazzled viewers in the Northern Hemisphere. I received several amazing photos from my friends in the US.

Sadly, by the time it got to the Southern Hemisphere it had dimmed off considerably. Yes, it was visible from Melbourne – but how visible is “visible”...?

What's worse, Melbourne was in lockdown for the Coronavirus, so people like me couldn't get out from under the dome of light pollution. The whole thing was a bit of a bust, really.

Comets are unpredictable. They regularly brighten when far distant, something that gets astronomers jumping up and down. But just as often they fizzle out – or even break up – before getting close. Once (well, in my memory at least) a comet even crashed into Jupiter. Let’s call that a bad day.

This is NASA's daily pick from back in July.

Comet C/2020 F3 (NEOWISE)

What's going on here? Why does the comet have two tails?

Nearly all comets develop a twin tail. It's because they leave behind two types of particles as they are warmed by the sun.

The broad white tail you see on the right is mostly made up of plain dust, blown away from the nucleus by the solar wind. Because dust particles are relatively heavy, they start slowly, gently accelerating as they are blown by the solar wind.

Incidentally, in the photo, the comet is travelling from right to left. The curve in the dust tail is a result of this motion. The ripples are a matter of debate, but might be due to the nucleus spinning.

But the comet is also shedding ions, individual molecules that have directly collided with a particle from the sun. These are much lighter particles than dust, so when they enter the solar wind, they match its speed almost instantly. As a result, the ion tail is much straighter.

Think of it as a blue motor cycle and a white semi trailer waiting at the traffic lights. On green, the light motor cycle speeds away, leaving the heavy semi trailer lumbering along behind.