Why do comets have tails?
They might look like a flame coming out the back of a comet, but their tails are actually a trail of debris that’s been flung off the surface of comet when it’s heated up as it gets closer to the Sun.
They’re *not* some kind of smoke nor do they thrust the comet forward like the exhaust from a rocket engine.
Do comet whiz through the sky?
If you see a bright object that suddenly appears and leaves a trail behind it before disappearing, then you’ve seen a meteor. This is a piece of dust, a small or rock or possibly even a bit of human made space junk that’s hit the Earth’s atmosphere and burned up. These are often called shooting stars.
Comets don’t appear to move and stay in the same spot – tail and all. They do move through the sky, but so slowly you’re not going to notice their changes. They appear stationary, much like the Moon does in the night sky. They’re also vastly larger and much, much further away than the meteors that cause shooting stars. If a comet were to approach or even hit the Earth, it would be a bad day in the office!
How can a comet have more than one tail?
You might have heard comets described as “dirty snowballs”. It’s not a bad description. They’re a collection of dust, rocks, water ice and frozen gases that are leftovers from the formation of the Solar System and held together by the comet’s weak gravity. (There’s a more detailed BINTEL article about comets here.)
As they head towards the inner Solar System and heat up, the water ices melt and some frozen materials will turn from solids straight into gases. These various materials are flung into space and form the comet’s tail as they’re pushed away from the comet by emissions from the Sun.
The reason comets often have two tails is the way energy from the Sun interacts with the material being thrown off the comet.
Gases that escape into space are ionised by UV Solar radiation. (Ionisation happens when the electrons are stripped away from a molecule). The Solar Wind – which is a collection of charged particles being emitted from the Sun – push these ionised gases out away from the comet. The straight tail you see trailing behind a comet is called the “Ion Tail” because it’s made up of ionised gases. (Bear in mind that while these gases reflect light from the Sun and appear to shine brightly, there’s not a lot of gas in the vast area the tails occupy. It would probably be regarded as vacuum here on Earth.) Ion tales appear blueish or white when photographed from Earth.
The “Dust Tail” is made up of larger particles, many around the size of those found in cigarette smoke. While these particles are tiny and spread very sparsely, they’re enormous compared to the size of the ionised gas molecules. The charged particles of the Solar Wind has much less effect on them. Rather they’re pushed by away from the nucleus of the comet by the pressure of light from the Sun. (A tiny number of comets have also show a “sodium tail” but we won’t go into that here.)
Comet Hale-Bopp in 1997 showing both dust and ion tails.
What? Sunlight has pressure?
Yes! It’s not much, but light falling on any object places a minute, but constant pressure. This is called “radiation pressure”. The photons in light have a small amount of momentum and this pushes on the surface they falls on. In the case of a dust tail, light from the Sun falls on the tiny particles that have by ejected from the comet.
The dust tail tends to trail away behind the comet tracing a curved path slightly away from the direct line of sight, unlike the straight line that the ion tail traces.
What’s an anti-tail?
The comet currently in the sky, Comet c/2022 E3 (ZTF) has also shown an “anti-tail”. This is a trail of matter that appears to be in front of the comet. Seeing comet tails are matter thrown off a comet and pushed back by energy from the Sun, how does it end up looking like this?
A comet ant-tail – Image credit Miguel Claro
A comet ant-tail is actually an optical illusion. This happens when the comet is close to the Sun and moving rapidly. What we see is the part of the tail that has already been left behind before as the comet moves and is lit up again by the Sun. It’s not matter being sent from the comet towards the Sun
Comet tails are complex!
Comets are unpredictable to put it mildly. They change night to night as they swing by the inner Solar System. Their brightness can change rapidly. Parts of the tail can be become disconnected from the body of the comet. Their shape, length and even number of their tails can vary. It’s also common for them to break up entirely! If you’ve seen a comet one night, it’s well worth going to a few nights later to observe it again to check out how it’s changed.
Comets are a fascinating part of the Universe. Please feel free to add comments and more information if you’d like.