10/15/2024
last night, when I posted my photo of comet Comet Tsuchinshan-ATLAS, I noted the presence of the "anti-tail" -- a faint spike of light appearing to point in the opposite direction of the comet's main dust tail. The first photo I included in this post is of ATLAS last night, and if you look carefully you can see a faint spike of light facing away from the main tail.
I also promised that I'd come back and post an explanation, once I felt I understood the phenomenon well enough to explain it correctly.
So anyway... here goes nuthin', as they say.
First of all, let's start with "why do comets have tails in the first place?"
A comet is just a big ball of ice and rock floating through space, that has been captured by the sun's gravitational field. They generally have very long elliptical orbits -- in the case of ATLAS, the orbit takes 80,000 years to complete. ATLAS is believed to have originated from the Oort Cloud, a band of icy rock chunks surrounding the sun, and gravitationally bound to it -- but nearly 100x as far away from the sun as Pluto.
As the comet approaches the sun, the heat from the sun's light begins to melt some of the ices (remember, we're not just talking about water ice, but also frozen methane, ammonia, CO2, and other gasses, some of which melt at very low temperature). The resulting gases and the dust that is released as well, all erupt from the comet initially in random directions, leaving a dusty path along the comet's orbital plane (think of the comet's orbit as a record album -- maybe an old vinyl copy of "Dark Side of the Moon"-- with the sun in the center and the comet orbiting along the outside edge of the album. The album itself describes the orbital plane.)
Note that I said the dust erupts from the comet's head (aka "the coma") "initially" -- upon leaving the comet's surface, the pressure exerted by the solar wind (streams of particles that come from the sun and give us aurora) pushes most of this dust into a dust tail, that projects in the direction away from the sun. This tail can be millions of miles long, and the dust and ice particles in it reflect the sunlight, making it visible to us here on earth. This tail often has a curve to it, though not always. It is the tail we see with our naked eyes when we go out and view ATLAS in the evening sky.
A second tail, the so called "ion tail" or "gas tail" also projects out from the comet, but in this case it doesn't leave particles along it's path, but the gas molecules/atoms are energized by the sun's radiation and glow -- this tail always points directly away from the sun, so that a line drawn along the ion tail through the head of the comet will point directly at the sun. This is seen in the second photo I posted, of comet neowise in 2020 -- the ion tail is the fainter tail.
So what about the "anti-tail"? If the other comet tails are caused by solar wind pushing material AWAY from the sun, how does a tail form of particles pointing TOWARDS the sun??
Turns out the anti-tail is a bit of an optical illusion.
See, as the comet travels along it's orbit it leaves behind a trail of of dust particles in it's orbit (it's these cometary dust trails that causes annual meteor showers when the earth just happens to pass through one).
Normally this dust is too diffuse and faint to see, but if the earth just happens to pass through the comet's orbital plane (i.e., we're now looking at the comet's orbit "edge-on" like looking at that Pink Floyd album edge-on) we are looking through a lot of the dust, and because there's more of it along our line of sight, the faint bits of reflected light from this dust appear brighter than if seen from other angles. This illuminated dust is what appears as the "anti-tail"
Not every comet will present an anti-tail, and even those that do will only show it for the brief period that the earth is traveling through the comet's orbital plane. So we are fortunate in this case that we are passing through the comet's orbital plane at the same time it is close enough to be visible in our sky.
Anyway, it's a cool phenomenon, and I've been seeing it in a lot of people's comet images so I figured I'd try explaining it as a way of better understanding it myself.
clear as mud, right?
