What's a Comet? A Short Guide on the Most Exciting Objects to Capture in the Sky

Antoine & Dalia Grelin
Oct 15
3 min read

Comets are, in my opinion, some of the most exciting objects to capture. They are very unpredictable, and their shapes are often unique and change every single night!

Deep sky objects (like galaxies, nebulae, and clusters) stay in the same place year after year, whereas comets pass by and are gone within weeks.

What are comets? — Comet depicted in the 16th century The Book of Miracles

What is a Comet?

A comet is an icy body made of rock, dust, and frozen gases like water, carbon dioxide, and methane. Most of them spend billions of years in the Oort Cloud or the Kuiper Belt, extremely far from the Sun, where it’s cold enough for their materials to remain frozen.

Sometimes, a comet's orbit will bring it close to the Sun. As it approaches, the heat will cause the ice to turn from solid to gas. As the gas is released along with dust, a bright fuzzy atmosphere will form around the nucleus of the comet, as well as a tail.

The Different Parts of a Comet

Comets normally have four main components you can capture in your images:

The Nucleus

This is the solid core of the comet. It is, in most cases, only a few miles/kilometers long. It's way too small to see well in images, even if you use a large telescope.

The Coma

This is what you see when you look at the core. It is a bright (most of the time green) and hazy fluff of gas and dust that surrounds the nucleus.

The Dust Tail

This is a tail full of dust and ice that follows the comet and looks white or yellowish. Sometimes, it can be seen curved as it follows the comet's orbit.

The Ion Tail

This is a second tail completely different than the Dust tail. This tail is made of gas that glows bluish and is always pointed directly away from the Sun due to the solar wind.

How a comet changes each day — Credit: Shengyu Li and Shaining

Both of the tails can change shape or brightness from one night to another, which is one of the reasons why comets are so exciting to capture... and recapture again and again while they're visible! The image above shows how the Ion tail of Comet 12P/Pons–Brooks looked from March 6 to March 14 2024!

Get the Comet Imaging and Processing Online Course!

Learn how to capture and process comet in the Comet PixInsight Processing Guide! This is an easy-to-follow workflow that not only includes the full step-by-step processing guide, but also lessons on understanding and imaging comets. This course is relevant for both color and monochrome camera setups! Also, it is available with closed captions in four languages!

A Comet's Orbit

Comets follow elliptical orbits around the Sun, meaning they spend most of their time in the outer Solar System, moving very slowly. As they approach the Sun (the perihelion), they speed up very fast and brighten. This is when they become visible to us here on Earth, for a few weeks or months before fading away.

Some comets (known as short-period comets) return on a regular basis, like Halley's Comet (1P/Halley), which visits every 76 years. It was last seen in 1986, and is expected to return in mid 2061.

Others, called long-period comets, take thousands of years to return, if they ever do at all.