Chasing Comets

There are two (hopefully) remarkable comets coming up in 2024, and I thought a nice, concise blog on chasing comets would be just what the doctor ordered. 

 

Let’s start off with why you don’t have tons of heads up like other celestial events; comets are unpredictable. It’s very difficult to predict how bright a comet will actually be while it’s flying by in the distant edges of our solar system. Oftentimes comets don’t make it past their close approach to the Sun and end up burning up completely instead of shining spectacularly in our night sky. Between solar winds and gravitational systems pulling them apart, comets will eventually break up and disperse over time. Most comets are too faint to see with the naked eye and require specialized equipment to see them. Every now and then we get lucky with a comet bright enough to be seen with the unaided eye.

The makeup of a comet has four main parts: the nucleus, the coma, the plasma/ion tail, and the dust tail.

The nucleus is the solid body of the comet primarily made of volatile ice and silicate and dust. Comets are thought to be remnants of the creation of our solar system and that they come from 2 places: the Oort Cloud or the Kuiper Belt. Located roughly a light year from the Sun, the Oort Cloud is where the long-period comets come from. On the other hand, the Kuiper Belt is beyond Neptune and is the main origin point of short-period comets. Long-period comets' orbital periods span several million years, whereas the short-period ones are more around 200 years.

The coma is a comet’s local atmosphere, composed mainly of sublimated gas and dust. A coma starts as a teardrop shape that turns into the tail when it’s exposed to solar winds as it approaches the Sun. Though it is harder to detect and predict comets when they are far away from the Sun, as they get closer, they inevitably will show tell-tale signs that they are, in fact, a comet. Between the heat and radiation of the Sun, the ice from the comet sublimates (turns directly into gas) and increases the size of the tail of the comet which can be thousands of miles long.

Comets often have two tails pointing in different directions. One being the dust tail which is made up of the dust breaking up off of the comet. Dust tails are often broader and curved due to gravitational pull and radiation pressure from the Sun, putting the dust particles on a different orbital trajectory than the comet. The plasma/ion tail is the other and that is made up of the gasses coming from the coma. Commonly, the ion tail is the one that tends to go in a straight line away from the Sun.

Comets are considered “Great” comets based on their apparent brightness, and a comet's brightness is measured in magnitude. Most of the great comets are visible to the naked eye and are at a magnitude of 1 or below (lower is brighter). Another thing that makes them stand out as great is that they become well-known outside the astronomical community.

 

Object

Magnitude

The Sun

-27

The Full Moon

-13

Venus

-3.7 to -4.5

International Space Station

0 to -3.9

Jupiter

-2 to -2.9

Sirius (The brightest star)

-1.4

Arcturus, Vega

0

Polaris (The Pole Star)

2

Andromeda Galaxy (closest galaxy)

3.4

Uranus (Faintest naked eye planet)

5.9

Faintest magnitude visible to the naked eye

6.5

Faintest stars visible through 10×50 binoculars

9

Faintest stars visible through a small telescope (3″)

11

Faintest stars visible through a large telescope (12″)

15

Pluto

16

Let’s cover the apparent magnitude scale (brightness of celestial objects). The objects in the night sky have their brightness measured based on the view from earth. We measure the brightness of all astronomical objects such as the stars, planets, moon, galaxies, and even our Sun. On this scale the lower the number, the brighter the object is. For example, the Sun is -27 mag. The integer decrease in magnitude represents an increase in brightness of roughly 2.5 times. So with the Sun being a magnitude of -27 and our full moon being a magnitude of -13, the integer difference is 14. So, we would do 2.512^14 = 398,359 (roughly) meaning that the sun is 398,359 times the apparent brightness than the full moon. The magnitude scale is logarithmic and originally only had 6 degrees of brightness when the Greek astronomer Hipparchus classified the system with 1st magnitude stars being the brightness and 6th being the faintest he could see in the sky. The system was based with the star Vega at a magnitude of 0. Obviously, over the years we have developed more precise ways of measuring apparent brightness (like photoelectric photometers) than our eyes and the table has been expanded with our local celestial objects, along with many other fainter objects that measure out to the 30th degree. Keep in mind that apparent magnitude is not absolute magnitude. Apparent magnitude is the brightness observed from Earth regardless of distance, where absolute magnitude is what the magnitude would be at a standard distance of 10 parsecs. 

Comets have two different naming systems: the original and the current IAU system that the International Astronomical Union put into place starting in 1994. Originally, it was common that a comet was named after their discoverer or the scientist who discovered that the comet’s orbit was periodic. Off of the new naming system, the name of a comet actually gives you a lot of information about it. If the prefix of the comet’s name has a “C/” at the beginning it denotes that it is a long-period comet, while a “P/” means it is periodic. A prefix of “X/” indicates a comet with no reliable orbit, and a “D/” is given to comets that have disappeared, died, or have been lost. The number following the prefix is the year the comet was discovered followed by a letter (A-Y and not using I to avoid confusion with interstellar objects and the number 1) representing the half-month of the discovery and a number indicating the order of discovery. Let’s go over an example below.

My first comet that I was able to see and chase around was Comet Neowise. Its identification name was C/2020 F3 (NEOWISE). As we break this down we can tell the C/ means it’s a long-period comet (has an orbit around the Sun of over 200 years). Next is the discovery time being 2020 and F3. The comet was discovered in 2020, and the F is given because it was found in the second half of March (best to use a table for letters to half months), and the 3 because it was the third discovery in that part of March. At the end of the designation, you will find “NEOWISE.” It was given that name due to being discovered by the NEOWISE mission of the wide-field infrared survey explorer space telescope (WISE).

Photographing comets can prove to be a little challenging as they often are quite close to the Sun, so your window of time to photograph them is limited. Your window is either early in the morning before sunrise or just after sunset, depending on where the comet is compared to its perihelion. Once a comet has been announced, like Neowise, you want to figure out where and when you will see it in the sky. Neowise was found in the northern sky and was close to the horizon just before sunrise before it hit perihelion and just after sunset once it passed. This meant I wanted to find locations looking north in the sky, getting there before sunset so I could be set up for the short amount of time before Neowise was below the horizon.

 

Per usual, to photograph any night sky object you want a sturdy tripod, a remote shutter release, a camera up to the job, and (the hardest thing to get) good weather. The apps I use to help find where the comet will be in the sky are PlanIt Pro, Stellarium+ and PhotoPills. Using these apps in conjunction, I am able to predict where and when the comet will be so I can make sure my location of choice will work. I also research the hell out of a comet before I go hunting, and I find a good chunk of that information on EarthSky.org. You know a comet is going to be really good to chase when it is hitting magnitude 2 and less and the weather is looking good. Once you found your spot and you have confirmed that the comet will be above your horizon, you gotta get set up for shooting. It’s good to know that comets move at a different speed than the stars in our night sky. This is because the comet is orbiting our Sun and has a very elliptical orbit. Most comets, if they do end up being visible to the naked eye, will only be that way for a few weeks or months before they head back out to the far reaches of our solar system.

 

Let’s cover the settings I suggest using for catching comets. Start off with your shutter speed around 30 sec, ISO 1600, and f2.8. From there you should be looking to see that your comet shot is in focus, so you will be looking mostly at the nucleus to see a sharp edge or if the dust tail has definition. It can be quite easy to blow out the highlights on the coma and nucleus of the comet, so you might need to experiment with your settings a bit depending on when the comet is out. Neowise was first visible after sunset from blue hour to astronomical twilight. This meant the amount of light in the sky was constantly changing so I had to keep adjusting my setting as it got darker. To pull out definition in the ion tail you may need to up your ISO as you can’t really increase your shutter speed without blurring the comet. While chasing Neowise all over the PNW, I had a chance to use a few different lenses to photograph it. I used a Nikon 50mm prime, 24-70mm 2.8, and 70-200mm 2.8. There really isn’t a lens that does better than another when it comes to this. You just want to make sure you have a low light lens like a 2.8 and pick your focal length based on what you want in the image. As you can see, when I use the longer focal lengths like 200, the comet is the whole focus of the shot. Verses, when I was using my 24-70 @24, you get a nice balance of comet detail balanced with the landscape. Particularly when I was photographing Neowise, I was trying to catch the comet with the landscape and see the full ion tail which is why I often went for my 24-70mm.

 

Fun fact: Meteor showers actually steam from either an asteroid or comet.