Hunting for comets

By Chris Fellows, Serenity Mobile Observatory

See if you can spot a comet with your binoculars.

Did you know that at any given time there are normally several visible comets in the sky? These icy visitors, from the extreme suburbs of our solar system, are usually so far away, so small, and moving so slowly that we scarcely think about them. These frozen denizens of deep space are leftovers from the formation of our solar system and consist mostly of water ice, methane, nitrogen and other ices with a little dust and rock thrown in for good measure.

Comet C/2017 T1 Heinze (dot in center) by Chris Fellows

Most are rather small, ranging anywhere from a few meters up to a few kilometers in diameter, and spend most of their time in stately orbits way out in the Kuiper Belt or even farther out in the Oort Cloud. Occasionally and completely randomly one or more will be perturbed by a close gravitational interaction with another object or even a rare collision with another comet in the area. Once this happens the comet is either banished from the solar system and becomes an interstellar wanderer for all eternity or is doomed to a slow or violent destruction in the inner system. Comets in this latter category are the ones we will concern ourselves with today.    

Once a comet has had its orbit changed and has started its long fall into the inner solar system several things will change. As it gets closer and closer to the sun it will become brighter, it will start to sublimate (like dry ice boiling in the sun), and it will speed up. The comet now has a new orbit around the sun that is highly elliptical, which means it will speed through the inner solar system, gaining speed as it falls in, whip around the sun reaching its maximum velocity at its closest approach or perihelion, and then head back out, slowing as it climbs out of the sun’s gravity well. At this point it can spend a few years or maybe hundreds of years in the outer solar system before it starts its fall back in.

Each time this happens there are a few possibilities that determine its fate. It can shoot straight through, not getting close enough to anything to drastically change its orbit, get heated up and lose mass to sublimation with each trip. Eventually it will lose so much mass that it will totally evaporate or break up and be lost forever. Or it can encounter any of the large bodies in the inner solar system– things like planets, especially the gas giants like Jupiter or the sun itself. A close approach with these objects will either change the orbit yet again or break the comet into smaller pieces, also changing its orbit and life expectancy.

Finally, it can have a direct collision with any of the inner solar system objects and be immediately and spectacularly removed from the picture. This is what happened to Comet Shoemaker–Levy 9 (formally designated D/1993 F2) when, in July of 1992, on a close approach to Jupiter it broke apart, and then in July 1994 the fragments hit the gas giant and was photographed by Hubble and many other ground-based observatories. This observation provided the first direct evidence of an extraterrestrial collision of Solar System objects.

From Wikipedia

Contrary to popular belief, a comet’s “tail” doesn’t stream out behind its direction of travel but points away from the sun and is formed by the solar wind. A typical comet will have two tails: one of dust (this is the one most people can see) and one of ionized gases, normally much dimmer and hard to spot. With the right equipment you can see them both as shown in the picture. Also notice the direction on the comet and the direction of the sunlight in the photo.

OK, so how do you find these things? My favorite resource is The Sky live, where you can find some really neat tools like a hot topics scatter map and a 3D interactive solar system map with inner system comet positions. On this site you can search for comets that you have heard about and get detailed information with sky charts on where to find them in real time. I suggest that you visit their “Night Guide” section, change location to your location using the easy map tool, and make sure your observing date/time is set to when you will be out. This will generate a whole new page, customized to your location and observing times with extensive lists of planets, asteroids and comets that will be visible on that date. From that list you can click on “sky chart” next to the object you are interested in to have it draw you a custom chart showing you where in the sky to find your target.

You can also use several cell phone apps to find comets. I like Celestron’s Sky Portal, but there are many more out there to choose from.

No matter how you find them, comets are always fun to catch and fun to watch move across the background stars while you watch them. Get out to your favorite dark sky site and see if you can catch a comet tonight!

Till next time … 

Clear Skies,
Chris Fellows, Serenity Mobile Observatory

Find Chris on Facebook (or, if you’re lucky, at your campground). (Editor: Check out his amazing photos on his Facebook page!)

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