Comets are the most unpredictable and fascinating objects to observe. They seem to appear out of nowhere totally unannounced. Most of the time their brightness is tough to predict. Sometimes they become unexpectedly spectacular like Comet Holmes in October 2007, or they totally flop like Comet Kohoutek in January 1974. It is this unpredictability, variability, and hence the surprise factor that makes comets a favorite for many to observe.
Comets are irregular lumps of rock and ice coated with fine dust particles and frozen gases usually no more than a few miles across. Many comets travel in elliptical orbits that bring them closer to the Sun than Mercury and farther than Pluto. Out beyond Pluto the surface temperature of a comet is not much warmer than absolute zero or about -459ºF. Comets develop a cloud of dust and gas called a coma that surrounds the nucleus as it nears the Sun and warms up causing the frozen gases to sublimate. The closer a comet gets to the Sun, the larger and brighter the coma becomes. Comets develop two types of tails: a dust tail and an ion tail. The dust particles are liberated from the surface of a comet as it heats up. Internal pressure builds and fractures the frail crust, which releases jets of gas and dust. The dust particles have more mass than the ionized gas, therefore they appear thicker and yellower than the bluish-white ion tail. The solar wind pushes the tails away from the comet’s nucleus. The ion tail has very little mass, therefore it appears straight, but the dust tail can curve from the rotation of the nucleus since the particles are heavier and travel much more slowly away from it. Comet Hale-Bopp is a classic example; it had a dramatic spiral-like pattern in its coma and a distinctly curving, yellowish dust tail separated by a straight bluish-white ion tail. If the comet is active with numerous jets erupting on its surface, it usually means lots of particles are escaping to create a large, bright coma and a very long, bright tail. It is the tail that gives each comet its own personality.
It is always exciting to hear about a bright comet coming as they are so unpredictable. How will it evolve as it approaches the Sun? Observing comets can be a lot of fun because nothing more than the eyes are needed to see them. A simple pair of binoculars improves the view and makes it easier to see even fainter comets. Make note of which constellation the comet resides because most comets do not follow the Moon and planets along the ecliptic, and thus can be seen in unusual constellations in odd areas of the sky, even near Polaris. Their orbits can make them move in an opposite direction in the sky from night to night compared to the planets. Make note of the size and brightness of the comet along with any variations in brightness in the coma and tail. Are both the dust and ion tails visible? Is the coma thick or diffuse? Is the coma thicker near the center or is it nothing more than a “fuzzy ball”? Are there any striations in the coma and tail? Make note of any color since brighter comets can be colorful such as the yellow-green Comet Holmes and greenish Comet Lulin. If the comet is being sketched, be sure to notate the date, time, and location of the comet along with any other interesting observations.
Comets are diffuse objects unlike the pinpoints of stars or the discs of planets, therefore light pollution from any source, including the Moon, will hamper viewing. Dark skies are mandatory and high magnification will help as it darkens the background a little and shows better detail in the coma. Binoculars provide a decent magnification while providing a wide field of view. Sweep along the tail and see how much further along it can be seen compared to the eyes. A telescope is useful if the comet is very bright. A bright point of light might be visible in the heart of the coma and is often confused as the nucleus. The nucleus is actually hidden behind all of the dust and ice because the higher concentration of these particles closer to the nucleus appear brighter and give the illusion of a star-like point of light. It is especially fun to watch the comet move among the field of stars within a few hours. A telescope is perfectly suited for this type of observation and it is even possible to detect motion within minutes. The coma can be examined in finer detail with a telescope. By using different magnifications it should be possible to see if there are any areas within the coma that are brighter than the others and if there are any indications of jets or a spiral pattern should the nucleus be rapidly rotating. Take note if there is any fragmenting, which might be visible as several brighter areas within the coma or tail. Sometimes the tail can disconnect due to variations in the solar wind, so pay attention to anything unusual along the tail such as brighter streaks or dark gaps.
High power is best for observing fine detail as the eye can detect faint structures when the object is bigger, even if it is fainter. Move the eye around and focus on a faint star in the field if there is one as this will make it easier to focus on the diffuse parts of the comet. Also jiggling the telescope will allow the eye to detect faint detail. It always pays to cover the head with a dark cloth and be sure to avoid breathing on the eyepiece, which fogs it up in cold weather.
Another project is to observe the comet through weeks and months of visibility; watch it evolve as it approaches the Earth and Sun and then depart. For a fun challenge, try and observe a comet for as long as possible before it is too faint to be seen with the eyes, binoculars, and finally a telescope.
Astrophotographers have a lot of fun with comets as comets can pass near or even cross over several interesting nebulae and star clusters. This makes for rare and beautiful photographs and such photos abound as astrophotographers are proud to display them. Beautiful photos have also been made with scenery such as historic landmarks, churches, trees, or even people. Photos are priceless keepsakes because no two comets are alike and no two apparitions of the same comet are ever quite alike. Comet Halley is such a comet. It is predictable, but looks different at each apparition because our viewing angle varies with the different distances it is from the Earth and Sun. A comet may have a long, bright tail, but will look like nothing more than a bright, fuzzy ball when seeing it head on since the tail streams away behind it out of our view. If a bright, active comet is observed lengthwise, the tail can stretch along a huge length of the sky as did Comet Hyakutake’s.
Comets are hauntingly beautiful in their delicate appearance and are by far the most interesting to observe. Like ghosts they seem to silently come and go, almost without prediction, and transform before the eyes with their tails seemingly whispering in the chilly depths of the dark, starry night.