Preparing for Comet ISON: A Comet Primer for Casual Stargazers

October 2013  :  Dave Huestis

In my January preview of astronomical highlights for 2013, I noted the predictions for a “Comet of the Century” to grace our skies in November and December. Descriptions like “as bright as a Full Moon” and “may possibly be seen even in broad daylight” were catch phrases bandied about the internet concerning Comet ISON. Well, as the comet approached, I followed the news updates on its progress. Soon there were news items saying something to the effect of “best comet in the last few decades.” And a more recent analysis of the comet’s changing expectations prompted a Skyscrapers member to comment “comet of the week!”

That’s the way it is with comets. The only thing predictable about them is that they are unpredictable. A recent example was Comet PANSTARRS back in March. It was predicted to be brighter than it actually became. Up here in the northern hemisphere in the hazy and light-polluted skies of Southern New England PANSTARRS was just barely visible to the naked-eye.

At the end of May, while Comet ISON was still out beyond the orbit of Mars, it was lost to view because it was in the direction of the Sun. When the comet finally moved out of the solar glare and was imaged on August 12, it was still fainter than what was originally forecast. At the time some comet experts thought it was still too early to make a reliable call on what we could expect to see as ISON neared the Sun. Others simply didn’t believe it would become visible to the naked-eye. And there are those experts who do not believe Comet ISON will survive its close passage to the Sun. Despite these disparate opinions, we may yet get a good look at this cosmic visitor to the inner solar system.

To prepare you for the potential Comet ISON experience (whatever that may be), today’s column (Part I) is a primer that will provide some basic facts about comets in general. Part II will serve as an ISON observing guide, noting dates, times and where to look in the sky.

Let’s hope Comet ISON doesn’t become another Comet Kohoutek, which fizzled in 1973.

Part I: An Introduction to Comets

What’s the first thing that comes into your mind when you read or hear the word comet? Most people will likely think of Halley’s Comet, one of the more famous comets. (Edmund Halley was the first astronomer to predict the return of a comet in 1769, the one now named Halley in his honor. Unfortunately he died before it returned.) If one asks them to describe a comet they will tell of a star-like object with a long tail. That answer would actually be a good start. However, for many that would be the extent of their knowledge on these interplanetary visitors from the depths of our solar system.

The above description of a comet is fairly reminiscent of a centuries old depiction. Comet actually means “long-haired,” and Greek philosopher and scientist Aristotle (384 BC – 322 BC) is the first known person to use it to describe these “hairy stars.” Comets were very often considered bad omens, supposedly foretelling the death of royalty or the onset of some calamity. They seemed to appear out of nowhere, and then disappear just as quickly. Astrologers must have flourished when a comet appeared in the heavens.

Comets were initially thought to be manifestations of the atmosphere. But in 1577, Danish astronomer Tycho Brahe (1546-1601) and others took precise measurements of the motion of the Great Comet of 1577 from widely separated locations and determined that the comet did not show any parallax (difference in position as seen from multiple locations). These observations indicated that the comet was well outside of the Earth’s atmosphere. But if these celestial objects were out roaming amongst the planets of our solar system, where did they come from?

Most all comets originate in the Oort Cloud, a theoretical spherical cloud or halo of perhaps several trillions of comets encompassing our solar system and extending up to 465 billion miles from the Sun. Comets are leftover material from the creation of the solar system 4.6 billion years ago. They are irregularly shaped objects, estimated to range in size from several hundred feet to 25 miles and over, composed of ice, small rocks, dust, various gases, and organic compounds.

In fact, during the 1910 apparition of Halley’s Comet, astronomers spectroscopically discovered cyanogen gas in the comet’s tail. With this news came reports of what cyanogen gas was and what effects it could have on the human population. It became blown way out of proportion. The gas detected was so tenuous that Professor Mitchell of Columbia University stated, “I believe that if a cubic mile of the comet's tail could be gathered and put into a beaker, it would require the most minute chemical tests to detect anything more than a trace of the poisonous gas.” But the damage had already been done. Some people were panicking.

Providence’s resident astronomer Frank E. Seagrave had many queries on the comet's possible destruction of the Earth by collision. Things went from bad to worse when Mr. Seagrave announced that the Earth would pass through Halley’s tail! That news really got everybody frightened. In fact, one prominent businessman from Providence refused an invitation to spend the night of May 18, when the comet’s tail would sweep over the Earth, at Frank's observatory, insisting that the place for a conscientious man was at home with his family.

“Later in the evening throughout the city, people were outside waiting for something to happen at the appointed hour of the Earth’s passage through Halley’s tail. Providence at the time was wrapped in a hazy fog, again attributed to the comet by some, while others “in the vicinity of the Great Bridge, catching a whiff of the familiar Providence smell, were certain that they were able to trace the cyanogen gas of the comet’s tail.”

Well, nothing happened. No collision. No death by gas. Hopefully we are much more enlightened today, though all sorts of doomsday scenarios can be found on the internet each time a new comet is forecast to appear. The more things change, the more they stay the same.

In 1950, the late Dr. Fred Whipple (1906-2004) of Harvard College Observatory and an old friend of Skyscrapers, theorized that comets are like “dirty snowballs.” Just how “dirty” are the surfaces of comets? They have the darkest surfaces of any object in the solar system. In fact, Comet Halley, being a very old comet, is so dark it reflects only about four percent of the light that bathes it. I can’t stress enough how dark that is. A little quick research revealed that asphalt reflects seven percent of the light it receives! For those of you who live around New England, think about the snow on the side of the road in mid- to late-March. It’s full of sand, dirt and small rocks. It’s so dirty you almost want it to snow again just to cover over the unsightly mess. Well, a comet’s surface is many times more “dirty.”

When comets are in their cold and cozy orbits within the Oort Cloud they are inactive. Occasionally one of them gets nudged by the gravity of a nearby star, sending the comet tumbling on its long journey towards our Sun. Initially it may take many hundreds of thousands of years for a comet to journey to our region of the solar system. The outer gas giant planets may alter the comet’s path and drastically shorten its orbit, and the Sun will do likewise.

Comets with orbital periods of 200 years and greater are called long-period comets. Less than 200-year orbit comets are called short-period comets. Though short-eriod comets had their origins in the Oort Cloud, over time they have been “herded” by the Sun or the gas giants of the outer solar system into a region called the scattered disk, which lies from 5.1 billion to 9.3 billion miles or so from the Sun.

There are even comets like Encke, which orbits the Sun once every 3.3 years, since at aphelion (farthest from the Sun) it is just shy of Jupiter’s orbit, and at perihelion (closest to the Sun) just comes inside of Mercury’s orbit. It too started its existence long ago out in the Oort cloud, but successive travels through the inner solar system modified its orbit substantially.

In fact, in 1993 a comet was discovered in orbit around Jupiter. Comet Shoemaker-Levy 9 had made a close approach to Jupiter in 1992 and was captured by this giant world. Jupiter’s gravity caused the comet to fragment into at least 21 pieces. But what was more significant was the fact the calculations showed that these fragments would crash into Jupiter’s cloud tops. In July 1994, over a six-day period, the pieces plummeted into Jupiter’s atmosphere. While we could not see the impacts happen because of our viewing angle, once the impact zones rotated into view even a small telescope revealed the huge impact scars. The impact zones were visible for several weeks. This incredible collision was the first time astronomers had ever witnessed such an event.

The Earth can also have a close encounter with a comet, though most don’t come anywhere near our home world. I’m sure the Earth has had close encounters with comets throughout the four billion years our planet has been in existence, but most come no closer than several tens of millions of miles to us. However, in 1908, it is believed by some scientists that a fragment of Encke’s Comet exploded high in the sky over the Tunguska region of Siberia. The resulting concussion wave leveled approximately 800 square miles of forest. Had it occurred over a more populated area the toll on life and property would have been devastating.

When a comet reaches the orbit of Jupiter (mean distance of approximately 483,500,000 miles), it starts to feel the influence of solar radiation. The comet reaches what is known as the H2O turn on point. It begins to heat up. Subsurface ice melts and the pressure forces the material out through cracks or vents on the surface of the comet. Jets of this material spew out into space like geysers (remember the scene in the movie Armageddon?). This process is called outgassing. These jets can not only affect the rotation (tumbling) of the comet, but also they can affect the comet’s orbital path through space.

All of this expelled material forms a cloud of nebulous material, called the coma, which envelopes the nucleus. Cometary comas can be many tens of thousands times the size of the nucleus. In fact, in late 2007, Comet Holmes developed the largest coma in recorded history. It set a record by becoming the largest object in the solar system – amazingly surpassing the diameter of the Sun.

The solar wind (a stream of particles radiating from the Sun) not only sandblasts loose material off the comet’s surface, but also starts pushing dust, gasses and rocky material away from the comet, forming two tails. One is the curved dust tail, which is responsible for producing a trail of debris along the comet’s orbital path. If the Earth happens to pass through this debris we experience a meteor shower. You probably know a few of the major ones by name, such as the Perseids and the Geminids. The second tail is the ion tail, comprising gases that always point directly away from the Sun. So unless you observe the comet over a period of time, you really can’t tell if it’s coming or going!

Some comets develop extensive tails, which can be many tens of millions of miles long. Also, as the comet gets closer to the Sun it will usually brighten very rapidly. If this is the comet’s first encounter the Sun, it most likely has a lot of loose material on the surface and much fresher subsurface material that will react to the solar heat, which will be blown off by the solar wind creating a large and bright coma, as well as a lengthy tail. An old comet that has made many trips through the inner solar system may have exhausted a lot of this material and any activity may be minimal. This is especially true for short period comets. Eventually most of the volatile material contained in a comet nucleus evaporates away and the comet “dies out.”

As a comet nears the Sun, one of several things can happen. A comet can fall into the Sun. The SOHO spacecraft has observed many suicidal comets. A comet, called a sungrazer, may pass so close to the Sun that the tidal forces will fragment the comet into several pieces. Or a comet may simply evaporate after a close encounter. Comets have also been known to be ejected from the solar system as they are sling-shotted around the Sun.

Our view of a comet depends almost entirely upon our position in space in relationship to the position of the comet and the Sun. A comet may be better observed in the northern or southern hemisphere of the Earth. Also, depending upon the circumstances, one may be able to see a comet in the morning sky before sunrise and in the evening sky after sunset for a period of time.

Once the comet travels around the Sun and begins the trek back towards the depths of the solar system, the tail is pushed out in “front” of the comet, since the solar wind blows the coma into a tail regardless of which direction the comet is moving. After a short time the comet becomes invisible to the naked eye, requiring binoculars to observe it. Soon binoculars will be useless and only telescopes will show the rapidly fading comet. Amateurs with large instruments will be able to watch the comet for quite some time, but for the casual observer the comet will be soon forgotten.

When a new comet is discovered astronomers do their best to predict how the comet will perform based on its orbital path and how bright it is at the distance it was discovered. Occasionally the predictions fall far short of the mark. Kohoutek in 1973 was one example. Even though the recent comet PANSTARRS back in March was better in the southern hemisphere as it approached perihelion (closest to the Sun), by the time it became visible to us here in southern New England it could barely be seen with the naked-eye. It was fainter by two full magnitudes (or 6.3 times fainter) than predicted.

David Levy, famed comet discoverer (of Comet Shoemaker-Levy fame), addressed the forecasting of comet behavior by saying, “Comets are like cats. They have tails, and they do precisely what they want.” For instance, Comet Kohoutek in 1973 was predicted to be the comet of the century … and turned into the dud of the century. Then three years later in 1976 Comet West put in a pre-dawn appearance. It was a beautiful comet, easy to see with the naked-eye. However, despite efforts to publicize it in the local media, it received virtually no press. Why? News outlets felt they had been duped by the hype over Kohoutek, and they weren’t going to be taken in again. The results: the general public missed a wonderful comet. Fortunately the spectacular appearances of Hyakutake (1996) and Hale-Bopp (1997) greatly helped the astronomical community fully recover from the Kohoutek curse.

Many historic comets have special names like The Great Comet of 1577 and The Great Comet of 1882. However, newly discovered comets now bear the name of their discoverer(s) – up to three names can be assigned. Plus, a comet is also assigned an astronomical designation, like C/2012 S1 for comet ISON.

While amateur astronomers have discovered many comets, today many are being discovered by automated search projects with and without space-based telescopes. They can be detected because they shine by reflected sunlight, just like all the planets and moons in our solar system.

On any clear night there are tens of comets visible to various sized telescopes. Once in a while a new comet will be discovered that shows potential for putting on a good show that anyone in a dark sky can observe and appreciate. However, naked-eye comets are fairly infrequent. Those that do not require a telescope to observe them are the best. Hyakutake (1996) and Hale-Bopp (1997) were prime examples. For the casual stargazer the best tools were the naked-eye and binoculars, for the tails of these visitors spanned a greater area of sky than a telescope could encompass.

With less spectacular comets binoculars will reveal a fuzzy patch of light with a small extension (the tail). It will resemble a small (very small) triangular wedge of pie in the sky. Once you find it in binoculars you can try your luck with a telescope if you’ve got one. The image will not be spectacular (we’ve all been spoiled by the likes of Hyakutake and Hale-Bopp), but the increased light gathering ability and magnification will certainly reveal more of the cloud-like fuzz-ball and its nebulous tail.

If the amount of material on the surface is limited, the comet will quickly brighten, and then diminish after the material has been depleted. If there is a more abundant frost layer, the comet will maintain its brightness. Also, if the material below the frost is loose, and not hard-packed, it too will contribute to the brightness of the comet. Increased activity adds more dust to the coma and the tail.

Will Comet ISON recover and put on a grand display? It is estimated to be only three miles or so across, albeit a very big “dirty snowball,” but ISON will pass within 700,000 miles of the solar surface! I am not very optimistic about great naked-eye views at this point in time. Definitely do not expect anything like the spectacles of Hyakutake or Hale-Bopp. Despite the current prospects, I am hoping that ISON does not have a Kohoutek moment.

Next month in Part II, I will provide a Comet ISON viewer’s guide, providing the details of when, where and how you may get a glimpse of the comet.

Keep your eyes to the skies!