The Perilous Journey of Comet ISON

by Shane L. Larson

In the vast dark past Pluto, the fringes of the solar system are a deep freeze filled with a trillion fragments of ice and rock, the left-over detritus from the formation of the Sun and planets. Caught in the gentle embrace of the distant Sun’s gravity, these tumbling iceballs have drifted for billions of years, far from the neighborhood of the Earth. They are each comprised of a loose rubble of rocks and dust, bound together by water ice and other exotic ices, such as carbon dioxide, methane, and ammonia. The orbits are tenuous and fragile, but every now and then, a chance encounter with a neighbor or a close pass by a dwarf planet or distant star perturbs the orbit. When this happens, the inexorable pull of the Sun becomes more important, and one of these iceballs begins a long journey toward the inner solar system.

When the solar system formed, the most distant reaches, far beyond Pluto, were populated with the left-overs. This cloud is the origin of comets.

At the outset the journey is uncertain, since it is unknown what the iceball may encounter during its travels. Maybe another encounter will send it tumbling back around the outer darkness with the other flotsam and jetsam, or maybe a tug from the gravity of one of the large, gas giant planets will bend its course, sending it into a wide orbit among the planets. But should it make it into the deepest reaches of the solar system, in the neighborhood of the small, rocky planets that huddle around the Sun, then the iceball may make a miraculous transformation. The heat from the Sun will warm the iceball, initially raising a faint fog of evaporated materials, rising over the cold surface like morning mist on a lake. As the Sun grows ever closer, basking the iceball in warmth, more ice and volatile materials will evaporate raising a great cloud that can be thousands of kilometers across. Some of this material is pushed away from the iceball by light and wind streaming off the Sun, stretching it out to form a vast tail that can be longer than the spacing between the worlds. Filled with water and dust, this enormous cloud and tail reflects sunlight very well, making the iceball visible to the watchers living on one of the rocks near the Sun. They call this spectacle a comet.

Long before humans ever walked the green shores of Earth, something shook the distant realm of comets up, and one tiny iceball, about 5 kilometers across, started to drift toward the inner depths of the solar system. From the iceball’s perspective in the outer darkness, it may not have been clear even where the long tumble would end up; the Sun seen from the fringes of our solar system is only a distant spark, brighter than all the other stars in the sky, but still only a fraction as bright as it appears from the sunny meadows of Earth. Initially it was drifting slowly, perhaps only a kilometer per second. By typical Earth standards, a kilometer per second sounds incredibly fast, but on a journey from the fringes of the solar system to the Sun, a distance of almost 7 and a half trillion kilometers, these speeds are excruciatingly slow. For this comet, the journey in will take almost two million years.

(L) Astronomer Jan Oort, who proposed the idea of the distant cloud of iceballs that become comets. (R) Astronomer Fred Whipple, demonstrating the “dirty snowball” model of comets with a 500 pound snowball!

The distant cloud of frozen comets is known as the Oort Cloud. Its edge is thought to lie somewhere around 50,000 times farther from the Sun than the Earth (or, in astronomer-speak, “50,000 astronomical units”). That means it takes light from the Sun 288 DAYS to reach the Oort Cloud. Space is vast, and the motherland of the comets is far away. For most of the journey, for most of the comets, we on Earth are completely unaware of these tiny iceballs. Only if the comet gets close enough, only if the comet heats up and puts on a show, do we become aware of them at all.

On 21 September 2012, after millions of years falling closer and closer to the Sun, one iceball was about 6 astronomical units away, cruising in the vast emptiness between the orbits of Jupiter and Saturn. At this distance, the Sun had started its gentle warming of the surface, and the low-level fog had started to grow around the iceball. A small robotic telescope located near Kislovodsk, Russia, was concentrating its gaze in the vicinity of the constellation Cancer and stumbled on the faint, growing glow. Suddenly, we were aware of this new comet. A name was quickly assigned: C/2012 S1. It is more colloquially known as Comet ISON, after the International Scientific Optical Network, which discovered it. As it turns out, we had seen this particular comet before — a survey system on Mount Lemmon in Arizona had stumbled across the comet on 28 December 2011 and not recognized it, and a month later on 28 January 2012 the Pan-STARRS system on Haleakala in Hawaii had similarly found but not recognized the comet. By September, it had grown bright enough for us not just to notice, but to realize what it was we were seeing.

Comet ISON in April 2013, as seen by the Hubble Space Telescope (NASA, ESA, and the Hubble Heritage Team [STScI/AURA]).

After much study, including observations by the Hubble Space Telescope, we’ve learned a lot about little Comet ISON. The iceball core, which astronomers call the nucleus, is about 5 kilometers across — not terribly large, but not terribly small either. Like many comets, it has a lot of frozen carbon dioxide (what you and I call “dry ice”) that is evaporating off the surface and forming the foggy cloud around the core (what astronomers call the coma). These are ordinary properties that ISON shares with most comets. But Comet ISON has a secret — it is really a member of an elite group of comets known as sungrazers.

ISON’s projected path near the Sun, on an image taken by NASA’s SOHO satellite. The Sun is shown overlaid on the black disk that SOHO covers it with; the streaming red and blue are the strong winds outflowing from the Sun that ISON will fly through. (Image from NASA/SOHO.)

Comet ISON is plunging almost directly toward the Sun, destined to skim low over the surface then slingshot around and be flung back toward the outer dark of the Oort Cloud. It will make its closest approach to the Sun on 28 November 2013, when it will be only 1.2 million km above the surface of the Sun (about 50 times closer to the Sun than the planet Mercury). To an observer riding on the back of Comet ISON, the close pass will be excruciatingly intense. This close to the Sun, the temperature will soar to more than 2700 degrees Celsius on the comet’s surface, hot enough to melt iron. Furthermore, ISON will be close enough to the Sun that it will experience gravitational stresses — the Sun’s gravitational pull on one side may be much stronger than on the other side (what astronomers call a tide). Comet ISON, like most comets, is a loosely packed iceball of rubble, held together by the ice. If the Sun’s gravitational tide is strong enough, ISON could disintegrate from the stress, much like a poorly packed snowball on a too-warm winter day.

(Top) Our understanding of the structure of comets. (Bottom) A model of a comet you can build on your own (link) [Photo by Latoya Flowers, Adler Planetarium]

Comet ISON is plunging toward the Sun, on a long and perilous journey from which it may not emerge unscathed. But why are we so fascinated by the tale of this voyage? There are many reasons, not the least of which is the spectacle. Comet ISON will come closer to the Sun than most comets, so the heating on it will be more extreme. The prospect of extreme heat has led to the possibility that vast amounts of volatile ices will melt and be released, generating a much larger and brighter coma and tail. If that happens, the spectacle from Earth could be awe-inspiring, literally visible to anyone who just steps outside and looks at the sky. Some comets are visible from Earth almost every year; if you have a friend who is an amateur astronomer, you may have stood outside on a cool evening in the fading light of day and seen a distant streak of light as a comet sails toward the Sun. These cosmic visitors are quiet passersby, noticed by a few, but not raising the awareness of the majority of the inhabitants on the third rock from the Sun.

Some of my own comet observations of average comets, that went largely unnoticed by most people. (L) Comet McNaught in 2007. (R) Comet PanStarrs in 2013 (see a video I made of PanStarrs setting over the Wellsville Mountains of Utah).

But sometimes, maybe only once in perhaps a hundred or more years, a comet will grow to spectacular brightness, hanging in the sky like a chandelier of cosmic proportions. Stretching across the sky, its tail streaming away from the Sun, these Great Comets are spectacles that can’t help but be noticed — they are quite clearly new, different, and rare. Many famous examples of comet spectacles have been recounted, whispering through the ages of Nature’s awesome beauty and unpredictability. Among the famous stories is the tale of the Great Comet of 1680. Sometimes called “Kirch’s Comet” or “Newton’s Comet”, the Great Comet of 1680 has the singular distinction of being the first comet discovered by telescope, which had only been invented a scant 70 years before. The storied tales of its appearance recount its enormously long tail, captured in Lieve Verschuier’s famous painting of the comet in the sunset over Rotterdam. Perhaps most notable about the Great Comet of 1680 is that its orbit carried it to within 930,000 km of the surface of the Sun, similar to ISON’s own perilous path.

The Great Comet of 1680 over Rotterdam (painting by Lieve Verschuier).

One of the brightest comets of the 19th Century was Comet Donati, which famously was the first comet to be photographed, though many paintings of Donati are more famous, such as Amèdée Guillemin’s image of the comet over the Plais du Justice in Paris. Abraham Lincoln recounted having seen Comet Donati, sitting on his hotel porch in Jonesboro, Illinois in September 1858, on the eve of his third debate with Stephen Douglas. Like the Great Comet of 1680, and like ISON, Comet Donati passed by the Sun then returned to the outer darkness. It is not predicted to return to our vicinity for millennia yet to come.

(L) A stero photograph of Comet Donati over London in 1858. (R) Amèdée Guillemin’s painting of Comet Donati over the Plais du Justice in Paris.

There also is a definite sense of extreme adventure associated with ISON’s journey around the Sun — just a tiny comet, all on its own, constantly fighting uphill against the lashing solar wind until finally, elated, it skims over the surface of the Sun. After blazing a trail across the solar limb, the comet skips off and away, looking backward with a gregarious laugh and a tip of the hat saying, “See ya, Herbert!” as it streaks back toward home. If I hadn’t of said “comet”, you might think the tale has all the trappings of an awesome movie about a wayward kid hoboing around the world, and not about our cousin, the little Comet ISON.

Like most things we see in the Cosmos, ISON is spectacularly unpredictable. The hopes and dreams and stories that we weave around this little cosmic voyager are synthesized from watching other great travelers, like the Comet of 1680 and Comet Donati. Perhaps ISON will grow to spectacular brightness (we hope so), but perhaps it will skate by the Sun, and survive the blistering passage with little more show than any of a thousand other comets that have occasionally graced our skies. Either way, it will be visible to those who are interested in looking, though maybe not obvious to those who are focused on the normal trappings of everyday life.

So for the moment, we are waiting. Will ISON shine brightly in the night, or simply suffuse the Earth with a subtle, diaphanous shower of comet light? Will ISON survive its close slingshot ride around the Sun, or will the intense heat and gravity shatter it into a million tiny bits, never to be together as Comet ISON again? We don’t know the answers to these questions; all we can do is wait. But one thing is certain. If ISON does survive its encounter with our parent, the Sun, it will head back out into the distance darkness of the Solar System, never to be seen again by the human race. If it does return, it will not be for millions and millions of years, so this is our chance to see Comet ISON.

For those who do look, it will be a chance to look at one of Nature’s great spectacles — a cosmic sky show featuring a little pile of rubble left over from the construction of the solar system. Left over rubble from the assembly of everything that we see around us — the Sun, Jupiter, Mars, the Moon, Earth, petunias, raccoons, and you. I often daydream of an imaginary adventure, following ISON on its perilous journey, sneaking up behind it and grabbing a Mason jar full of the stuff. I’d love to have it sitting on my shelf, right next to my jar full of Pacific seawater and seashells, so I could stare at it and wonder about how it’s all connected together. I’ll be out looking at Comet ISON this winter — to observe it, to wonder about its origin and destiny, and to discover how it fits into the great jigsaw puzzle that is slowly emerging to describe the Cosmos and our place within it. I hope you are out looking too.

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I’d like to thank my friend, Annie Vedder, for the poetic turn of phrase that became the title of this post. 🙂