Farewell Comet ISON, We Hardly Knew Ye

Seth Jarvis

Comet ISON’s 15 minutes of fame have come and gone.

Come from… where?  Gone to… where?

Far, far from our Sun, as in about 1,000 times farther from the Sun than Pluto and a significant fraction of the distance to the next closest star, is a region of space surrounding our solar system astronomers call the “Oort Cloud.”  It’s called a “cloud” because it consists of trillions upon trillions of lumps of varying sizes of water ice mixed with bits of rock, dust, and a dash of organic materials left behind by the formation of our solar system roughly 4.5 billion years ago. Some of these icy objects are the size of planets, but mostly the Oort Cloud is populated by lumps of dirty ice about the size of a middling-sized mountain, i.e., a few miles wide.

Even at this extreme distance these lumps of dirty ice are held, just barely, in orbit around the Sun.

These mountain-sized lumps of dirty ice spend eon after eon traveling slowly around the Sun.  But occasionally something happens to bump, nudge, tug, jostle or otherwise perturb one of these things in such a way as to cause it to begin a long, and I mean LONG, fall towards the Sun.

Nearly three million years ago one of these mountain-sized lumps of ice experienced a perturbing event, and then in September of 2012 this lump of dirty ice that was headed for the center of our solar system came to the attention of astronomers operating the International Scientific Optical Network (ISON) when it was about as far from the Sun as the orbit of Jupiter.

Astronomers eagerly studied it, figured out its orbit, and concluded it was a comet, fresh (as comets go) out of the Oort Cloud.

Most surprisingly, this comet, now designated “C/2012 S1 (ISON)” was calculated to pass tremendously close to the Sun.  Whereas most other comets only get no closer to the Sun than the orbit of Mercury before being slung back out into the deep freeze of farther-than-Neptune outer space, this comet would get less than a million miles from the Sun.  This made Comet ISON a “sun-grazing” comet, the largest sun-grazer yet discovered, and therefore an object of intense interest to scientists.  Every major observatory on Earth started to observe this new comet, and spacecraft orbiting Earth, and orbiting the Sun, and even one orbiting Mars were instructed to aim their instruments at Comet ISON.


This is how comet C/2012 S1 (ISON) wished to be remembered. Image credit: Damian Peach


Some media outlets breathlessly described Comet ISON as “The Comet of the Century!” which made astronomers very uncomfortable. Why? Because comets are maddening things. Their orbits can be calculated with great precision, but how they’ll behave when they get close to the Sun can be highly unpredictable. Some comets are spectacular, others are duds. There’s no way to really know what they’ll do when they get close to the Sun until they’re actually close to the Sun.

Since this was ISON’s first passage past the Sun nothing was known of how it would behave and astronomers were forced to make educated guesses about the comet’s potential visibility based on what they knew of the behaviors exhibited by other comets.

Certainly all the elements were in place for ISON to be a truly spectacular comet.  Although it was on the smallish side, it was going to get fantastically close to the Sun. That meant the solar heating of the comet’s ices would be severe (an estimated 5,000° F), and that much heat had the potential to cause the comet to form a large and very bright coma and tail.

If ISON could manage to hold itself together against the blast-furnace heat and extreme gravitational stresses of so close a passage to the Sun, it would almost certainly emerge from its encounter with the Sun as a brilliantly bright comet, possibly so bright it could be seen in daylight.

But that was a mighty big “if.”

Comet ISON, having spent nearly three million years falling towards the Sun, spent roughly six hours traveling less than a million miles from the Sun, and that was long enough for the Sun’s heat and tidal forces to tear Comet ISON to shreds and leave behind nothing but a puff of water vapor and a few bits of dust and gravel. Some folks who were up on their Greek mythology took to referring to it as “Comet Icarus.”


Comet ISON’s demise as observed by the Solar and Heliospheric Observatory (SOHO) spacecraft.

So long, ISON.  You could have been a contender, but it was not to be.

BTW, NASA has a nice “In Memoriam” article about Comet ISON.  It’s worth reading.

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