ISON’s Chances for Survival 50/50

New studies suggest that the comet's nucleus has a good chance of surviving its close passage of the Sun, but there are a lot of unknowns that could swing the result either way.

Comet ISON refuses to show its hand. Scientific pundits’ predictions have ranged from two pair to a straight flush, but for now the comet is playing it close to the vest.

Comet ISON
Observations of Comet ISON in April suggest that its pole is pointing at the Sun. The nucleus is hidden inside the bright white dot in this image, and the blue color is for aesthetic appeal.
NASA / ESA / J.-Y. Li (PSI) / Hubble Comet ISON Imaging Science Team
Work presented at this week’s American Astronomical Society’s Division for Planetary Sciences meeting in Denver adds to speculations about the comet’s upcoming performance. These new observational and theoretical studies suggest that the comet’s nucleus has a good chance of surviving its sungrazing pass but hint that, like a sneaky card player, it might have a trick up its sleeve.

Matthew Knight (Lowell Observatory) and Kevin Walsh (Southwest Research Institute) took two approaches in their attempt to determine whether ISON will survive perihelion. First, they looked at the nucleus’s size. Both theoretical calculations and observations of sungrazing comets by the SOHO and STEREO spacecraft show that comets smaller than about 400 meters (>1,000 feet) across should disintegrate. The best estimate for ISON’s size is between 1,000 and 4,000 meters, so it shouldn’t completely evaporate, even in the 2,800&deg C (5,000° F) it will feel at its closest approach on November 28th, Knight said in a press conference on October 9th.

But the comet also has to contend with tidal forces. Close to the Sun, the comet will experience a stronger gravitational pull on the Sun-facing side than on its far side. These tidal forces could stretch the nucleus out like a cigar and, if the pull is strong enough, break the nucleus apart. Solar radiation will erode these smaller pieces more effectively, lowering the comet’s chances for survival.

The nucleus’s density and rotation have a big say in what the tidal forces will do to ISON, Knight explained. He and Walsh ran simulations using likely densities and spin speeds for comets and found that, in most scenarios, ISON should survive. But survival is less likely if the nucleus is spinning very fast or prograde (i.e. forward toward the Sun). He puts the odds of survival above 50%, but jokes that the error bars on that estimate are “plus or minus 100%.”

The prediction is so uncertain in part because astronomers don’t know ISON’s spin. Hubble observations by Jian-Yang Li (Planetary Science Institute) and his colleagues indicate that the comet’s rotational axis is pointing at the Sun. The team detected a slight uptick in brightness that they interpret as a jet from the nucleus, and because this jet appears in all the images they took over 19 hours (and in ground-based observations over a two-month period), its stationary-ness suggests it’s quite near the nucleus’s pole. But no one’s seen changes associated with which way the nucleus is spinning.

The sunward-pointing pole is where ISON’s mystery deepens. The comet is following a parabolic orbit, meaning that, thus far, it’s basically been on a straight-shot trajectory toward the Sun. If it’s pointing only one hemisphere at the Sun, the other hasn’t been heated yet — and won’t be until maybe a week before perihelion, when the comet passes within Mercury’s orbit. When that happens, pristine surface material such as carbon dioxide ice could “sublimate like crazy,” releasing a lot of dust and causing an outburst, Li said. But he cautions that his team determined the pole’s location from only one set of observations in April, and the result could change with follow-up. Hubble observed ISON this week, so that follow-up is imminent.

Any outburst could wreak havoc with the nucleus's spin, making its survival even harder to predict.

Of course there’s always the wild card. Knight noted that comets in the solar system spontaneously disrupt for unknown reasons maybe 1% of the time. That’s beyond researchers’ ability to simulate.

In other words, we’ll have to wait and see what we’re dealt.


See our updates page for the latest on Comet ISON. And don't forget to submit your images to our gallery.

13 thoughts on “ISON’s Chances for Survival 50/50

  1. Bruce

    Sorry if I’m behind the curve on this story, but since this comet is on a parabolic trajectory, does that mean that it has never come near the Sun before? Also, if it survives perihelion, will it never return?

  2. Anthony Barreiro

    Bruce — this is Comet ISON’s first pass through the inner solar system. Its orbit is *not* parabolic, it is hyperbolic, meaning this will be ISON’s only pass through the inner solar system. Whatever survives perihelion will continue out of the solar system never to return. (By the way, I’m amused by all the speculation about what Comet ISON may or may not do. Ancient astrologers foresaw all sorts of disasters from the locations and shapes of comets. Now we’ve got CCD cameras and spectrometers, but the darn comets are still doing exactly as they please!)

  3. Anton Szautner

    @ Anthony Barreiro:

    As far as I am aware, the latest measured eccentricity of Comet ISON is 1.000004, which is so close to parabolic as to make a claim for hyperbolic trajectory laughable. To be absolutely sure, it is certainly nowhere near anomalously beyond the range of trajectory regimes that are quite permissible for objects falling in from the far reaches of the Oort cloud, those which can have easily been very slightly perturbed (with a change of under a meter per second many hundreds of millions of years ago) to allow such an object at such a large aphelion distance to fall in and pay a close visit to the Sun. MANY examples of near parabolic trajectories for comets are well documented; its nothing new. A claim of a hyperbolic trajectory would require a much larger departure from an almost perfect parabolic course, not to mention one that so closely approaches the Sun at perihelion, just on statistical grounds. If it was a random visitor from interstellar space, you can be sure its trajectory would STRONGLY depart from the parabolic…and we would marvel at why such an object was aimed so closely at the Sun to boot.

  4. Anton Szautner

    One more point. The period of an object plying a near parabolic trajectory (eccentricity near 1.0) is obviously very long, if we concern ourselves with its CURRENT orbit. But it can remain perfectly consistent with the age of our system. We understand that the extreme periphery of the sun’s gravitational influence is necessarily dictated by our Sun’s gravitational interactions with passing stars (and perhaps even passing molecular clouds) over the course of its 4.5 billion-year history as it has orbited around the galaxy with the rest of the stellar swarm, and that would tend to determine the outer extent of the current Oort Cloud of objects. Moreover, most models suggest that almost all of the objects populating our Oort Cloud (and such icy objects attending most every other star) are coeval to the formation of the parent system. That only means that objects that condensed as part of our system and which may have been perturbed outwards to comprise the Oort Cloud can be perturbed back into the inner system with very little change of velocity, as I’ve alluded to earlier. In fact, any very slight perturbation (basically on the order of human walking speed!) can as easily provide some impetus TOWARD the Sun as mostly cancel its feeble ‘sideways’ velocity at such huge distances, which is why we have in fact seen so many comets that have near parabolic trajectories. None so far with unambiguously hyperbolic ones: there has never yet been a strong candidate for an interstellar interloper.

  5. Jacques Millet

    Why are we still thinking that a passing star is needed to send comets towards the sun? To me the extremely long parabolic orbits of comets are telling me that no star has passed close to the sun since the formation of comets. Any perturbation from a passing star would fling them AWAY from the sun!

  6. Bruce

    Thanks Anthony, and thanks for the additional info Anton. I used the word parabolic to describe ISON’s path because that’s the word Camile used in her article. So indeed I was going to ask him about his calling the trajectory hyperbolic. But Anton, you sort of went all hyperbolic on him for using the term, I think. Tectonically speaking, a conic with an eccentricity greater than 1 is the very definition of the math term hyperbolic, and as you very helpfully pointed out, the eccentricity of this comet is 1.000004, which as all can see is indeed greater than 1. So technically, Anthony is right, so it is funny (laughable) that you would call his use of hyperbolic laughable. Lol.

  7. Bruce

    My apologies to all. “Tectonically” above should have been “Technically”. But a better word choice in that sentence would have been “Mathematically”, but I’ve misspelled that one before too. But I hope some have at least gotten a chuckle out of this.

  8. Mike W. Herberich

    I certainly did, Bruce! Tectonically funny! Thank you, and, in addition, as almost always, instructive and resolving (and conciliatory :-}). Imagining cutting through a (mathematical, thus double, to infinity "up" and "down") cone by means of a plane illustrates this. Cutting "across" produces ellipses at different eccentricities (including a circle and a point). Tilting the plane until parallel to the cone’s "wall" exactly gives a up- or down-parabola or line. Tipping even more results in a hyperbola (two separate branches). I hope this is not too hyperbolicly parabolic … (ellipses)__.

  9. Anthony Barreiro

    Thanks Anton and Mike for correcting my error and adding to my understanding. Proof once again that a little bit of knowledge is a dangerous thing. The universe is a big place, and I will always have a lot to learn.

  10. Dangerous?

    Anthony, if everything were as dangerous as what you have written by means of your knowledge, only in this blog here alone, the world would be a veeeery peaceful place! Please, do not cease from posting as often as you do. I always read you, and, may I say, joyfully.

  11. mortimerzilch

    at perihelion comet ISON is supposed to be traveling at near 1,000,000 miles an hour, or so I’ve read. That means it can go from one side of the Sun to the other in one hour! And THAT means that the dark side of the comet – it’s heading in with its rotational axis pointing toward the Sun – will become almost immediately exposed to maximum solar energy. That’s a very interesting scenario. Zero to maximum in one hour. I am not at all sure I can grasp what that might do to the comet nucleus. Neither can I imagine ANYBODY else has a good idea of what that will be like. But wasn’t there someone arrested at LAX for making "dry ice bombs" very recently? The comet might explode is what I am suggesting…while traveling at one million mph…and that might be really quite a show to see.

  12. mortimer zilch

    I think it might explode. Right now it is reported that the North Pole of Ison is pointed at the Sun. That means the front is in constant daylight, and the back in constant night-time. So, that explains its underperforming brightness – only have it’s surface is reacting with the solar radiation. As Ison accelerates around the Sun – up to 1,000,000 mph I’ve heard tell, it will quite suddenly expose the heretofore frozen dark side to intense, proximate solar radiation, and that at solar maximum, albeit a weak maximum.
    That is the precise prescription for an explosion. Rapid heating of a cold dark frozen mass. I think Comet Ison will make a spectacular apparition from around the back of the Sun…maybe an event like none other ever seen…an exploded comet. Wouldn’t that be something!?

  13. Bruce Mayfield

    I like your pair of comments Mortimer. Of course all would love to see the grand display that should happen if your prognosticating pans out, especially in light of ISON’s under-whelming show so far. But your not just making a wild, baseless prediction, for you were able to take the facts at hand and give what seem to be plausible reasons for your forecast. Your stock will rise greatly in this group if “ISON Explodes!” becomes a headline in about 40 days or so.

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