What might happen to Comet ISON from here on out?

This post was written by Matthew Knight with input from Dr. Carey Lisse.

November is here and Comet ISON is just a bit over three weeks from it closest approach to the Sun (astronomers call this “perihelion”). As we’ve said from the beginning, it is impossible to predict exactly what ISON will do. However, given numerous observations of previous comets, we do have a pretty good idea of the range of possible outcomes. Now that ISON has almost completed its journey from the Oort Cloud to within a million miles of the Sun’s photosphere, it seems like a good time to go over what might happen.

I’ve grouped the possible outcomes into three scenarios, discussed in chronological order for when we would observe them occur. It is important to note up front that no matter what happens, now that ISON has made it inside the Earth’s orbit, any or all of these scenarios are scientifically exciting and we would learn a lot (although I’m clearly rooting for case 3)!

Case 1: Disintegration well before perihelion


D/1999 S4 LINEAR as viewed by Hubble Space Telescope. Image credit: NASA, Harold Weaver (The Johns Hopkins University), and the HST Comet LINEAR Investigation Team.
The first scenario, which could theoretically happen at any time, is that ISON spontaneously disintegrates. A small fraction (less than 1%) of comets have disintegrated for no apparent reason, with C/1999 S4 LINEAR in 2000 (pictured at right) and C/2010 X1 Elenin in 2011 being two recent examples. Speculation about ISON’s possible demise by disintegration has been around for months, and while these predictions have so far not come to pass, disintegration is probably far more likely during the next three weeks than it has been at any point up until now. As others have discussed, ISON’s activity has not been increasing particularly quickly recently, and ISON is now reaching the region of space, within ~0.8 AU (1 AU, or astronomical unit, is the distance from the Earth to the Sun) of the Sun, where comets like 1999 S4 and Elenin disintegrated.

If ISON does disintegrate in the next two weeks or so, we would likely see the central condensation get less distinct and eventually dissipate into nothing. Depending on the nature of the disintegration, the brightness of the inner coma might spike briefly then fade rapidly, or it might just gradually decrease in brightness. Since ISON is currently being observed by a tremendous variety of telescopes on Earth and around the solar system, this would be the best-observed case of cometary disruption in history and would likely contribute vast new information about how comets die. While it would be extremely disappointing to miss out on a potential naked-eye comet, the scientific return from a disintegration would be phenomenal as we will be able to learn a lot about how the comet is put together. This is particularly interesting because the formation and construction of comets is still one of the major mysteries concerning how the planets in the solar system were built.

Case 2: Destruction near perihelion


Comet C/2011 W3 Lovejoy seen over Australia. Image credit: Alex Cherney, TWAN
Assuming ISON survives the next few weeks in tact, it faces an even more daunting challenge: making it around the Sun. As you are no doubt aware, ISON will face extreme temperatures as it nears perihelion. At its closest point to the Sun, the equilibrium temperature approaches 5000 degrees Fahrenheit, hot enough to cause much of the dust and rock on ISON’s surface and in its coma to vaporize. While it may seem incredible that anything can survive this inferno, the rate at which ISON will likely lose mass is relatively small compared to how big it likely is (think of how a large pile of snow can last for weeks after a snowfall, even when the outdoor temperature has gotten much warmer than freezing). Furthermore, because it is moving very fast, about 400 km/sec at perihelion, it will not spend very long at such extreme temperatures. Assuming that ISON is bigger than about 200 meters in radius (current estimates suggest it is 500-2000 m in radius), it will likely survive mass loss due to sublimation of ices alone.

Unfortunately for ISON, it faces a double whammy from its proximity to the Sun: even if it survives the rapid vaporization of its exterior, it gets so close to the Sun that the Sun’s gravity might actually pull it apart! I discussed this in more detail in a previous post, but simulations found that ISON is more likely to survive than be pulled apart, although there is a very real chance that it could be pulled apart by these “tidal forces.”

If ISON is destroyed within a few hours to days of its close approach to the Sun, the most likely cause will be the temperature and gravitational stresses of the near-Sun environment. However, spontaneous disintegration (Case 1) could still be the culprit, it would just be hard to prove. In any event, if ISON meets it demise before its close approach to the Sun, it will likely be among the most spectacular comets observed by SOHO and STEREO, but there won’t be anything left to see from the ground after the close approach because all of the dust released during the breakup will be vaporized. On the other hand, if it is destroyed a few days after close approach when the temperatures are less extreme, the dust released when it disintegrates will likely form a spectacular tail visible from Earth. A good analog was comet C/2011 W3 Lovejoy (pictured at left), a sungrazing comet that passed within 100,000 miles of the Sun’s surface in December 2011. Lovejoy had not been discovered yet when it was at ISON’s current distance from the Sun (it was discovered just 19 days before perihelion) but it nonetheless became a spectacular naked eye object for Southern Hemisphere observers a few days after it completely disintegrated.

Whether it disrupts before or after close approach to the Sun, in this scenario, ISON would have been observed from nearly the orbit of Saturn to within a few solar radii of the Sun’s surface. Such an event has never been observed before, so the scientific return would be excellent. Again, if the break up happens after perihelion, there is a very good chance ISON would become an excellent naked object, so I think most everyone would be happy with the outcome.

Case 3: Survival


The Comet of 1680 over Rotterdam. Painting by Dutch artist Lieve Verschuier. Image credit: Wikimedia Commons.

C/2006 P1 McNaught observed over Paranal Observatory, Chile. Image credit: Emmanuel Jehin
The final case is the most straightforward: ISON survives its brush with fiery solar death and reemerges with enough nuclear material to continue to be active. It could still, of course, disintegrate for no apparent reason at it journeys back to the outer reaches of the solar system, but this would likely be a spontaneous, unpredictable event. The CIOC has spent the better part of a year refusing to speculate on how bright ISON might get near perihelion so I won’t make any predictions here. However, if it survives in tact, it would likely lose enough dust near the Sun to produce a really nice tail. In a realistic best case scenario the tail would stretch for tens of degrees and light up the early morning sky like comet C/2006 P1 McNaught in 2007 (pictured at near right). In a “pie in the sky” scenario, we might be dazzled with a tail spanning half the sky and a coma that remained visible for months, like the sungrazing comet of 1680 (pictured at far right). The best of all possible worlds for everyone would be if ISON broke up just a bit, say into a few large pieces, enough to throw out extra material that will make the comet really bright from the ground, while giving astronomers pieces of a comet to study for months and months.

Regardless of whether ISON became the comet of the century or just a naked-eye object, everyone would be thrilled with the apparition. Astronomers would have had the chance to study a unique comet traveling straight from 4.5 billion years of deep freeze into a near miss with the solar furnace, with perhaps the largest array of telescopes in history, and arm chair astronomers across the northern hemisphere would have been treated to a long awaited easy naked eye comet (making up a bit for having missed out on the best views of both C/2011 W3 Lovejoy and C/2006 P1 McNaught).

Wrapping it up

As I’ve detailed above, just about any conceivable scenario for how the next 3-4 weeks play out for ISON results in very interesting scientific results. While there is a non-trivial chance that ISON might not make it all the way to perihelion and leave us disappointed that we missed a chance to see a naked eye comet (and leave people like myself stuck with a plane ticket to an observing run we no longer need), in no way should ISON be considered a dud. The very fact that you are still reading this blog post after some 1400 words attests to the impact ISON has had. While ISON probably won’t become the “Comet of the Century,” the process of hundreds of professional and amateur astronomers trying to piece together an understanding of this unique comet in real time has achieved something rare in today’s society: teaching people how science works.

So, hang on, the ride isn’t over yet. By all measures, ISON is certainly NOT a boring comet!