Why I’m not particularly worried about ISON’s brightness

Comet ISON is currently hiding behind the Sun and, with no observations of it since early June, there has been a fair bit of speculation recently about what it has been doing. Unfortunately, much of this has been has been of the negative variety, and I’d like to address it clearly here.

It’s premature to write ISON off.

The bulk of the critiques of ISON’s prospects are due to the flatness of its lightcurve since early 2013. This is something that I and many others have been paying close attention to for quite some time. In fact, we have been showing the magnitudes published by the Minor Planet Center since this website went live, and I have been showing similar plots at talks I’ve been giving on ISON since November 2012.

The fact that ISON’s brightness began flattening out early in 2013 did not go unnoticed, with JPL's Horizons updating their lightcurve parameters (what I use to draw the curve on the lightcurve plot we display on this website) as sufficient new data were obtained. Updates to the lightcurve parameters have continually lowered the predicted brightness. As astronomers plan their observations, these lowered expectations have been continually taken into account. We’d obviously love for ISON to outperform these projections and live up to the unfortunate hype that is still floating around on the internet. However, most (and hopefully all) observations are being planned around the most conservative estimates for the brightness.1

Furthermore, the flattening out of ISON’s brightness was not terribly surprising for a couple of reasons. First, by a quirk of the Sun-comet-Earth viewing geometry, ISON’s distance from the Earth remained nearly constant from January through June, and actually increased from mid-February through mid-May. Coupled with that, the phase angle changed from mid-January through mid-April in a way that steadily lowered the brightness. These two effects worked together to offset much of the gains in brightness that ISON made due to the decreasing distance to the Sun during the interval, and is in stark contrast to the behavior from discovery in September 2012 through January 2013, when all three effects worked in unison to increase ISON’s brightness.

Even when normalizing for these effects, ISON still hasn’t brightened as much as initial projections suggested it might. But, again, this shouldn’t have been shocking news to those familiar with observations of comets making their first passage through the inner solar system. University of Maryland professor emeritus Mike A’Hearn gave a fantastic discussion of this at last week’s Comet ISON Observer’s Workshop (skip to the 15:40 mark). For reasons that are not entirely understood, these “dynamically new” comets tend to brighten at a much slower rate than comets that have been around the Sun before. ISON’s initial rate of brightening was much higher than is typical for dynamically new comets and, sure enough, it slowed down dramatically soon thereafter.

Given the evidence that I’ve detailed above, why am I still optimistic about ISON’s prospects? Because all of the observations obtained so far of ISON have occurred beyond the “frost” line. The frost line -- also sometimes called the “ice line,” “snow line,” or “water line" -- is the distance at which water ice begins to sublimate vigorously. The distance depends on a few variables, but is roughly between 2.5 and 3.0 AU, about the distance of the asteroid belt. Beyond this distance, it is too cold for water ice to sublimate efficiently and it remains almost completely frozen. Inside of this distance, it is warm enough that water ice sublimates into a gas easily.

Comets are active because ices at or near the surface are heated enough to sublimate into gas. The gas drags dust grains along with it, creating the fuzzy coma and tail characteristic of comets. Beyond the frost line, the main gases responsible for sublimating and causing the comet to be “active” are those that sublimate more easily (e.g. are more “volatile”) than water. The principal drivers of activity in comets beyond the frost line are carbon monoxide (CO) and carbon dioxide (CO2). While we don’t know the exact ratio of these to water in ISON yet, we do know the ratios in more than a dozen comets, and in all cases, the amount of water dwarfs the amount of CO or CO2. Thus, when the comet passes inside the frost line, there is a whole lot more material capable of driving activity than there was beyond the frost line. As a result, I expect ISON to begin to increase in brightness more quickly the rest of the way into perihelion than it did the first half of this year.

One final point I’d like to address is the idea that ISON has already run out of volatiles and is already beginning to fade. Comets are notoriously unpredictable so there is certainly a chance that this is true. However, I see this as unlikely. As discussed above, ISON hadn’t yet reached the frost line during all observations to date, so unless it had little to no water to begin with (something that is unheard of in a comet), it should not have already lost its water. Comparisons to comets that had flat lightcurves before disappearing aren’t really comparing apples to apples if one comet is in the region where water should have been vigorously active for a while, whereas ISON hasn’t even begun water ice sublimation yet.

Also, short-lived gases have been detected in ISON which would only be expected to be around if there was some ongoing activity producing them. Cyanogen (CN) gas was detected by Dave Schleicher at Lowell Obsevatory in March and May (unfortunately subscriptions are required to read these links). NASA’s Spitzer Space Telescope detected CO or CO2 (the bandpass includes both gases, hence the uncertainty) on June 13. The fact that these gases were detected argues strongly that ISON was still active at these times and hadn’t already run out of volatiles.

This whole article will likely be moot in a few weeks. ISON is currently at 2.57 AU and should therefore pass comfortably inside of the frost line by next week. As soon as ISON is next observed, its brightness will reveal if water-driven activity has “turned on.” As I mentioned at the start of the article, ISON is currently behind the Sun as viewed from Earth. Who will next observe ISON is a topic too long for this article, but keep a look out for new results from either the Deep Impact Flyby spacecraft2, one of the fleet of Mars missions, or humble observations from the surface of Earth. The new observations will soon give us a much better idea of what to expect from ISON from here on out.


1Close inspection of the current lightcurve plot reveals that the data ran consistently below the predicted curve in April and May. I would argue that the JPL Horizons estimate is still too optimistic and should be revised downward a bit more, resulting in a further reduction of the peak brightness near perihelion.

2As an admittedly biased aside that solely represents my own opinions and not those of the rest of the CIOC, NASA, or anyone else, Deep Impact Flyby is a really cool mission. The same spacecraft that flew by Comet Tempel 1 during the Deep Impact encounter then took amazing images of Comet Hartley 2 (and did some exo-planet studies to boot) during EPOXI is now being used as a remote telescope. Because it is in a heliocentric orbit, it is often far from the Earth and can observe things from different viewing geometries than the Earth and, at times like now, things that are unobservable from Earth. It also has the only instrument anywhere capable of simultaneously observing CO, CO2, and H2O which, as I discussed earlier, are the primary drivers of cometary activity and are therefore very interesting to study in comets. It's currently running on a shoestring budget and is in constant threat of being shut down for good. If you think missions like this are awesome, please contact your senator and/or congressperson and tell them to keep funding planetary science!