As can be seen on the Current Status page, C/2013 A1 Siding Spring’s reported brightness declined considerably more than expected over the last week or so. We chose not to speculate about what might be causing the decline on that page, but I’m going to take advantage of the freedom provided by a blog post to expand on what I think might be going on.
For the purposes of this article, I’m specifically talking about how Siding Spring behaves in comparison to the “prediction,” the black line on the lightcurve plot that comes from JPL Horizons. This line accounts for the changing geometry and how an “average” comet behaves as it approaches the Sun. The first part is very straightforward – when Siding Spring is closer to Earth it will look brighter, and when it is farther away it will look fainter. The second part is where things get tricky. It assumes that Siding Spring will brighten at a constant rate as it gets closer to the Sun and fade at that same rate as it recedes. On average, this is approximately how comets behave, but each comet is different. Some brighten more steeply, some less so. Some have rates that change with time, and some even get fainter for stretches while still approaching the Sun. Thus, this curve is really just an educated guess at how Siding Spring might behave.
Until late September, Siding Spring was behaving very well (e.g., predictably). It was running a bit brighter than the forecast, but doing so by a consistent amount. In other words, if you knew how bright it was at a particular time, you could reliably predict how bright it would be at a later time. Recently it has stopped cooperating; it very suddenly went from being a bit brighter than the prediction to being nearly a magnitude (or about a factor of 2.5 in brightness) fainter than the prediction.
This fading doesn’t seem to be an observational bias. Our lightcurve plots data from many different observers and the downward trend in brightness is seen when looking at individual observers’ lightcurves as well as the ensemble of measurements. Furthermore, we see it in both the “total” measurements that seek to record the brightness of the whole coma as far out as possible (the purple points) and in the “nuclear” measurements that record the brightness only in a small region near the middle (most of the red points). In other words, it doesn’t matter who is doing the measuring or where they are measuring, everyone seems to be measuring the same thing.
Why did it fade?
Part of Siding Spring's fading was predictable: due to the orientation of the orbits, the comet is now getting farther away from Earth. This is already included in the brightness prediction, but another factor is not: our view of Siding Spring's tail is changing so we are looking at it more from the side whereas before we were looking down the tail. This change in geometry means that from our point of view on Earth there is less dust close the nucleus. It has likely caused some of the drop in brightness, but is almost certainly not enough to account for the rapid decrease seen over the last week or so.
While not expected, this rapid fading is not unheard of. This sort of behavior is most often seen in “new” comets, those like Siding Spring that are entering the inner solar system for the first time. Just last year, comet C/2012 S1 ISON (also reaching the inner solar system for the first time) went through a long stretch where its brightness gradually decreased relative to the prediction, ultimately fading by about two magnitudes (a factor of ~6 in brightness) compared to a reasonable prediction of what it might do based on what was known at that time. While this was worrisome to those of us watching ISON closely, it recovered from this downturn and evolved quite nicely for another six months before its fatal plunge close to the Sun.
Now that we’ve agreed that fading relative to the prediction shouldn’t necessarily induce panic, let’s consider some possible explanations for the drop in brightness. Going from what I consider to be most to least likely:
1. Siding Spring may be starting to run low on the exposed ices near the surface that have been driving its activity. This could be because the ices are being depleted or that a layer of dust is building up on top of the ices (having been lifted off the surface very weakly by outgassing then fallen back to the surface) and is blocking sunlight from reaching them.
2. There may have been a population of icy grains in Siding Spring’s coma that were released at larger distances and survived until now but are starting to disappear due to the increasing heat.
3. Siding Spring may have been in a long, slow outburst over the last few months that is now ending.
4. Something catastrophic has happened to the nucleus, like having broken up, and what is left is now being rapidly destroyed.
Possibilities 1-3 are all relatively common and have been proposed to explain ISON's behavior when it was at similar distances from the Sun. I consider possibility 4 considerably less likely, as breakups are usually signaled by very sharp increases in brightness prior to fading, while Siding Spring’s lightcurve has increased smoothly. Furthermore, breakups are far more common at smaller heliocentric distances than Siding Spring is now.
Finally, there is the possibility that it’s none of these options. Comets are complex and mysterious objects that are not very well understood (although hopefully they will be much better understood after Rosetta). There’s a very good possibility that Siding Spring is fading for some reason we don’t understand or haven’t thought of yet!
What might happen?
Since I don’t have a good explanation for what has caused Siding Spring to fade in brightness over the last week or so, it is even harder than usual to predict what it will do in the future. This may seem silly, but there are three ways that its brightness could change relative to the predicted curve, and at this point I think they are all viable: fade, stay the same, or brighten.
The simplest case is that it will continue to fade. I say this is simple because I don’t have a good explanation for what is happening so it’s entirely possible that whatever is going on will continue to happen and Siding Spring will continue to get fainter than the prediction. This would be disappointing from the standpoint of observers from the Earth looking to get pretty pictures and from the spacecraft at Mars (like Maven) that are hoping to measure the effects of the comet’s coma interacting with Mars’ atmosphere. However, it would have one really nice benefit: it would make it much easier for the high resolution instruments at Mars to detect the nucleus (especially HiRise on MRO). The big problem with studying comet nuclei is that when they are close to Earth they are active so the nucleus is obstructed by a lot of gas and dust. If Siding Springs continues to fade, there will be a lot less gas and dust in the way!
The next possibility is that it will remain about the same distance below the curve as it is right now. In this scenario, Siding Spring will go back to behaving like an average comet, just one with a slightly lower level of activity than we saw before. This would have minimal effects on observations from Mars since it is still pretty close to the predictions from which the various Mars teams have been making their observing plans all along.
The final possibility is that Siding Spring might brighten and outperform the predictions. This isn’t necessarily as unlikely as it may seem. If the drop in activity was caused by a layer of dust building up over ices, there could still be enough heat penetrating below the dust that some ice sublimates to gas, causing pressure to build up below the dust and eventually becoming strong enough to blow the ice off and resume normal activity (think of this as a geyser). Alternatively, depending on the orientation of Siding Spring’s pole, the same hemisphere may have been pointing towards the Sun the whole inbound journey leading to loss of ices in the hemisphere that was in “summer.” Over the next few weeks, the orientation of Siding Spring’s pole relative to the Sun will change significantly as it reaches perihelion and begins to move away from the Sun. This could cause sunlight to shine in the hemisphere that was previously in “winter” for the first time, resulting in lots of fresh ice to drive more activity. If this happens, observers from the Earth will get a better show and the instruments at Mars prepared to study the coma interacting with Mars will have an easier time, but observations of the nucleus itself will be harder. It is important to note that even new, more vigorous, activity over the next two weeks is unlikely to be a significant hazard to the Mars spacecraft fleet. As Tony explained recently, “any dust grains that are going to hit Mars would have been ejected from the comet over a year ago.”
To wrap this up, I’ll remind everyone of the famous quote by David H. Levy of comet Shoemaker-Levy 9 fame: “Comets are like cats; they have tails and they do precisely what they want.” We can’t say with any certainty what Siding Spring will do between now and when it arrives at Mars, so we’ll just keep observing it and look forward to some very exciting results from the Mars fleet.