How Bright is Comet ISON?

The CIOC frequently gets asked how bright Comet ISON will be at a particular time in the future. Our standard response is that comets, especially ones entering the inner solar system for the first time (like ISON), behave unpredictably and it is therefore impossible to reliably predict what ISON will do. What we don’t often mention is that it is actually rather difficult to say conclusively how bright ISON or any other comet is right now!

You are probably saying to yourself “That’s ridiculous. Just take an image and measure how bright it is!” If only it were that easy…

I don’t want to go into too much detail because there are literally entire books written on how to accurately measure the brightness of celestial objects. However, I would like to briefly discuss a few of the reasons why it is difficult to come to a consensus answer for how bright a particular comet is.

1. The measured brightness of a comet depends on how large an aperture is used, with bigger apertures yielding brighter measurements.
2. Background stars can contaminate the aperture and bigger apertures have more background stars. Furthermore, two observations made at different times can yield different measurements due to different background stars being in the aperture because the comet moved.
3. The manner in which the background is calculated can affect the measured brightness, especially if the background is taken too close to the comet, the moon, or the Sun so too much background is removed.
4. Stars used to calibrate an image are generally of different spectral types and since different spectral types suffer differing amounts of extinction by the Earth’s atmosphere, the calibration varies depending on if the comet is observed overhead (the light passes through relatively little atmosphere) or near the horizon (the light passes through a lot of atmosphere).
5. The brightness depends on the wavelength of light in which the observation is made, so a measurement in an R-band image (which is mostly just reflected solar continuum) will yield a different measurement than a measurement in a V-band image (which has both reflected solar continuum and prominent emission from gases).
6. Different CCDs have different sensitivities and can therefore skew results. For observers making brightness estimates by eye, the human eye has a different response than a CCD and people’s eyes have different sensitivities.

I could go on, but you get the idea that there are a lot of variables that can affect the brightness measurement. Because each observer has their own method, uses their own equipment, and collects their data at a unique time and place, these various effects result in quite a bit of uncertainty in the brightness. This uncertainty is obvious when you look at the plot of the brightness of Comet ISON as reported to the Minor Planet Center. The vertical scatter in the red points for any given date is about 2-3 magnitudes. This corresponds to about a factor of 10 in brightness!

If a factor of 10 uncertainty in brightness is concerning, check out this plot I made of the reported brightness of Comet Hale-Bopp (C/1995 O1) using the Minor Planet Center’s database. Hale-Bopp was considerably brighter than Comet ISON at comparable distances. One would think that being brighter it would be easier to get a more accurate measurement for Hale-Bopp since many of the effects I mentioned above preferentially affect fainter objects. In fact, the spread in Hale-Bopp’s measured brightness is often about 5 magnitudes or a factor of 100 in brightness!

After looking at the plot of Hale-Bopp, I think we should feel happy that ISON’s brightness measurements are as close together as they are. I suspect that a lot of the decrease in scatter is because people are better at making accurate measurements than they were in the past. The internet allows observers to much more easily communicate with each other and learn the best techniques, and technological advances have made it possible for many amateurs to have equipment that rivals or even surpasses many professionals. We are seeing this pay off with data that are actually much more coherent than that collected for Hale-Bopp despite ISON being a considerably more difficult comet to measure accurately.

Measuring comet brightnesses is challenging. There are literally hundreds of people around the world forgoing sleep and doing their best to make reliable measurements in order to contribute their little part to our understanding of Comet ISON. On behalf of the CIOC, I’d like to say “thank you” to all those observers.