We should have known better. Early predictions called for Comet 2009 R1 (McNaught) to get as bright as 2nd magnitude. As recently as 2 weeks ago, I posted that it was on pace to reach magnitude 3.5 to 4.0. Now even that fainter magnitude was too bright. For the past 2+ weeks the comet has failed to brighten as seen from Earth. The plot below shows a fit (red line) to visual observations from early April to June 10. If the comet had continued to brighten at that rate it would be brighter than magnitude 4 by now. The blue curve is a fit to observations since June 10 and shows a comet that is at its peak and may even fade a little before reaching perihelion.
How can the comet fade before reaching perihelion? The next plot shows how we can more accurately measure the brightness behavior of a comet. First the comets magnitude is normalized to a distance of 1 AU from Earth and to a phase angle of 0 degrees. Normalization to a geocentric distance of 1 AU is done by subtracting 5 log (Δ) from the magnitude, where Δ is the distance from Earth in AU at the time of the observations. Normalization to 0 degrees phase angle is done with Joseph Marcus’ function for dust scattering. I’m not going to go into any more detail on the Marcus function but interested readers will find it in the journal International Comet Quarterly, Vol. 29, No. 4.
The lightcurve shows a comet brightening at a normal and healthy rate until the comet reached a distance of 0.7-0.8 AU from the Sun. A brightening rate of 9.0 log r is in the ballpark for a long-period comet like McNaught. (Note, I discarded a few April observations from the fit because they seem to be a few magnitudes to faint. At this preliminary stage, their exclusion produces a more realistic fit to the April/May/early June data.) Sunward of ~0.7-0.8 AU the comet’s brightening slows considerable.
Another way to plot the data for McNaught suggests that the comet is producing less gas and dust and in an absolute sense has been fading for 2 weeks now. The below plot is not only normalized to 1 AU from the Earth and to o degrees phase angle but also to 1 AU from the Sun. In this case a 5 log (r) term is subtracted from the visual magnitude in addition to the 2 normalization terms mentioned above. An object that was releasing a constant amount of gas and dust would have a horizontal line with no slope on the plot, an object releasing more gas/dust as it approached the Sun would show a positive slope (line going up and to the right) while an object releasing less gas/dust would have a negative slope (line going down and to the right). For some reason Comet McNaught saw its gas/dust production start to decrease after reaching a distance of 0.7-0.8 AU from the Sun, or roughly around the date of June 10.
Extrapolating the latest brightness trends gives us a better idea of where the comet stands when compared to other comets that had perihelia of ~0.4 AU.
So what’s happening to Comet McNaught? The comet is a dynamically “new” comet. In layman’s terms the comet is on its first pass through the inner Solar System after spending the past 4.5 billion years in deep freeze in the outer reaches of the Solar System. We can tell this by its orbit which is nearly parabolic. The early brightness was due to very volatile ices which sublimate at low temperatures. These very volatile ices have now been depleted and the comet is relying on the usual assortment of cometary ices (water ice, carbon monoxide, etc.) to drive the creation of a coma and tail. So in a way, the comet faked us out by acting bigger and brighter than it really was.
“New” comets have been seen before and they have disappointed before. The most famous was Comet Kouhotek in 1973/74. Back in my early years of comet observing, C/1989 X1 (Austin) was predicted to be a bright comet. I even remember an issue of Sky and Telescope magazine with a cover tagline of “Monster Comet Coming”. As the lightcurve below shows, the comet brightened normally until it was about 1.2 AU from the Sun. After that the comet’s dust/gas production decreased even though perihelion was at a small distance of 0.35 AU. Rather than a brilliant naked eye comet, Austin was a 5th magnitude fuzzball even though it passed much closer to Earth than Comet McNaught ever will.
As for the future of Comet McNaught, it will poorly visible very close to the horizon for the next week or so. After that the comet will be forever invisible to northern observers. Perhaps observers in the southern hemisphere will get a shot at a rapidly fading McNaught in a few months. After that the comet will probably never be seen again. Its current orbit is hyperbolic which means the comet will be ejected from the Solar System never to return.