Comet 2009 R1 (McNaught) is the center of attention in the early dawn sky. The 5th magnitude comet is brightening as it plunges toward the Sun. Though not an impressive sight to the naked eye or in small binoculars, observers with large binoculars and telescopes (or dark skies) will be well rewarded.
I’ve been able to observe the comet a few times over the past few nights. On Monday morning I drove to the outskirts of Tucson to get a glimpse of the comet under dark skies. Though the sky is dark enough so the Milky Way is bright and distinct, the comet was just barely visible to the naked eye. Even then it appeared as nothing more than a faint star where one shouldn’t be. Based on that observation I put its brightness at magnitude +5.3.
In small 10×50 binoculars, the comet was a small fuzz not much bigger than the stars around it. The coma is only ~6 arc minutes across in the 10x50s so this is not a large comet. The gas tail was about a degree in length but was very faint and barely noticeable.
It took my giant 30×125 binoculars to really show the comet in all its glory. The coma (or head) of the comet was a brilliant blue-green. The color being due to the fluorescence of cyanogen (CN) and diatomic carbon (C2) in the comet’s atmosphere. The faint gas tail extended for over 2 degrees. Even a short 0.3 degree dust tail was visible. Interestingly this comet doesn’t show much yellow coloring which is the color of dust (really the color of the Sun reflecting off the dust). The lack of dust makes it a dust-poor or, conversely, a gas-rich comet.
The comet is currently 0.61 AU from the Sun and 1.14 AU from Earth. In 2 weeks it will be at perihelion and only 0.41 AU from the Sun though at a more distance 1.30 AU from Earth. There have been some published reports that the comet will become a brilliant object at 2nd magnitude. Unless there is an unexpected outburst the comet will not get that bright. Based on brightness measurements over the past few months, the comet is on track to peak at a magnitude of 3.5 to 4.0 (see figure below).
Unfortunately the comet will also be dropping closer and closer to the Sun. Even though 3rd magnitude is rather bright for a comet, its close location relative to the Sun will make it very difficult to observe. Most observers will be unable to observe the comet in another week or so, if not already. Even from my dark sky location, the mountains to the north of Tucson are already making it difficult to see the comet before the start of dawn.
With a little more analysis we can determine if Comet 2009 R1 (McNaught) is a typically bright comet. The plot below shows the peak brightness near perihelion for comets with perihelion distances less than 1.5 AU. The brightness is not the apparent magnitude (how bright the comet looked to observers on Earth) but the heliocentric brightness (brightness of the comet normalized to 1 AU from the observer, 1 AU from the Sun and at a phase angle of zero degrees). This is done to “place” the comets at the same distance and orientation from the Sun and observer and allow a direct comparison between each comet. For example a candle a few feet from you looks much brighter than a distant star but it is the distance that makes it appear brighter. If both candle and star were located at the same distance the star would be many times brighter.
Since Comet McNaught is not yet at perihelion and we don’t know how bright it will be at that time, we plotted its brightness and orbital circumstances for June 14th. It is obvious that it’s a comet of just above average brightness for a comet at that distance from the Sun (the average heliocentric brightness is the blue line). Note that it is about 2 magnitude (6 times fainter) than Comet Halley. I also labeled Comet Hale-Bopp which is an obvious outlier on the plot and truly a once-in-a-lifetime comet.
CCD observers have been producing some great images of the comet and its 2 tails. Check out Michael Jager’s website “Comet Pieces” for some of the best images out there.
Thanks to Gianluca Masi of Rome, Italy for sharing some images he took with a Celestron C14 at the Bellatrix Observatory in Ceccano, Italy. I was able to apply a radial gradient filter to bring out the fine detail. The image below shows a multitude of rays in the gas (ion) tail. These rays are very dynamic and can change on hourly time scales. Though there is some evidence of structure in the head of the comet, it is very subtle. This is too bad because well defined dust jets can be used to measure the rotation period of the nucleus and made out dust-producing active areas.