COMET HIGHLIGHTS for NOVEMBER 2013
2013-Oct-30
November marks the perihelion passages of two bright comets, the dependable 2P/Encke and this month’s big question mark C/2012 S1 (ISON). Two other comets will also be brighter than 10th magnitude this month including one that may approach naked eye visibility.
Comet Spotlight
C/2012 S1 (ISON) [T = 2013 Nov 28, q = 0.012 AU]
As of the end of October, ISON is still only the 4th brightest comet in the sky. But with a November 28 (Thanksgiving Day in the US) perihelion passage that will bring the comet within 1.2 million km of the Sun’s surface, ISON may become a spectacular sight near perihelion and for weeks afterwards. I wish I could say with some certainty what the comet will do ,even a short four weeks from now, but it’s on-again, off-again personality continues. The comet was observed to rapidly brightened between September 2011 till January 2013. This was followed by a slow decrease in intrinsic activity between January and June of this year. Lost in the glare of the Sun till August the comet seemed to be back on track when it reappeared.
As of mid-October CCD photometry was raising a big red flag. Even though V-band photometry showed a healthy comet brightening at a normal, or even above normal, rate, R-band photometry showed a comet that was barely brightening at all. It was if the comet had a split personality, increasing gas production but little increase in dust production. This divergence couldn’t last and visual and CCD observations over the past two weeks show that is hasn’t. Unfortunately the the comet’s brightness in V has now slowed down to match what we’ve been seeing in the R images.
All bets are off as to what ISON will do in the coming weeks. What seems to be certain is that the comet will not become a brilliant object pre-perihelion and may not even reach naked eye brightness prior to being lost in the glare of dawn.Will the comet experience a surge in brightness around perihelion, will it split, disintegrate, or just solider on as a healthy but intrinsically faint comet? We can wait and watch.
ISON starts the month in Leo at 0.98 AU from the Sun and 1.23 AU from Earth. It will travel the length of Virgo and Libra before encountering the Sun near the head of Scorpius. Depending on how bright the comet gets, observers with clear skies and horizons may be able to follow the comet to within a week or less of perihelion. If the comet becomes brighter than ~ -4 at perihelion it may even be visible to telescopic observers during the day for a few hours around perihelion. Assuming it survives perihelion, the comet will once again become visible in early December.
InboundComets
2P/Encke [T = 2013 Nov 21, q = 0.34 AU]
Encke has the shortest known period of any comet at 3.3 years. Since it was first seen in 1786, the comet has been observed to orbit the Sun over 60 times. On October 16/17 it reached a minimum distance of 0.48 AU from Earth. With perihelion on November 21, Encke will also be quickly descending into the glare of dawn as it races along the length of Virgo into Libra.
At the start of the month, Encke will be 0.62 and 0.60 AU from the Sun and Earth, respectively. Most observers will lose sight of the comet by mid-month as the comet drops to within ~0.4 AU of the Sun. As of late October, the comet was between magnitude 7.5 and 8.0 and it should reach magnitude 5.5 to 6.0 by mid-month.
C/2013 R1 (Lovejoy) [T = 2013 Dec 22, q = 0.81 AU]
Perhaps the brightest comet in the sky right now (magnitude 7.0 to 7.5) is a relatively new discovery. On September 7 Terry Lovejoy, an amateur astronomer from Queensland, Australia, discovered comet C/2013 R1 (Lovejoy) on CCD images taken with a HyperStar equipped 8″ SCT. The comet will close to within 0.40 AU of Earth on November 19/20 and perihelion on December 22 at a distance of 0.81 AU from the Sun. Unlike ISON, Lovejoy is a dynamically old comet and has been rapidly brightening since discovery. If its brightening rate continues the comet may be a borderline naked eye object by the end of the month at magnitude 5.0 to 6.0 when it will be 0.91 and 0.41 AU from the Sun and Earth. The comet starts the month in Cancer and quickly traverses Lynx, Ursa Major, Canes Venatici, before ending the month in northern Boötes.
C/2012 X1 (LINEAR) [T = 2014 Feb 21, q = 1.60 AU]
Comet LINEAR was discovered back on December 8, 2012 at a faint 19th magnitude. The comet has always appeared intrinsically faint and was never expected to get much brighter than ~12th magnitude when it reaches perihelion early next year. Imagine the surprise of CCD observer Hidetaka Sato when he found the comet nearly six magnitudes brighter than predicted on Oct. 20. Like a miniature version of Comet Holmes in 2007, the comet currently sports a rapidly expanding shell of dust with evidence of jets and other coma features.
The comet is a morning object low in the northeastern sky as it moves slowly eastward from Coma Berenices through southern Boötes. Recent visual estimates place it between magnitude 7.5 and 8.5. Though the comet’s total magnitude may change little over the next few weeks, the surface brightness of the coma will decrease as the coma expands making the comet harder to see with time. With perihelion 3-4 months away, additional outbursts are definitely possible.
The Section is always collecting observations (both visual and CCD) and magnitude estimates for all comets. An image gallery has been set up and now contains images of 40 different comets. Over the next few weeks, I will be rolling out dedicated pages to some of the brighter comets. These pages will be updated as needed with new observations and lightcurves.
All ephemerides/positions for the above comets and all other comets can be generated at the Minor Planet Center and JPL/Horizons websites.
- Carl Hergenrother (Comet Section Acting Co-Coordinator)
