COMET HIGHLIGHTS FOR August 2013
2013-July-30
Here in Tucson (the home of your Comet Section C0-coordinator), August marks the end of the observing year. In most places, observing slows down in the winter due to bad weather and the cold. But in usually sunny southern Arizona, the summer is the time when our mirror covers stay on. For almost two months, the monsoon rains force the complete shutdown of many professional telescopes and most amateur ones as well. But by September the rains are moving on and we start planning the upcoming observing season.
In a way, the field of comet observation is having a similar month to Tucson. The bright comets of the Spring are now receding from view while we await the “hopefully” rapid brightening of this Fall’s cometary duo.
C/2012 F6 (Lemmon)
As has been the case for the past two months, Comet Lemmon remains the brightest comet in the sky and the only comet that is a relatively easy object for visual observation. Now over 4 months past perihelion (T = 2013 Mar 24, q = 0.73 AU), the comet continues its steady fading as it recedes from 1.88 to 2.70 AU from the Sun and 2.04 to 2.38 AU from Earth this month. The comet should be magnitude 9.7 on the 1st, 10.0 on the 11th, 10.3 on the 21st and 10.6 at the end of the month. Lemmon is a northern circumpolar object and starts the month in Cepheus before entering Draco.
C/2011 L4 (PANSTARRS)
Comet PANSTARRS is much faded from its March glory days when it peaked at 1st magnitude (T = March 10, q = 0.30 AU). August sees the comet moving south through the constellation of Bootes. Based on recent magnitude estimates the comet should fade from magnitude 11.4 to 12.3. Though this will probably be the last month this comet will be listed in the Comet Highlights, CCD and large aperture visual observers are encouraged to keep track of it as it slowly fades over the coming months and years.
Inbound Comets
C/2012 S1 (ISON)
I wish I had some real news to report about Comet ISON (T = 2013 Nov 28, q = 0.01 AU). The supposed ‘Comet of the Century’ has been hanging out too close to the Sun since mid-June to be seen from Earth. When last spotted it was around 15th magnitude. Unfortunately it’s been stuck at 15th magnitude since January. Of course, that’s 15th apparent magnitude (the brightness the comet appears to us on Earth). Intrinsically, the comet has actually faded by ~2 magnitudes since January.
Though this doesn’t bode well for a -8, -12 or -16 magnitude comet as reported in the press, comets fading or at least stalling in their brightening is not unheard of. In fact it is fairly common for dynamically new comets making their first trip into the realm of the planets after billions of years. What is odd about ISON’s behavior is that it is experiencing its stall or fading so far out from the Sun. Most dynamically new comets are closer to the Sun when they ‘misbehave’ and most only see their rate of brightening slow down rather than go into reverse.
ISON will reappear in the morning sky this month as it transverses Cancer. Hopefully the first CCD observations will roll in around mid-month. By the end of the month the comet will still be low in the sky (~10 degrees up at the start of astronomical twilight for northern mid-latitude observers, southern comet watchers probably won’t be able to image ISON this month). It’s anyone’s guess as to how bright ISON will be when recovered. When last seen the comet was 3.7 AU from the Sun. It will be 2.42 AU from the Sun at mid-month and 2.18 AU at the end of the month. If the comet is imaged around the middle of the month, accurate photometry will give us a good clue as to what the comet has been up to since June. If it’s still only magnitude ~15.5 it means the comet has continued fading at the same rate as it has since the start of the year. If it is around magnitude 14.5 then the comet is intrinsically the same brightness as it was in early June. On the other hand if the comet is brighter than ~14.5 then it means the comet has started intrinsically brightening once again.
2P/Encke
If ISON is a new comet inbound likely for the first time in billions of years, the other bright comet of the Fall is its opposite. Comet 2P/Encke (T = 2013 Nov 21, q = 0.34 AU) is the comet with the shortest known period at 3.3 years. Since it was first sighted in 1786, the comet has been observed to orbit the Sun over 60 times. Even though the comet is starting the month only 1.94 AU from the Sun (that distance decreases to 1.57 AU at the end of the month), it is a late bloomer and may still be nearly inactive this month. In fact, the most recent observations place it around 19th magnitude. CCD imagers are encouraged to routinely image this comet over the next few months as it rapidly transforms from a 19th magnitude “asteroid” to a 6th or 7th magnitude “fuzzball” in late October.
C/2012 K1 (PANSTARRS)
While ISON may be getting all of the headlines due the possibility of becoming a brilliant object, C/2012 K1 is actually a much brighter comet intrinsically (T = 2014 Aug 27, q = 1.06 AU). Based on recent CCD observations C/2012 K1 is intrinsically ~2 magnitudes brighter than ISON and very similar in brightness to C/2011 L4 (PANSTARRS) at a similar distance from the Sun (5.2 AU). Current projections have it reaching 5-6th magnitude next summer which is impressive considering the comet will be located over 2 AU from Earth on the other side of the Sun at the time. We can only imagine how bright this comet would be if it came closer to Earth.
This August the comet decreases its heliocentric distance from 5.05 to 4.74 AU located over 5 AU from the Sun. Even at this distance it is ~14th magnitude and already showing significant jet activity.
As explained above in the ISON section, K1 is a dynamically new comet. Such comets routinely brighten rapidly when far from the Sun only to see their rate of brightening slow, stall or even turn into a fading trend as they get closer to the Sun. Observers are encouraged to monitor K1 as it nears the heliocentric distances when C/2011 L4 and C/2012 S1 experienced their change in brightening.
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.
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)
