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Two Naked-eye comets for February 2013

posted Feb 8, 2013, 9:45 PM by Mike White   [ updated Mar 1, 2013, 1:44 PM ]

The big comet of the summer is supposed to be comet C/2011 L4 (Panstarrs). When discovered back in June of 2011 the brightness of the comet at a distance of ~8 AU (halfway between the distances of Jupiter and Saturn) suggested this comet could be a brilliant negative magnitude at perihelion(closest point to the sun) in March of this year.

There were some questions whether this comet was on its first passage through the inner Solar System or if it had been through the neighbourhood before. This is an important distinction because comets fresh from the Oort cloud have a tendency of being very active while far from the Sun. Then as the most volatile ices are sublimed off, the comet settles down into a less active state and never gets quite as bright as predicted. We have seen this many times in the past with comets being lauded as “great” or “future” comets, only to disappoint when they finally reach perihelion. Comet Cunningham in 1941, Kohoutek in 1973/74 and Austin in 1990 are prime examples.

Comet PANSTARRS has an orbit that is almost indistinguishable from parabolic meaning the comet is likely to be a fresh comet from the Oort cloud. This fact had many people doubting whether it would really reach magnitude -1 as predicted by its early behaviour. Now that the comet is once again observable (though only from the Southern Hemisphere) it does appear the comet‟s brightening has slowed down and the comet will only reach a magnitude of +2 to +3 if that.

Recent visual observations place the comet between magnitude 7.0 and 7.5. It will continue to be a southern-only comet till mid-March when it will become visible in the early evening sky for northern hemisphere observers. Even with apeak brightness of “only” 2nd or 3rd magnitude, the comet will be quite a sight in binoculars and small telescopes.

Last month Comet PANSTARRS looked on pace to peak around magnitude -1 near its March 10th perihelion. But over the past few weeks the comet has not been brightening as quickly as hoped. As a result it is likely that the comet will be much fainter than -1 at its peak brightness and more along the lines of 2nd to 4th magnitude. The comet will still be a nice sight especially in binoculars and telescopes.
The comet was first seen by the Hawaiian based PanSTARRS asteroid survey on June 6, 2011 at a large distance of 7.9 AU from the Sun. At perihelion it will approach within 0.30 AU of the Sun. The comet is a new Oort cloud comet meaning it is making its first passage through the inner Solar System. The fact that it is a new Oort cloud comet explains its failure to brighten as quickly as first predicted. These sorts of comet often appear relatively bright when far from the Sun because they still contain a large amount of very volatile ices. As the comet approaches the Sun, these ices sublimate and the comet brightens at a slower rate.

This month, the comet starts at a distance of 1.0 AU(Astronomical Unit = distance from the earth to the sun) from the Sun with that distance dropping to 0.43 AU at the end of the month. A few recent observations place it at magnitude ~6.5-7.0. If it continues to brighten at its current rate it should become a naked eye object by mid-month. The comet will only be observable from the Southern Hemisphere this month as moves from the constellation of Sagittarius to Piscis Austrinus. Northern observers will have to wait till late March when the comet should be a naked eye object.

Ephemeris for C/2011 L4 (PANSTARRS):
Date RA DEC Delta r Elong V
2013 Feb 01 19h 21m -45d 22' 1.649 1.009 34 6.8
2013 Feb 10 20h 26m -45d 15' 1.427 0.822 34 6.0
2013 Feb 19 21h 49m -41d 06' 1.238 0.626 30 4.5
2013 Feb 28 23h 16m -29d 13' 1.118 0.431 22 3.0
RA = Right Ascension, DEC = Declination, Delta = distance from Earth in AU, r = distance from the Sun in AU, Elong = elongation from Sun in degrees, V = Visual magnitude

C/2012 F6 (Lemmon)

If Comet PANSTARRS is an example of the “over-hyped” comet that is “under-performing”, Comet Lemmon is the exact opposite. When discovered by the Mount Lemmon Survey last March, the comet was nothing special. Even with a perihelion distance of only 0.73 AU, the comet looked too faint to amount to much. I even placed it on my watch list for small comets that were likely to disintegrate and not survive perihelion. Instead this comet has been brightening at a much faster rate than predicted and is now the brightest comet in the sky.
Back on January 9 I spotted it in my 30×125 binoculars at magnitude 7.9. It was an easy object even though it only got ~10° above the horizon as it was speeding to the south. Its southward motion now means the comet is only visible for Southern Hemisphere observers. The most recent visual observations place is around magnitude 6.5. If it continues like this it could rival or even surpass Comet PANSTARRS in brightness at magnitude +2 to +3 when it reaches perihelion at the end of March. For those of us up north the comet will again become visible in late April/ early May. At the time the comet will be fading but should still be around 5th magnitude.

Everyone was expecting C/2011 L4 (PANSTARRS) and C/2012 S1 (ISON) to be the two naked eye comets of 2013. No one was expecting C/2012 F6 (Lemmon) to be vying for the status of naked eye comet when it was discovered by Alex Gibbs of the Mount Lemmon survey back on March 23, 2012. At that time the comet was a faint 20th-21st magnitude and 5.0 AU from the Sun. It also appeared to be an intrinsically faint comet. So faint in fact that I thought it had a good chance of disintegrating near perihelion. Instead the comet has brightened at a rapid rate. If this brightening trend continues the comet may be a fine naked eye object this February through May. Perihelion will occur on March 24, 2013 at a distance of 0.73 AU from the Sun.

Over the past few days visual observers have estimated the comet at magnitude 6.2 to 6.5. The comet is already too far south for most northern observers and the comet will continue to travel deeper into the southern sky this month. As a result, this comet will only be visible to southern observers till May. The comet starts the month around magnitude 6.3 and will continue to rapidly brighten all month. By the end of the month the comet may be as bright as magnitude 6.0. It will be traveling through the far southern constellations of Musca, Octans, Tucana and Phoenix.

Ephemeris for C/2012 F6 (Lemmon):
Date RA DEC Delta r Elong V
2013 Feb 01 13h 32m -81d 08' 0.993 1.221 76 6.3
2013 Feb 10 23h 00m -81d 07' 0.993 1.098 67 5.5
2013 Feb 19 23h 52m -65d 58' 1.053 0.981 57 4.5
2013 Feb 28 00h 03m -52d 43' 1.154 0.877 47 4.0

Not good news that PanSTARRS may no longer be a brilliant comet. With the dust tail evident in recent photos this will still be a visually impressive comet especially in binoculars and small telescopes. As stated above Comet Lemmon may even be brighter and will be an obvious fuzz ball with a razor thin ion (gas) tail. This was similar to what happened in May 2004 when we had two naked eye comets in the sky at the same time Comet C/2002 T7 LINEAR and C/2001 Q4 NEAT. The former was very much like our new friend Lemmon. The gas tail stretched out to 43 degrees photographically. Very impressive. Q4 NEAT was somewhat more squatter, but had greater dust production. The big difference this time is that two comets are in the same part of the sky coming as close as 25 degrees apart, and PanSTARRS will offer a better viewing angle of its tail than NEAT back in „04 . Whatever happens we should be in a great position to watch the display!

Feb 2013 Comets

with additional comments and images by Ian Cooper.