Latest on Comet Pons-Gambart

I last reported on newly (re)-discovered comet C/2012 V4 back on December 8. Though comet C/2012 V4 has been linked with Comet Pons-Gambart which was last seen in 1827, it still has not been officially named Pons-Gambart.

Earthbound observers estimated it at magnitude 8.5 to 9.5 during the first half of December. Unfortunately the comet is now too close to the Sun to be seen from Earth. It won’t be seen again till sometime in mid to late January. How bright it’ll be at that time is debatable. There has been some talk on the comets-ml list that the comet continued to brighten after perihelion in 1827. If true and if the comet behaves the same way, the comet may still be bright enough at the end of January for small telescope observers.

As mentioned in my December 8 posting, one of the STEREO sun watching spacecraft has not only had a clear view of the comet but the comet passed within ~0.28 AU of the spacecraft on December 12. The movie below shows the comet rapidly brightening and displaying a lengthening tail as the comet zips past STEREO-B and off its camera FOV. Note that the tail on the last few frames almost extends from one edge of the field to the other. It is a clear reminder of how our view of a comet (faint and not much to get excited about from Earth while a naked eye object with a 20+ deg long tail from STEREO-B) is dependent on our viewing conditions.


C/2012 V4 Spotted by STEREO-B

Comet C/2012 V4 (which is mostly likely a return of Comet Pons-Gambart) was discovered on images taken by the solar observatory spacecraft SOHO (though the SWAN instrument used for the discovery watches the entire sky not just the region near the Sun). But SOHO isn’t the only spacecraft watching the vicinity of the Sun. The comet is currently crossing the field of the HI2 instrument on the STEREO-B spacecraft. Thanks to Marco for posting a link to a YouTube video showing the comet in the STEREO images.


The comet is first visible on images from November 21 and then rapidly brightens and develops a long thin tail by the time of the last high-resolution data from STEREO on December 6. The distance between the comet and STEREO is rapidly decreasing from ~0.9 AU on the 21st to ~0.4 AU on the 6th. The distance will continue to drop to a minimum of 0.28 AU on December 12. This means Pons-Gambart will get much closer to STEREO-B and, as a result, much brighter than seen from Earth. As you can see in the plot of the inner Solar System to the right, STEREO-B is located along the Earth’s orbit but trailing the Earth by 120° or 1/3rd of the way around the Sun.

I downloaded images taken by STEREO-B from their public website and created a movie showing the motion and brightening of C/2012 V4. The comet is very difficult to see at first but becomes much easier to notice halfway through the movie.

The single image below has been annotated to show the direction of motion of the comet as well as the outline of some of the more prominent constellations in the field like Orion, Taurus and Auriga. Also the Earth and Jupiter are also in the field. The Sun is located just off to the left of center. After the comet passes near the Sun it will pop back into view again except this time in the FOV of the HI1 camera which images the space on the other side of the Sun.


Last night I estimated the brightness of C/2012 V4 at magnitude 9.4 from my backyard. At that time the comet was 0.84 AU from the Sun, 1.54 AU from Earth and 0.32 AU from STEREO-B. The phase angle (Sun-Comet-Observer angle) from Earth was 35° and 131° from STEREO-B. Correcting for the difference in distances and phase angles means magnitude 9.4 from Earth translates to a magnitude of [9.4 – 3.4 (due to distance) – 0.8 (due to phase angle)] = ~5.2 from STEREO-B.


Note, the movie above was created from differenced images. In order to pull out more details on the solar wind one image is subtracted from another. That is why the image shows each star and even the comet as positive-negative pairs. The solar wind is the wispy material seen blowing from the left to right.

Comet C/2012 V4 at magnitude 9.4

Last evening (Dec. 7 UT) the sky was finally free of cirrus and I was able to see newly discovered comet C/2012 V4, the first probable return of Comet Pons-Gambart since 1827. This evening (Dec. 8.06 UT) I was able to spot the comet again using Mars as a guide since the two bodies are within a few degrees of each other.

In short, this was a very difficult comet to observer. It was invisible in my 30×125 binoculars and I could only see it with averted vision in a 12″ dobsonian. Though the sky is relatively dark at my house (Milky Way always visible and LM of ~+6.0), the comet was located within 10° of the horizons and in the direction of the skyglow from Tucson. I estimated a brightness of 9.4 and a coma 1′.5 across. If C/2012 V4 were located overhead it wouldn’t have been a problem. For example, 168P which was also around magnitude 9.4 in early October was visible in 10×50 binoculars but it was much higher and located away from city lights.

Unless the comet undergoes a rapid brightening soon it will only get harder to observe as its elongation slowly decreases.

Comet Pons-Gambart Seen Again After 188 Years

On June 21, 1827, French astronomers Jean Louis Pons and Jean-Félix Adolphe Gambart discovered a comet among the stars of Cassiopeia. Both men were prolific comet finders. Pons was the most prolific discoverer of comets up until the modern era and still holds the record for most visual discoveries. A record that is unlikely to ever be broken. Between 1801 and 1827, Pons found 26 comets. Comet Pons-Gambart was his second to last comet find. Though not as prolific as Pons, Gambart is credited with 5 comet discoveries between 1822 and 1834. Comet Pons-Gambart was his 3rd find.

As the comet was already a few weeks past perihelion at discovery, it was only observed for ~1 month before it faded. Over the years, orbit computers have noticed that Pons-Gambart was on an obvious elliptical orbit and determined periods between ~45 and 65 years. The only problem was with periods that short the comet should have returned at least 2 to 4 times since 1827. Perhaps the comet was fainter now or even broke up in the intervening years to explain why it was constantly being missed.

Fast forward to this year… Robert Matson of Newport Coast, CA found evidence of an unknown comet on images taken with the SOHO spacecraft. SOHO’s SWAN imager is used to map the Lyman-α emission of the solar wind. SWAN is also very good at detecting hydrogen was dissociated water molecules released by comets. As a result, SWAN has been used to discover comets. Matson noted the presence of a comet on SWAN images from Nov. 7, 10, 11, 13 and 19. He then informed a number of observers about the new find and on Nov. 29 Terry Lovejoy of Australia found the comet.

Before the comet was even formally announed, Maik Meyer of Limburg, Germany noticed the similarities between the new SWAN/Matson comet and long-lost Comet Pons-Gambart. There is little doubt that the two are related and are probably the same object. Only problem is the 2012 observations don’t exactly match the 1827 observations assuming orbital periods of 45-65 years. A recent MPEC released by Gareth Williams of the Minor Planet Center found that the 2012 observations are consistent with a much longer period than previously assumed. It is likely that Pons-Gambart wasn’t really missed before because with a 188 year orbit this is actually its first return since 1827. Since there is still some uncertainty in whether this comet is really Pons-Gambart, it is currently nameless and goes by the designations C/2012 V4.

C2012V4_orbitVisual observers have estimated the comet to be as bright as magnitude 9.0 to 9.5. Unfortunately the comet is located in eastern Sagittarius low in the southwest right during evening twilight. For the next few nights it will be located within a few degrees of Mars which will help with finding the comet. (As you can see from the plot to the right, the comet appears along the line-of-sight between the Earth and Mars but is not actually close to Mars). This will make for difficult observing due to its very low elevation. To make matters worse the comet is only going to get even lower in the future. It is possible the comet may brighten some as it approaches perihelion on Dec. 19 and 0.81 AU from the Sun. By then the comet will be almost 1.7 AU from Earth.

There has been lots of cirrus during the evening hours here in Tucson so I haven’t been able to see the comet. I’ll post my impressions when/if I am able to spot it.