Sunday, 2 September 2012

The faint tail of Comet P/2012 NJ (La Sagra)

Comet P/2012 NJ (La Sagra) was originally designated as an asteroid when announced by the Minor Planet Center on MPEC 2012-N19 as 2012 NJ. It had been discovered by the La Sagra Sky Survey team from the mountains of Andalucia, Southern Spain early on 13 July 2012 and appeared to be physically unusually big for a newly discovered NEO these days, estimates of its size, derived from assumed asteroidal values of its albedo or reflectivity indicating a diameter of approximately 4-12 km. Coupled with an unusual orbit, inclined almost perpendicular to the Earth's orbit and very elongated, with perihelion just inside Mars' orbit at 1.29 AU but with aphelion further out than Saturn's orbit, it could well be expected to be a comet but none of the initial observers reported any cometary activity, several observers using telescopes of 0.5 - 0.6-m diameter.

However, on July 18th circular CBET 3178 (subscription required) was issued, announcing that the object was indeed a comet and re-designating it as Comet P/2012 NJ (La Sagra). Gerhard J. Hahn (Institute of Planetary Research, German Aerospace Center, Berlin) had reported it as showing a 35" tail in p.a. 235° on stacked and single images taken by Stefano Mottola using the 1.23-m telescope on Calar Alto on July 16, 17, and 18 UT. A post by Gerhard on the Minor planet Mailing list on July 19th gave further details and a link to a document containing images from July 17, 18 and 19th, showing a thin straight tail. An animation of portions of the individual images from that document has been made, used with permission and is shown here:

Comet P/2012 NJ (La Sagra) on July 17 ,18 & 19th 2012
Images taken by Stefano Mottola using the 1.23-m Calar Alto telescope
(c) Institute of planetary Research, German Aerospace Centre, Berlin

During the days following a few other images showing the tail were posted, including a good sequence  covering July 21-25 by Jean-François Soulier from France here, showing a very faint thin tail.

The comet reached perihelion at a distance of 1.29 AU from the Sun on 2012 June 14 and reached its closest to the Earth (0.59 AU) on 2012 July 22.

At Great Shefford Observatory, my first opportunity to image the new comet was on July 21 but with clouds affecting images it was difficult to detect the tail at all. However, on the nights of July 22, 24 and 25 the skies were better and 30 minute sequences of exposures were made once or twice per night to examine the appearance of the tail. Here, 30 minute stacks from the three nights are shown, the original field of view being cropped to 7'x9' and the scale doubled from the original 2.2"/pixel:

Comet P/2012 NJ (La Sagra) on July 22, 24 & 25th 2012
0.40-m Schmidt-Cassegrain
Each of the three frames is a stack of 90 x 20 second exposures
(30 minutes total)
(c) P. Birtwhistle, Great Shefford Observatory 

On the night of July 24/25th, 180 images were combined for a total exposure of 1 hour and in the resulting image below the very faint tail can be traced for 6' 30" in p.a. 210°

Comet P/2012 NJ (La Sagra) July 25-26 2012
7'x9' field of view, 1 hour exposure
0.40-m Schmidt-Cassegrain
(c) P. Birtwhistle, Great Shefford Observatory

The tail showed a marked drop in brightness between the nights of July 24/25th and 25/26th. An initial sequence of 90 images exposed on the evening of July 25th did not show the tail at all, further sets were obtained later in the night, eventually revealing the faint straight tail. Similar sets the night before had revealed the tail much more readily.

On 31 July 2012 Artyom Novichonok posted on the Comets Mailing List message 19737 that he and Otabek Burhonov had imaged the comet using the 1.5-m f/8 telescope at Majdanak in Uzbekistan with a total exposure of 2.5 minutes a few hours earlier than the image above.

The weather stopped any more attempts from Great Shefford before the full moon on 02 Aug 2012 and in the subsequent dark of the moon no further deep attempts were made, the comet itself receding from both Sun and Earth and predicted to be a full magnitude fainter than in the last half of July.

However, Artyom posted again on the Comets ML with message 19774 on 27 Aug 2012 with details of an image taken 2 weeks earlier on 13 Aug 2012 showing a very faint trace of the tail pointing in the anti-solar direction (p.a. 150 deg, down and to the left) and what appears to be a separate tail in p.a. 96 deg, though this second tail has yet to be confirmed.
Comet P/2012 NJ
From Majdanak Observatory in Ukbekistan
The tail observed in July is now very faint
and points down and to the left.
Another possible (shorter) tail points to the left,
though this tail is yet to be confirmed.

It is understood that the team at Calar Alto may target P/2012 NJ in mid-September, it will be interesting to see whether any trace of the tail is detectable by then.

During the current apparition, the anti-solar direction (the expected direction for an ion tail) rotated from p.a. 242° at discovery, 238-234° during July 16-18, 215° on July 25 and 147° on Aug 14th and those directions agree to within a few degrees of the measured angles from the images above. By mid September 2012 the anti-solar direction will have reached p.a. 100 to 90°.

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