Updated 2024 December 24

The equations for the light curves of comets that are currently visible use only the raw observations and should give a reasonable prediction for the current brightness. If the comet has not yet been observed or has gone from view a correction for aperture is included, so that telescopic observers should expect the comet to be fainter than given by the equation. The correction is about 0.033 per centimetre. Values for the r parameter given in square brackets [ ] are assumed. The form of the light curve is either the standard m = H0 + 5 log d + K0 log r or the linear brightening m = H0 + 5 log d + L0 abs(t - T + D0) where T is the date of perihelion, t the present and D0 an offset, if L0 is +ve the comet brightens towards perihelion and if D0 is +ve the comet is brightest prior to perihelion.

Observations of new and periodic comets in 2015 are given in ICQ format. 

Full details of recently discovered objects will not appear until they are available on the CBAT web pages. The actual accuracy of preliminary orbits is often (nearly always) much worse than the published accuracy implies.  In part this is because each orbital solution is treated as a mathematical construct and does not take account of observational error.  JPL does publish the errors, whereas the MPECs do not.

Rob Matson provides some discovery details:

I first spotted the comet in SWAN imagery on Sunday, February 22, when composite images were available for 2/15, 2/16, 2/17, 2/18 and 2/19. It wasn’t bright enough to be reportable yet (particularly the 2/15 image), but I was pretty confident it was real so started the process of converting raw pixel coordinates to RA/Dec and grabbing SOHO satellite ephemeris data for the dates in question. I could find no other known comets predicted to be at that location, so I computed a preliminary orbit. It didn’t take long to realize that my chances of acquiring it in 20x110 binoculars from southern California were poor due to the low solar elongation combined with northern hemisphere winter.  It would take more aperture and darker skies.   

By Tuesday I had the comet in 8 images, and it was time to find someone on the ground who could confirm it, and to report rough positions to Dan Green and Gareth Williams. I made my usual epoch assumption for SWAN composite images (12 UT), figuring that +/- 12 hours uncertainty is better than the other alternatives. In reality, SWAN composites are stitched together from much smaller images collected over a roughly 24-hour period, so different parts of the celestial sphere are imaged at different times. (I’ve never been able to find information about SWAN’s scan pattern, or if there is ancillary data that provides the epoch of each pixel.) I sent the eight positions to Dan & Gareth.     

I explained that my +10 estimate for the magnitude was just an educated guess based on the comet’s moderately easy detectability in the last several images. I computed a preliminary parabolic orbit and generated search ephemerides for Terry Lovejoy and Rob McNaught, which I sent to them just prior to emailing Dan & Gareth. The comet appeared to be moving at about 1.6 degrees per day, so I advised Terry and Rob that the position could be off by +/- 0.8 degrees (mostly in declination) just based on the SWAN timing uncertainty alone, and more like +/- 1 degree when SWAN’s coarse angular resolution is folded in. 

Terry got back to me almost immediately, and mentioned that he was concerned that it might be too low for him with the possibility of local obstructions, but that weather permitting he’d give it a try.   At 1:39 am Wednesday morning (Pacific Time), Terry emailed me to say he’d found it close to the predicted position, estimated the magnitude at +11 with a 2’ coma, and said it had a short stubby tail. At this time he informed me that Michael Mattiazzo had also contacted him shortly after I had, which wasn’t surprising as Michael and I have shared discovery credit on a number of prior SWAN comets.  Once the CBET was out, I saw that Vladimir Bezugly (another veteran SWAN comet finder) had also spotted the comet.   

It will be interesting to see if the comet turns out to be periodic; when I combined ground-based astrometry with SWAN positions, I got solutions that seemed to favour a 100- to 200-year orbit, depending on how much I shifted the epoch times of the SWAN positions.

Karl Battams notes that the comet's image appeared tiny at first (just above the noise at about 9th magnitude) and gradually brightened as it neared the sun, but was surprised to see that it then dramatically brightened to magnitude about 1.5 as it crossed into the C2 field-of-view, when a short, faint tail appeared (February 19) and remained visible for the rest of the comet's visible passage (fading to magnitude around 6 by the time it left LASCO's field-of-view (February 21.77).

Matthew Knight, Lowell Observatory, has analyzed the SOHO images and reports that the comet was first visible late on February 17 right at the detection limit in C3 (magnitude about 9.5-10), and peak brightness occurred around February 19.9 at an apparent V magnitude of about 1.3.  It showed much more cometary appearance during the second half of the visible apparition, and the tail seemed more distinct as it was fading.

Pre- and post-perihelion solutions were required to fit the observations; together with the appearance of the tail near perihelion and rapid brightening, it is possible that some large physical change (perhaps splitting) occurred around that time. [CBET 4067, MPEC 2015 D73, 2015 February 24]

The CBET gives the discovery time as February 18.004. No error-bars are given for their orbit solutions. As there is a relatively small arc, and the SOHO positions are of low accuracy, it is not clear how significantly different are the pre and post perihelion orbits. The JPL orbit based on a three day arc classes it as a Jupiter family comet with a period of around 11 years.

The comet has been recovered from the ground, however it only shows as a dusty streak on images taken by Justin Cowart.

Observations in 2015 December and 2016 January showed that the object had cometary features and a coma was then noted in images from 2015 June.  It was therefore re-designated as a comet on MPEC 2016-C01 [2016 February 1]. Unfortunately the comet doesn't actually approach us particularly closely.

Juan Gonzalez recovered the comet after solar conjunction on December 6.25 at 10.4.  The comet brightened fairly slowly, but underwent a short-lived outburst in early April when it briefly reached 6th magnitude.  Overall it peaked at 7th magnitude in early May.  It faded more rapidly after perihelion than it had brightened before and clearly this was an effect of the outburst.

A secondary component was detected by Belgian amateur Erik Bryssinck on 2017 June 13.41.

1996 Q1 was discovered by an Australian amateur Vello Tabur on 1996 August 19.70. After reaching naked eye brightness in mid October, the comet suddenly faded dramatically. The orbit is very similar to that of 1988 A1 (Liller), which reached perihelion on 1988 Mar 31. It seems likely that these two comets separated at their last perihelion passage, about 2900 years ago. The comet is one of those known to have undergone nuclear splitting according to the list of Marcos & Marcos [Dynamically correlated minor bodies in the outer solar system, MNRAS, 474, 838, 2018 February]. They link it with 1996 Q1, but do not mention 1988 A1.

8 visual observations received so far suggest a preliminary light curve of m = 9.8 + 5 log d + 6.9 log r

43 visual and electronic observations received so far suggest a preliminary light curve of m = 5.6 + 5 log d + 25.9 log r

2 visual observations received so far suggest a preliminary light curve of m = 12.5 + 5 log d + [10] log r

The text of the original CBET 4092 was replaced by CBET 4104 issued on May 23.

81 visual observations received so far suggest a preliminary light curve of m = 8.2 + 5 log d + 7.1 log r

The discovery was announced later the same day in CBET 4105. The comet was one of around 50 potential objects found in images taken by with the MASTER (Mobile Astronomical System of the Telescope-Robots) 0.4-m f/2.5 reflector at the South African Astronomical Observatory on May 17.14. Denis Denisenko noted that it showed a diffuse appearance and this was confirmed by other astrometrists. [CBET 4105, MPEC 2015-K70, 2015 May 23]. The comet was at perihelion at 2.6 au in 2014 October.

5 visual observations received so far give a preliminary light curve of m = 11.8 + 5 log d + [10] log r

This object was reported as a comet found on May 27.36 (without any details on appearance) by the Cerro Tololo DECam team on August 14. It was reported as a NEOCP candidate found on August 15.71 on August 16 by the ISON-Siding Spring team. Details on the cometary appearance in May were submitted on August 18, following prompting by Gareth Williams. The linkage of the two sets of observations was made after the object was removed from the NEOCP and designated. None of the August reports have mentioned detecting cometary features, hence the announcement under a minor-planet designation. There is a roughly two-magnitude discrepancy in the absolute magnitudes determined from the May observations alone and from the August observations alone. Confirmation of any possible cometary activity is therefore very desirable.

It was re-designated as a comet [CBET 4251, MPEC 2016-C02, 2016 February 1], although no further evidence of cometary activity had been detected.  It has a Jupiter MOID of 0.11 au.  The comet name was changed from ISON-Cameron to Cameron-ISON following a vote by the Working Group on Small-Body Nomenclature in 2016 April.

Gareth Williams notes on MPEC 2015-T32, 2015 October 9]:

Richard Wainscoat reports that 2015 RK245 looked entirely asteroidal on images obtained with the 3.58-m CFHT on Mauna Kea on October 9. The seeing was not particularly good: for the first two images, it was 1".1; for the third, it was 0".85. The telescope was tracked on the motion of the object. In the direction perpendicular to the motion, the PSF of 2015 RK245 looked identical to that of adjacent stars. Wainscoat concludes "that there is no morphological evidence from the images of this being cometary."

The comet is one of those suspected to have undergone nuclear splitting according to the list of Marcos & Marcos [Dynamically correlated minor bodies in the outer solar system, MNRAS, 474, 838, 2018 February]. They link it to 10P/Tempel.