Updated 2017 October 18
When observing a comet please try to forget how bright you think the comet should be, what it was when you last viewed it, what other observers think it is or what the ephemeris says it should be.
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.
Radio observations from Arecibo between mid December and early January show no clear detection of OH emission in the 1667 MHz line. This may suggest that the cometary appearance was due to an impact event, or simply that gas emission was very weak.
Jewett et al in a paper submitted to ApJL suggest that the activity was most likely due to impact with a 35m diameter body.
The "Dictionary of Minor Planet Names" notes that (596) Scheila was discovered on 1906 February 21 by A. Kopff at Heidelberg. Named in honor of an acquaintance of the discoverer, a female English student in Heidelberg. (596) Scheila is a main-belt asteroid inclined roughly 14 degree on the ecliptic and it is now 3.1AU from the Sun and 2.5AU from the Earth. It is next at perihelion in 2012 May and has a period of 5.0 years. Its distance from the sun varies between 2.4 and 3.4 AU. It is about 117km diameter and has an abledo of 0.036.
K. Battams, Naval Research Laboratory, writes that A. Watson (Werribee, Victoria, Australia) has commented that the minor planet (3200) was visible in SECCHI HI-1A images during June 17-22, noting a very short radial elongation (perpendicular to the direction of motion) that was possibly a line-of-sight effect related to its passage through a reasonably dense, higher-speed solar outflow stream. Battams adds that the apparent brightness of (3200) increased significantly (about 2 mag or more), peaking at mag perhaps 10-11 a few hours after perihelion (T = June 20.302 TT, q = 0.140 AU); 36 hr later, the object's had faded to magnitude roughly 13-14. Phaethon was also visible in HI-1B images during June 21-22. More formal photometry will be performed later. [IAUC 9054, 2009 June 29]
Alan Watson recovered Phaethon in STEREO images from 2012 April 30 on May 3, noting that it seemed a little fuzzy.
Dave Jewett and Jing Li suggest Phaethon is a "rock comet". They published a paper on the subject in The Astronomical Journal, with an an on line summary. Jewett et al in a paper submitted to ApJL suggest that by contrast the activity seen in (596) Scheila was most likely due to impact with a 35m diameter body.
The asteroid has a period of 5.6 years and was at perihelion at 2.9 au in 2013 March. It was discovered by LINEAR on 2000 September 28.
The SPA ENB provided additional information: A new active asteroid, numbered 62412, has
been discovered in the main asteroid belt between Mars and Jupiter. It is the first
comet-like object seen in the Hygiea family of asteroids. Active asteroids are a newly
recognized phenomenon and 62412 is only the 13th known active asteroid in the main
asteroid belt. It is estimated that there may be about 100 of them in the main asteroid
belt. Active asteroids have stable orbits between Mars and Jupiter like other asteroids;
unlike other asteroids, however, they sometimes have the appearance of comets, when dust
or gas is ejected from their surfaces, creating a sporadic tail effect. Astronomers
recently discovered a tail on 62412, an object which had been known as an ordinary
asteroid for over a decade. The reasons for the loss of material and the formation of a
tail in active asteroids are unknown, although there are several theories such as recent
impacts or sublimation from solid to gas of exposed ices.
In the past, asteroids were thought to be mostly unchanging objects, but an improved ability to observe them has allowed scientists to discover tails and comas, the latter being like the thin envelopes of atmosphere surrounding comets' nuclei. Discoveries such as this one can help researchers to determine the processes that cause some asteroids to become active. They found that 62412 has a very fast rotation that may shift surface material, some of which may leave the surface and form the comet-like appearance. The tail may be created directly from material ejected from the fast-rotating body, or from ice within it subliming into water vapour after being freshly exposed on the surface. The density of 62412 has been found to be typical of primitive asteroids and not consistent with the much lower densities comets.
R. Kracht suggests that the Kracht-group comet C/2008 N4 is a return of C/2002 S7, principally on the assumption that C/2002 S7 was itself a return of one of C/1996 X5, C/1996 X4 or C/1996 X3 (see MPEC 2006-C49). The derived orbit links C/2002 S7 and C/2008 N4. This gravitational linkage leads to a previous perihelion time of T = 1996 Dec. 6.00, earlier by a few hours than the values for the aforementioned 1996 comets.This possibly suggests the presence of non-gravitational effects, which would confirm the cometary nature of these objects. Further work by Brian Marsden confirmed my suggestion and in MPEC 2009-J14 [2009 May 4] he noted:
Following up on MPEC 2008-P60, R. Kracht has suggested that the correct linkage for C/2002 S7 = C/2008 N4 is with C/1996 X3, on the assumption that the comet was affected by small nongravitational forces (see also MPEC 2009-H56). The nongravitational linkage, with parameters A1 = 0.0000, A2 = +0.0027, is based on Kracht's work.
The IAU decided to ignore the discovery and orbit computation sequence, which would give a designation of 2012 B1 because pre-discovery observations made earlier in January became available, and gave the designation of 2012 B1 to a comet discovered by PanSTARRS on January 25.
The comet was recovered by Worachate Boonplod in STEREO-A images from 2016 February 20. A linked orbit was published on March 12 [MPEC 2016-E131]
A non-group SOHO comet discovered in C2 images by Zhijian Xu on 2014 May 17 was quickly linked to 2008 Y12 by Michal Kusiak and the orbit confirmed by Reiner Kracht. A linked orbit by Gareth Williams was published on MPEC 2014-K37 on May 24. The comet has a period of 5.4 years and perihelion 0.07 au.
Published by Jonathan Shanklin. Jon Shanklin - firstname.lastname@example.org