Comet 168P/Hergenrother (P=6.89a) surprised the observers in fall 2012. At this apparition it was predicted to reach only magnitude 15. However, observers reported it at magnitude 13 on Sep. 6, with the brightness increasing by more than 0.1 mag per day during the following days. Finally it peaked at magnitude 9.3 on Oct. 7 (6 days after perihelion). Thereafter the comet faded continuously, however much slower than the brightening took place. The following formulae confirm that the comet had experienced a major outburst:

After Dec. 5 no further observations came to my knowledge, although the comet was still very well placed. Thus it has to be asumed that the comet faded dramatically during those days. This assumption may be supported by the last visual observation done by Uwe Pilz. On Dec. 5 he could barely detect the comet in his 32cm-reflector, estimating a brightness of only 13.2 mag - much fainter than the above formula predicts.

The coma diameter estimates show a rather large scatter, perhaps a result of the fact, that the bright tail wasn't properly distinguished from the coma with the observers estimating not always the small but also the long axis. The coma diameter increased from short of 1.5' (30.000 km) on Sep. 10 to 3.8' (75.000 km) in mid-October. This value was constant until the first week of November. Thereafter it decreased, measuring 2.5' (55.000 km) at the end of November.

Total Brightness and Coma Diameter

A bit surprising is the fact that the coma was not most condensed during the early stages of the outburst. Instead in those days it was still rather diffuse (DC 3), but condensed thereafter, reaching DC 7-8 at the start of October. During the following weeks it became more and more diffuse with the degree of condensation estimated at DC 2-3 at the end of November. Until mid-October the central condensation was very prominent, compared to the rather unimpressive coma. The central condensation was the source of an antisolar streak of brighter material, which merged with the very well visible short tail. According to the published observations the tail could be visually observed between Sep. 25 and Oct. 15, with a maximum length of 10' (600.000 km). During this period it rotated from p.a. 200° to p.a. 130°.

CCD images with the 2.0m Faulkes Telescope South obtained on Sept. 26.6 showed the comet with a sharp central condensation measuring nearly 3" across; the total coma was traced to a diameter of about 1.7' (total magnitude 10.2). On Oct. 3.6, the central condensation was somehow less sharp but had grown to size 8", while the total coma diameter was nearly 3' (total magnitude 9.4). On Oct. 9.6, the central condensation was ill-defined, and the total coma diameter was nearly 4'.5 (total magnitude 9.2). Their Af(rho) data supports the observed evolution: Sept. 26.6, Af(rho) = 670 cm; Oct. 3.6, 1210 cm; Oct. 9.6, 850 cm. R-band exposures, obtained remotely with the 2.0m Faulkes Telescope North on Oct. 26.4 UT under good seeing conditions, showed the presence of a secondary fragment (designated fragment B) placed about 2" in p.a. 188 deg with respect to the main central condensation of comet 168P; the magnitude of this fragment was measured to be about 17, and there is a diffuse coma nearly 2" in diameter. There was no evidence of this fragment in their previous stacked images obtained with the Faulkes Telescope South on Oct. 3.6, and with the Faulkes Telescope North on Oct. 22.4, to a limiting magnitude of about 19. Follow-up R-band observations with the Faulkes Telescope North on Nov. 7.43 and 7.45 UT showed that the fragment that they observed on Oct. 26.4 and Nov. 2.4 was no longer visible (limiting magnitude about 20); however, they noticed instead the possible presence of a second, extremely weak fragment, having mag about 19.5, placed nearly 8.4" in p.a. 142 deg with respect to the main central condensation of 168P.
Carl W. Hergenrother, Lunar and Planetary Laboratory, University of Arizona, reports that, prior to the detection of individual companions, a mass of material was observed moving away from the near-nuclear region towards the anti-solar direction. The feature was first detected on images taken with the Faulkes 2.0m reflector on Oct. 16.45 UT when it was comparable in brightness to the nuclear condensation of the primary. The Faulkes images from Oct. 26.42 no longer show the feature, although companion C is located near its expected location. Hergenrother also reports four separate companions to 168P observed in data taken between Oct. 26.42 and Nov. 8.29 UT.
Z. Sekanina reports that four of the five companions apparently broke off from the primary nucleus A. Based on 9 offsets from Oct. 26-Nov. 7, companion B was found to have separated on Sept. 17.5 +/- 2.5 UT with a velocity of 0.18 +/- 0.05 m/s. Very similar solutions were obtained on the assumption that the separation of B coincided with the onset of an outburst on Sept. 22. For companion C, from four offsets between Oct. 26 and Nov. 3, Sekanina derived a separation on Oct. 7.0 +/- 1.5 UT with a velocity of 0.3 +/- 0.1 m/s. Very similar solutions resulted assuming that the separation of C coincided with the onset of another outburst, on October 1. For companion D, the offset from Nov. 2 gave only very approximate parameters, a separation around Sept. 28 (probably coinciding with the same outburst as the separation of C). Companion E does not appear to have derived from A, but it may have split off from B within days after B broke off from A. Companion F was found to have separated from A on Sept. 24 (again at about the time of the outburst of Sept. 22). As the mass of material moved in the direction in which fragment C also appeared, and fragment C split off around the time of the Oct. 1 outburst, the mass of material was also a product of that same outburst. The apparent tendency of some companions to grow elongated with time suggests that they already represent clusters of subfragments subjected to a range of decelerations, thus entering an advanced phase of disintegration (CBET 3295). On Nov. 11 fragment G was discovered with the 2.1m-Kitt-Peak telescope. It was elongated in anti-solar direction, measuring 2.4x1.8" and positioned 9.7" (p.a. 168°) from the primary component. It had faded and shrunk (1.8x1.5") on the following day. According to Z. Sekanina this fragment separated with a very low velocity from the primary at the beginning of September - just at the time of the first outburst. Thus it existed for at least 10 weeks (CBET 3318).

Andreas Kammerer

FGK observations