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Interesting fainter comets


On Oct. 31, 2010 A. Boattini discovered a 20.0 mag comet with the 1.5m-Mt.Lemmon-reflector near the border of the constellations Cetus/Aries. Comet C/2010 U3 (Boattini) showed a medium condensed 6-7" coma, elongated in p.a. 330°, harboring a 3-4" central condensation. Additional observations showed a 20" coma and a 25" tail in p.a. 330°. At discovery the comet stood in the record solar distance of 18.5 AU! It will not reach perihelion before spring 2019 at a distance of 8 AU. Nevertheless it could become as bright as 15 mag, then moving through far-northern regions (Camelopardalis, Ursa Major, Draco) (IAUC 9182 / 2010-V109). This implies an inherently very bright comet which showed a coma diameter of already 250.000 km at discovery!
The small number of observations published until the end of July 2017 does not allow any definite statement concerning the brightness behavior of this comet. It should be observable with larger instruments as a 15.0-15.5 mag object in southern Camelopardalis during fall 2017, thereby exhibiting a well-condensed miniscule coma (of diameter 0.3-0.5').

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An asteroidal object of magnitude 20.5, discovered by T. Bressi on Spacewatch images taken on May 21, 2011 near the borders of the constellations Ophiuchus/Serpens showed cometary morphology on images taken in April 2012. Comet C/2011 KP36 (Spacewatch) displayed an 8" coma of total magnitude 20.0 and a very faint, 9" tail in p.a. 10°. The comet, intrinsically rather bright, will not pass its distant perihelion on its 235 years orbit prior to May 2016, then predicted to reach magnitude 14.5 (CBET 3109). It should be brighter than 15.5 mag between mid-2015 and the beginning of 2017. During this time it will move through the constellations Aquarius, Pisces and Cetus. The comet is expected to peak in September 2016.
Based upon 80 observations (until October 2017) the comet shows a brightness evolution clearly below average. The derived brightness parameters are m0=7.0 mag / n=2, yielding a maximum brightness of 13.5 mag in mid-September 2016. In 2015 the coma diameter was estimated to be 0.5' (80.000 km) and in summer 2016 0.7' (135.000 km) was derived, with a short-term increase to 1.2' (225.000 km) in fall 2016. In 2017 it measured 0.5' (120.000 km). Throughout the apparition the coma was medium-condensed (constant DC 3).

Total Brightness and Coma Diameter

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On images taken on July 19, 2015 the PanSTARRS team discovered a comet in the constellation Capricornus, near the border to Sagittarius. According to the team comet C/2015 O1 (PanSTARRS) displayed a 2.5" tail towards East and a miniscule coma of magnitude 19.5. Additional observers reported a coma of diameter 15x17" and of total magnitude 18.5 and a 4" tail in p.a. 100°. The comet will pass its rather distant perihelion (distance: 3.7 AU) in February 2018, expected to reach magnitude 13.5 (CBET 4119 / MPEC 2015-Q71). It should be brighter than 16 mag between January 2017 and April 2019. During this period it will move from Aquila through Hercules, Bootes, Ursa Major into Lynx. Thus it will be comfortably placed in the morning/night sky for mid-European observers during the interesting weeks.
Until end of October 2017 only 25 observations came to my knowledge. The reported brightness estimates can be represented by the parameters m0=5.0 mag / n=4. Thus the comet should peak at 13.5 mag in March 2018. At the end of October 2017 observers reported a magnitude of 14.2 and a diameter of the significantly condensed (DC 4) coma of 0.8'.

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On PanSTARRS images taken on Nov. 2, 2015 an object was discovered which was slightly larger than the surrounding star images. Follow-up observations of comet C/2015 V1 (PanSTARRS) showed a 6" coma of magnitude 19.5, but no tail. The comet will reach perihelion at the end of 2017, then moving in Pisces and expected to be of magnitude 15.5 (CBET 4160 / MPEC 2015-V61).
The dozen CCD-observations published until October 2017 report a brighter object than predicted. If the derived brightness parameters m0=5.5 mag / n=4 should be confirmed by additional observations the comet would have peaked at magnitude 14.5 in October 2017. The comet shows a coma of diameter 0.6'.

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On PanSTARRS images taken on Jan. 23, 2016 a team of observers discovered a 20 mag comet in the constellation Aries, which showed a miniscule coma and a short tail. It was shown, that this object was identical with the asteroidal object 2015 YY6, discovered by W. Yeung on Dec. 18, 2015 and with the object 2015 VL62, discovered on Nov. 2, 2015 at Mt. Lemmon observatory near the borders of the constellations Taurus/Auriga/Perseus. Additional observations of comet C/2015 VL62 (Lemmon-Yeung-PanSTARRS) showed a strongly condensed 8" coma of total magnitude 18.5m and a 15" tail in p.a. 80°. It will pass perihelion at the end of August 2017, then expected to be of magnitude 12-13 (CBET 4246/47). It should be brighter than 16.0 mag between end of 2016 and mid-2018, thereby moving from Aries into Scorpius. At maximum brightness it will move from Pegasus into Aquila, being observable for the whole night.
Until October 2017 I recognized 50 observations from 16 observers of this comet. The brightness estimates can be represented reasonably well by the parameters m0=10.0 mag / n=2, indicating a peak brightness of 13.5 mag around Aug. 20, 2017. Pre-perihelion the coma diameter increased from 0.4' (55.000 km) to 1.2' (90.000 km), only to decrease thereafter, reaching 0.4' (50.000 km) at the end of October. During the whole visual apparition the coma was rather diffuse, with the degree of condensation increasing pre-perihelion from DC 2 to DC 3-4, thereafter decreasing, reaching DC 2-3 at the end of October. Visually no tail was observed.

Total Brightness and Coma Diameter

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On June 22, 2016 the PanSTARRS project discovered a 19 mag comet in the constellation Draco. Additional observations of comet C/2016 M1 (PanSTARRS) showed a 10" coma of total magnitude 18.5 and a 12" tail in p.a. 180°. The comet will pass perihelion in August 2018, expected to peak at 9 mag (IAUC 4286 / MPEC 2016-N14). It will be brighter than 16.0 mag between April 2017 and the end of 2019. It will switch from the morning to the evening sky in December 2017 and should be observable for mid-European observers until end of June 2018 (then at 9 mag). During this period it will move through the constellations Draco, Hercules, Ophiuchus, Serpens, Aquila and Sagittarius.
The development of this comet is still rather indistinct, based on 25 observations (until the end of October 2017). The brightness estimates can be best represented by the parameters m0=8.0 mag / n=2, but the parameters m0=6.5 mag / n=3 are also plausible. Thus the comet should peak at magnitude 9.7-10.3m in mid-2018. At the end of October 2017 it was of magnitude 14.0, showing a 0.7' coma, which was significantly condensed (DC 5).

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On images taken on July 15, 2016 by the MASTER telescope on Teneriffe an asteroidal object of magnitude 17.0 was discovered in the southwestern part of Cetus. By scrutinizinig the images a cometary morphology was dectected. Comet C/2016 N4 (MASTER) showed a strongly condensed 15" coma of total magnitude 16.5 and a tail up to 60" long in p.a. 190°. It will pass perihelion of its elliptical orbit with a period of about 1.000 years in September 2017, expected to peak at about 14 mag. During the most interesting months (mid 2017 to opening of 2018) it will move through the constellations Andromeda, Cassiopeia and Cepheus (CBET 4291 / MPEC 2016-P06).
Only 20 observations came to my knowledge until the end of October 2017. Thus only very preliminary results can be presented. The brightness seems to develop according to the parameters m0=7.0 mag / n=4. However the covered variance in solar distance is very small, thus the values are rather unassured. If additional observations should confirm them the comet would have peaked at magnitude 14.5 in September 2017. At this time the coma diameter measured about 0.8'.

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On July 14, 2016 the PanSTARRS team discovered a comet of magnitude 21 in the constellation Hercules. Follow-up observations of comet C/2016 N6 (PanSTARRS) showed a strongly condensed 5x7" coma of total magnitude 20.0. The comet will pass perihelion at a solar distance of 2.7 AU in July 2018, expected to reach 14 mag (CBET 4309). It should be brighter than 16 mag between fall 2017 and spring 2019, with maximum brightness in spring 2018. During this period it will move through the constallations Bootes, Draco, Ursa Minor, Draco, Lynx (maximum brightness), Cancer, Hydra, Puppis, Canis Major and Lepus. Between mid-June and the start of September 2018 it will not be visible from mid-European sites.
In fall 2017 the comet was significantly brighter than expected. At the end of October it was of magnitude 14.5, showing a significantly condensed coma of diameter 0.6'. So far, the brightness estimates are only approximately represented by the parameters m0=7.0 mag / n=3. If these should be confirmed by additional observations the comet should peak at magnitude 12.5 in March/April 2018.

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On images taken on Aug. 26, 2016 the PanSTARRS team discovered a comet on the border of the constellations Capricornus/Aquarius, estimating the brightness to be about 21.5 mag, noticing a broad, 6" tail towards North. Detailed observations of comet C/2016 Q2 (PanSTARRS) showed a strongly condensed, 12" coma of total magnitude 19.0 and a very broad 12" tail in p.a. 338°. The comet (with an orbital period of about 440 years) will pass its perihelion at the large distance of 7.2 AU in May 2021, expected to reach about 15.5 mag. It should be brighter than 16 mag between spring 2020 and fall 2022, moving through the constellations Cygnus, Lyra, Draco/Hercules towards Ursa Major during this period (CBET 4311 / MPEC 2016-V116).

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On PanSTARRS images, taken on Sep. 7, 2016, a comet was discovered in the southern part of Eridanus. Comet C/2016 R2 (PanSTARRS) showed a very diffuse, miniscule coma of magnitude 18.0. It will pass perihelion (solar distance: 2.6 AU) at the start of May 2018 and should peak at about 12.0 mag around the turn of the years 2017/18 (CBET 4318 / MPEC 2016-U07). Between mid-2017 and mid-2019 it should be brighter than 16.0 mag. During this period it will move through the constellations Eridanus, Orion, Taurus, Auriga, Lynx, Ursa Major, Canes Venatici and Bootes. Thus it should be a well-placed object during the most-interesting months.
The comet shows a development clearly below average, as 22 observations (until October 2017) demonstrate. However, the observations are inconsistent. In particular several observations between end of 2016 and start of 2017 are problematic. If they are included, the comet shows a long-term decrease of its activity. If they are neglected the brightness development can be roughly represented by the parameters m0=8.0 mag / n=2, indicating a slow long-term increase of the activity. If these parameter should be confirmed by additional observations the comet should peak at magnitude 12.5 around the turn of 2017/18. At the end of October 2017 it was an object of magnitude 13.0 with a moderately condensed (DC 4) coma of diameter 1.3'.

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In the course of the LINEAR Sky Survey an asteroidal object of magnitude 19.5 was discovered on images taken on Jan. 26, 2017 near the border of the constellations Hydra/Corvus. Follow-up observations of comet C/2017 B3 (LINEAR) showed a significantly condensed 8" coma of total brightness 18.5 mag. This comet will pass perihelion at the beginning of 2019, expected to peak around magnitude 14-15 (CBET 4354). However, at this time it will be positioned in the far Southern Sky. It will not be observable from mid-European locations during the time interval when it is expected to be brighter than 16.0 mag.

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On Feb. 23, 2017 the Brazilian observer J. Barros discovered a comet with the 45cm-reflector of SONEAR observatory in the constellation Telescopium, near the border of Corona Australis. Follow-up observations of comet C/2017 D2 (Barros) showed a significantly condensed 11" coma of total magnitude 17.5. The comet will pass perihelion on July 14, 2017 at a distance of 2.48 AU and should then peak at magnitude 15.0 (CBET 4366 / MPEC 2017-J52). It should be brighter than 16.0m between mid-May and mid-October 2017, thereby moving from the western part of Grus into Aquarius. It will not be observable meaningfully from mid-European locations.
The few published observations hint to a maximum brightness of only 16.5 mag in August 2017.

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On Mar. 1, 2017 Gennadii V. Borisov discovered a comet, using a 40cm robotic telescope, near the borders of the constellations Aquila/Sagittarius/Scutum. He estimated the total brightness of the 20" coma to be magnitude 17.0. CCD follow-up observations of comet C/2017 E1 (Borisov) showed a strongly condensed 50" coma of magnitude 14.0. S. Yoshida observed the coma visually on Mar. 3, estimating the brightness of the 1.4' coma to be magnitude 12.2. The comet will pass perihelion on Apr. 10, 2017 at the solar distance of 0.90 AU (CBET 4369 / MPEC 2017-J52). Alas, it is positioned on the opposite side of the Sun, and thus will be visible at only small elongations and not brighten more than one magnitude. Until end of August it moves through Aquarius, Pisces, Cetus into Taurus. From mid-European locations it will only be visible until end of March – at low altitudes in the morning sky. It will reappear in August, but with an expected brightness of less than 16 mag.
Only 20 observations of this comet came to my knowledge, indicating brightness parameters of about m0=10.5 mag / n=6 (neglecting the observations of one observer prone to estimating too bright and too large comae). The comet peaked at magnitude 10.5 at the opening of April 2017, showing a weakly condensed (DC 3) coma of diameter 2.5'.

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On May 21, 2017 the PanSTARRS team discovered a comet of magnitude 21.0 with a miniscule coma in the constellation Draco. Follow-up observations of comet C/2017 K2 (PanSTARRS) showed a moderately condensed 12" coma of total magnitude 19.0 (CBET 4393 / MPEC 2017-N26). The comet was detected in the record distance of 16.1 AU, indicating an absolute brightness of 1 mag and an absolute coma diameter of 140.000 km. The comet will reach perihelion at a distance f 1.80 AU not prior to the Christmas days 2022. It should be brighter than 16.0 mag from early 2020 to the end of 2025. During this interval it will move from the northern parts of Hercules through Ophiuchus and Scorpius, diving far south between October 2022 and March 2023, then reappearing for mid-European observers in Eridanus, then moving through Lepus, Orion and Taurus into Auriga. Its maximum brightness of about 5.5 mag should be reached around the turn of the years 2022/23. During the most interesting months it will be observable from mid-European locations in the evening sky until the end of August 2022 (then of magnitude 6.0), then from early September 2023 (then of magnitude 9.5) onwards in the morning sky.

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The team of the "Asteroid Terrestrial-impact Last Alert System" (ATLAS) discovered a comet of magnitude 17.0 on May 26, 2017 in the southern part of Scorpius. Follow-up observations of comet C/2017 K4 (ATLAS) showed a strongly condensed 10" coma of total magnitude 16.0. The comet should peak at magnitude 15.5 in summer 2017, but will pass perihelion at a distance of 2.65 AU not prior to Jan. 8, 2018 (CBET 4397 / MPEC 2017-N58). It will remain brighter than 16.0 mag until mid-2018. During that interval it will move along the ecliptic right to the constellation Pisces. The observing conditions for mid-European observers are not favorable: too far south at first, then at decreasing elongations it will not reach altitudes of more than 10° above the southern horizon in the evening sky (until December 2017). It will reappear in the morning sky in May 2018, but again will be low, with altitudes of more than 10° not reached until June.

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On May 29, 2017 Cristovao Jacques discovered a comet of magnitude 17.5 near the border of the constellations Grus/Tucana, using the 45cm telescope of the SONEAR observatory. Comet C/2017 K6 (Jacques) showed a significantly condensed 20" coma and a 55" tail at p.a. 245°. It will reach perihelion on Jan. 3, 2018 at a distance of 2.00 AU, expected to peak at magnitude 14.5 (CBET 4399 / MPEC 2017-N58). During this time, however, it will be positioned in Phoenix/Fornax, being unobservable from mid-European locations. These could succeed in observing the comet on its way out from the end of January onwards. It should be positioned in the constellation Eridanus when it fades below 16.0 mag.

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On June 21, 2017 the ATLAS team discovered an asteroidal object of magnitude 18.0 in the constellation Cepheus, which revealed its cometary nature during follow-up observations. Comet C/2017 M4 (ATLAS) showed a moderately condensed 13" coma of total magnitude 17.5 and a short tail pointing SW. It will reach perihelion at a distance of 3.24 AU not prior to Jan. 15, 2019, expected to reach 14.0 mag in summer 2019 (CBET 4408 / MPEC 2017-N58). It will be brighter than 16.0 mag during the whole year of 2018 thereby moving from the border of the constellations Cygnus/Lyra through Hercules (maximum brightness) into Ophiuchus. From mid-European locations it will be observable until November 2018.

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On Sep. 23, 2017 the PanSTARRS team discovered a comet of magnitude 21 in the constellation Draco. Combinig the images with additional ones of Aug. 17 the orbit could be quickly determined. Comet C/2017 S3 (PanSTARRS) will pass perihelion on Aug. 15, 2018 in the solar distance of only 0.21 AU. Observations during the following days showed a rather diffuse 8" coma of total magnitude 20.8 (CBET 4432). This yields a preliminary absolute magnitude of 10.0 (assuming an activity parameter of n=4). This absolute magnitude is below the Bortle limit. Consequently, it is rather probable that this comet will extinguish while approaching the Sun. In the case it will develop according to the values mentioned and will survive it will peak at 3 mag at perihelion. However, in those days it will situated very near the Sun. Mid-European observers can follow the comet until the start of August in the morning sky. It should get brighter than 16 mag at the opening of May 2018, getting brighter than 10 mag in mid-July. During the days it will disappear in the twilight it should be of magnitude 5-6. During the mentioned time span it will move through the constellations Cassiopeia, Camelopardalis, Auriga, Gemini and Cancer. Post-perihelion it should reappear in the morning sky during the first week of October, expected to be an object of magnitude 13, reaching magnitude 16 in mid-December. During this period it will drift slowly through the constellation Coma Berenices.

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On images taken in the course of the Catalina Sky Survey on Sep. 30, 2017 a comet was discovered near the border of the constellations Lynx/Auriga. Comet C/2017 S6 (CATALINA) showed a strongly condensed 15" coma of total magnitude 18.0. It will pass perihelion on Feb. 26, 2018 in the solar distance of 1.54 AU, expected to reach maximum brightness of 15.0 mag already in December 2017 (CBET 4437 / MPEC 2017-V13). The comet is expected to be brighter than 16 mag between mid-November 2017 and the end of March 2018. During this period it will move through the constellations Camelopardalis, Cassiopeia, Andromeda and Pegasus. Mid-European observers can follow the comet until it will disappear above the western evening horizon at the end of January (expected to be of magnitude 15.5).

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On images taken in the course of the "Asteroid Terrestrial-impact Last Alert System" (ATLAS) on Oct. 2, 2017, Aren Heinze discovered a comet of magnitude 18 in the constellation Hydra, near the border to the constellation Monoceros. Follow-up observations of comet C/2017 T1 (Heinze), undertaken in mid-October, showed a medium-condensed 12" coma of total magnitude 17.0 and a 35" tail in p.a. 245°. The comet will pass perihelion at a distance of 0.58 AU on Feb. 21, 2018, expected to reach magnitude 9.5 (CBET 4444). However, the derived absolute magnitude is near the Bortle limit, making it possible, that the comet will fizzle while approaching Sun. Prior to perihelion it will pass Earth in a distance of only 0.22 AU on Jan. 4, 2018, expected to reach magnitude 9.0. Despite its brightness it will be a difficult object, showing a diffuse coma of diameter about 8'. Mid-European observers can glimpse the comet until the opening of March in the morning sky. Until the start of January it will reach altitudes of 35° which decrease to 10° between mid-January and the start of March. Between end of December and the start of February it will be visible in the evening sky too, reaching 70° above the horizon in mid-January! Between November 2017 and the opening of March 2018 it will move through the constellations Hydra, Cancer, Lynx, Camelopardalis (minimal Earth distance), Cassiopeia, Lacerta and Pegasus. Earth will cross the orbital plane on Jan. 3, 2018.

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Images, taken by the PanSTARRS project on Oct. 2, 2017 showed a faint comet with a miniscule coma in the constellation Eridanus. Follow-up observertions of comet C/2017 T2 (PanSTARRS) showed a significantly condensed 12" coma of total magnitude 19.3 and a 20" tail in p.a. 290°. The comet will pass perihelion on May 6, 2020 in the distance of 1.62 AU. It is expected to peak at magnitude 8.0 during several weeks (CBET 4445). It should be brighter than 16 mag during the years 2019 and 2020, being brighter than 12.0 mag between October 2019 and October 2020. During the most interesting months it will be better positioned in the morning sky until November 2019 for mid-European observers. Thereafter it will be better observable in the evening sky until the opening of September 2020. During this time interval it will move through the constellations Taurus, Auriga, Perseus, Cassiopeia, Camelopardalis (maximum brightness), Ursa Major, Canes Venatici, Coma Berenices and Virgo.

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In the course of the "Asteroid Terrestrial-impact Last Alert System" (ATLAS) an asteroidal object of magnitude 18.5 was discovered on Oct. 14, 2017 near the border of the constellations Cepheus/Draco. Follow-up observations showed the cometary nature of the object. Comet C/2017 T3 (ATLAS) showed a strongly condensed coma of diameter 15" and total magnitude 18.0 and a 60" tail in p.a. 155°. It will pass perihelion at a distance of 0.83 AU on July 19, 2018, expected to reach 9.0 mag (CBET 4449). However, it will then be positioned on the opposite side of the Sun and thus will not be observable well from any location on Earth. Mid-European observers can follow the comet until the end of March 2018 in the evening sky, then expected to be not brighter than 14.0 mag.

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Starting in mid-August 2017 comet 24P/Schaumasse (P=8.25a) should be detectable in larger instruments in the northeastern morning sky. During the following weeks it should brighten rapidly and should – using the parameters m0=7.5 mag / n=10 – peak at 10.5 mag in November 2017. In fall 2017 it will move from Gemini into Virgo, reaching greatest altitudes of about 40° in October.
Not more than a dozen observations can I incorporate in my analysis, spanning the period end of August and end of October 2017. The brightness estimates can be represented rather well with the parameters m0=10.0 mag / n=6. Thus the absolute brightness as well as the activity parameter are below the prediction, yielding a maximum brightness of only 12.0 mag in mid-November. During October the coma was very diffuse (DC 2) with a diameter of about 2'.

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Starting in October 2017 owners of large instruments could try to find comet 62P/Tsuchinshan (P=6.38a). It should brighten to 14.5 mag until the end of November. In October/November it will move from Cancer to the border of the constellations Leo/Virgo, thus being visible in the morning sky. However, this comet shows an erratic brightness behaviour.
The comet brightened rapidly between end of September and end of October 2017 from magnitude 14.0 to 11.0 (coma diameter about 2-3'). Because this development is based on only one dozen observations in the moment it cannot be decided whether the comet shows an extremely high activity (with the rough parameters m0=-1.0 mag / n=30) or if it experienced an outburst in mid-October. Assuming the first case the comet should peak at 10.5 mag in mid-November 2017, thereafter fading rapidly, reaching magnitude 16 already at the beginning of February 2018.

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Comet 65P/Gunn (P=7.64a) will pass perihelion again in October 2017. Due to its small eccentricity its development can be followed for two years in large amateur instruments. However, it will move through Scorpius and Sagittarius during the most interesting months, being a difficult object for mid-European observers.
I could use 35 observations, spanning December 2015 to July 2017. They indicate brightness parameters of about m0=7.5 mag / n=4, yielding a maximum brightness of 13.8 mag in June 2017. The maximum coma diameter of 0.8' (75.000 km) was reached in summer 2017, with the coma being rather diffuse (DC 2-3).

Total Brightness and Coma Diameter

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A small number of published observations of comet 96P/Machholz (P=5.29a) support a pre-perihelion (perihelion occurred on Oct. 27 at solar distance 0.12 AU) brightness development consistent with the predicted parameters m0=15.0 mag / n=5. The comet brightened to 11.0 mag at the time it disappeared in the twilight (in mid-October 2017). Thus it can be expected that it peaked around 3 mag during the days of perihelion. Post-perihelion it will be situated very near the Sun for many weeks and therefore most probably no further observations will be possible.

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Comet 103P/Hartley (P=6.48a) passed perihelion already in May 2017, but was not observable at that time due to small elongations. Starting in October 2017 owners of large instruments could try to detect this comet on the morning sky. Using the parameters m0=9.5 mag / n=4 the comet should fade from 14.5 mag to 15.5 mag during October/November. During this interval it will move from Sextans into Crater.

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Comet 213P/Van Ness (P=6.12a) will pass perihelion for another time at the end of September 2017, possibly peaking at magnitude 11.5. However, during the preceeding perihelion passage the comet displayed an outburst. Thus we better should expect a fainter object. Alas, the comet will move through Sagittarius during the most interesting months.
In this apparition the comet showed an average development, according to 25 observations. The brightness development can be well represented by the parameters m0=10.5 mag / n=4, yielding a maximum brightness of 13.5 mag at the end of July 2017. The coma diameter measured 0.5' (25.000 km) at the start of the apparition as well as in mid-October. It reached a maximum of 0.7' (35.000 km) at the end of July. The coma was not much condensed with the degree of condensation constant at DC 3. The comet may get fainter than 16.0 mag in mid-January 2018.

Total Brightness and Coma Diameter

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In fall 2017 comet 217P/LINEAR (P=7.87a) should become visible in midsized instruments. The small number of observations published until July 2017 hint towards the parameters m0=10.5 mag / n=3. The comet peaked at magnitude 12.0 at perihelion (on July 17), showing a moderately condensed coma of diameter 2'. Mid-European observers will first have a chance in August, with the comet positioned in Orion (morning sky). Until the end of November it will move along the ecliptic to the borders of the constellations Cancer/Hydra/Canis Minor. It is expected to fade from 12.5 mag to 13.5 mag.
Based on 25 observations the comet showed a very high activity in this apparition. Despite the small number of observations a different brightness development pre- and post-perihelion is evident. The comet brightened much more rapidly pre-perihelion than it faded post-perihelion. At magnitude 14.5 at the start of April it peaked at 12.2 mag in mid-July. At the end of October it had faded to 15.0 mag. The appropriate formulae are:

pre-perihelion: m = 6.4m + 5×log D + 55×log r
post-perihelion: m = 9.7m + 5×log D + 19×log r

Thus it should get fainter than 16.0 mag at the end of November. The coma diameter increased from 0.8' (50.000 km) at the beginning of the visual apparition to 1.6' (100.000 km) at the end of July, only to decrease thereafter, measuring 1.1' (70.000 km) at the end of October. The coma was medium-condensed with the degree of condensation at about DC 4 to DC 4-5. No tail was discerned visually.

Scheinbare Helligkeit und Komadurchmesser

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Comet 240P/NEAT (P=7.62a), which will pass perihelion (at a distance of 2.13 AU) in May 2018, brightened more rapidly than expected between July 18 and Aug. 28, 2017, according to H. Sato. Whereas on July 18 the 15" coma was of total magnitude 17.3m (w-band), the comet was of magnitude 13.9 mag on Aug. 28, displaying a 1.3' coma. The tail length increased from 75" to 3' (CBET 4427). Observations during the following weeks showed a constant brightness, about 4 mag brighter than predicted. At the end of October 2017 the medium-condensed (DC 3) coma of diameter 1.0' was of total magnitude 13.5. This should be the maximum brightness in this apparition. If the comet should develop according to the parameters m0=8.0m/n=4 it should show a constant brightness until it will disappear from the evening sky for mid-European observers at the opening of March 2018.

Andreas Kammerer


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