Comet Section        




The Geminid meteor shower is always a highlight for planetary observers in December. This month also provides a nice opportunity to observe the largest “Geminid” of all, asteroid 3200 Phaethon. The Geminid parent body will brighten to magnitude 10.7 as it passes within 0.07 au of Earth. Another object, 2017 VT14,  that could be cometary based on its orbit, may make a historic close approach to within 0.01 au of Earth though it may be too faint except for CCD imagers. As for active comets, C/2017 T1 (Heinze) will be interesting to watch as it approaches perihelion early next year. This intrinsically faint, dynamically new comet may brighten to 9-10th magnitude this month.

Evening Comets

29P/Schwassmann-Wachmann 1 [Perihelion on 2019 March 7 at 5.77 au]

The 2017 season for observing 29P is drawing to a close as the comet is slowly sinking lower in the wester sky after dusk. The comet continues its elevated activity level and is currently 13th magnitude. CCD imagers are asked to keep monitoring it as long as possible before it is lost till after solar conjunction. 29P is located near the border of Aquarius and Capricornus.

C/2017 O1 (ASASSN) [Perihelion on 2017 Oct 13 at 1.50 au]

The brightest comet of the past few months (that didn’t require a spacecraft to observe) was C/2017 O1 (ASASSN). After obtaining a peak brightness of magnitude 8-9 in September and October, the comet started to fade in November. As seems to be typical for large diffuse comets, there is quite a bit of scatter in the current magnitude estimates with most observers placing it around magnitudes 10 and 11. The comet should continue to fade as it moves away from the Sun (1.64 on Dec 1 to 1.84 au on Jan 1) and Earth (0.95 to 1.23 au). For those wanting to take a look at ASASSN, it will be located in the northern circumpolar sky as it moves through the Milky Way constellation Cepheus this month.

3200 Phaethon [Perihelion on 2017 January 25 at 0.14 au]

Asteroid 3200 Phaethon is the parent body of the Geminid meteor shower. Whether the shower was produced by long-lasting cometary activity or a series of splitting events, the Geminids are now one of the strongest annual showers. Recently Phaethon has been observed to display comet-like activity around perihelion. Due to intense heating (perihelion is 0.14 AU from the Sun or 7 times closer than Earth) some of the rocks on the surface may have fractured producing a cloud of dust which was knocked off the surface by solar radiation pressure. In effect, it is a acting like a “rock comet”. Still these sort of events are very short-lived and produce minimal amounts of debris, hence they are not large enough to create the large mass of existing Geminids by themselves.

This year Phaethon will pass 0.069 au from Earth on December 16 and brighten to magnitude 10.7. While little to no cometary activity is expected, such a close approach does provide an opportunity to directly study the surface of Phaethon via time-resolved and color photometry. Lightcurve, color and phase function photometry are requested for better understanding of this enigmatic object which is now also the target of a proposed Japanese-lead spacecraft mission, Destiny+ (see presentations on Destiny+ here and here). With a rotation period of 3.6 hours, an entire rotation can be observed during a relatively short observing session. This is the closest approach to Earth by Phaethon since 1974 (0.055 au) and till 2093 (0.020 au). Phaethon starts the month 1.25 au from Sun, 0.31 au from Earth and V magnitude 13.7. It rapidly moves across the evening sky constellations of Auriga (Dec 1-11), Perseus (11-14), Andromeda (14-17), Pegasus (17-19), Aquarius (19-21) and Capricornus (21-31) before ending the month at 0.73 au from Sun and 0.29 au from Earth.

2017 VT14 [Perihelion on 2017 Dec 11 at 0.98 au]

Phaethon won’t be the only “is it a comet or asteroid” object passing close to Earth this month. Though its orbit suggests no relationship to Phaethon, 2017 VT14 will pass close to Earth only hours after Phaethon.

2017 VT14 was discovered on November 13 by the NEOWISE satellite in low Earth orbit. As of November 23, when it was last observed, this object has not shown any cometary activity. Its orbit is similar to a Jupiter family comet and backwards integration of the orbit uncovers a number of recent close approaches to Jupiter. VT14 will pass 0.0098 au from the Earth on December 17. If it is recognized as a comet, it may make the closest cometary approach in recorded history. Comet D/1770 L1 (Lexell) passed within 0.0151 au of Earth in 1770. Only one other comet may have come closer though it is based on a very poorly determined orbit (C/1491 B1 at 0.0094 au).

VT14 starts the month at 0.99 au from Sun and 0.10 au from Earth. Perihelion occurs on December 11 at 0.98 au. By the end of the month, VT14 will be located 1.02 au from Sun and 0.09 au from Earth. If it remains inactive, it will start the month at V magnitude 20.5, peak at 16.0 on December 17 and end the month at 19.6. If it becomes active, it could be many magnitudes brighter.

The object will quickly move through the following constellations: Pegasus (Dec 1-12), Aquarius (12-15), Sculptor (15-16), Phoenix (16), Tucana (16-17), Hydrus (17, Horologium (17), Dorado (17-18), Volans (18), Carina (18-19), Vela (19-21), Pyxis (21-22), Antlia (22-31), and Hydra (31). It will be lost to most northern hemisphere observers just hours to days before closest approach. CCD observers are encouraged to closely watch this object for any sign of cometary activity. Similar to Phaethon, this is also a great opportunity to acquire lightcurve, color and phase function photometry for this object.

Morning Comets

24P/Schaumasse  [Perihelion on 2017 Nov 16 at 1.21 au]

Short-period comet 24P/Schaumasse is making its 11th observed return since its discovery back in 1911 by French astronomer Alexandre Schaumasse. Perihelion occurred last month on November 16 at 1.21 au and closest approach to Earth on November 21 at 1.46 au. On November 26, I saw 24P with my C14 at 98x at magnitude 11.2. It was small (2.5′) with a diffuse coma and no sign of a tail. This month, comet Schaumasse should slowly fade from magnitude 11 to 12 as it moves through Leo (Nov 1-8) and Virgo (8-30). Now past its November 16 perihelion, Schaumasse will recede from the Sun (1.22 to 1.34 au) and Earth (1.46 t o 1.50 au) this month.

62P/Tsuchinshan [Perihelion on 2017 Nov 16 at 1.38 au]

Discovered photographically in 1965, 62P/Tsuchinshan (Chinese for Purple Mountain which is the name of the observatory where the discovery was made) is making its 9th observed return. The comet was observed between magnitude 10.5 and 11.0 for much of late October into November. On November 26, I observed 62P with my C14 at 98x at magnitude 10.8. Similar to 24P, 62P was small (2′) with a diffuse coma and no sign of a tail. This month, its heliocentric distance increases from 1.39 to 1.47 au while its geocentric distance drops from 1.29 to 1.20 au as it moves through Leo (Nov 1-26) and Virgo (26-30) in the morning sky.

C/2017 T1 (Heinze) [Perihelion on 2018 February 21 at 0.58 au]

C/2017 T1 (Heinze) is a recent discovery by Ari Heinze with the ATLAS (Asteroid Terrestrial-impact Last Alert System) survey on October 2nd at 18th magnitude. Perihelion will be on February 21 at 0.58 au. The comet appears to be intrinsically faint but due to a small perihelion distance and a pre-perihelion close approach to Earth in early January at 0.22 au, it is possible that this comet may brighten to magnitude 9.5 to 10.0 by the end of this month. If Heinze is going to get that bright, it better get a move on. While we haven’t received any observations of Heinze in November, recent observations submitted to COBS show the comet had only brightened to 15th magnitude by the end of November.

This will be an interesting comet to watch this month. A dynamically new and intrinsically faint comet, Heinze may not survive its perihelion passage. CCD imagers (and if it brightens further, visual observers) should keep this comet under close watch. Heinze starts the month at 1.70 au from the Sun and 1.08 au from Earth. By the end of the month, the comet is much closer to the Sun (1.21 au) and Earth (0.25 au). It moves through Hydra (Dec 1-17) and Cancer (17-31) this month.

C/2016 R2 (PANSTARRS) [Perihelion on 2018 May 9 at 2.60 au]

C/2016 R2 (PANSTARRS) reaches perihelion in May at 2.60 au from the Sun. For much of 2017, C/2016 R2 showed little increase in activity. That all changed last month as the comet brightened from 14th to 11th magnitude. This month, PANSTARRS should continue to brighten as its perihelion distance drops from 3.09 to 2.94 au and it geocentric distance drops from 2.14 to 2.05 au before increasing back to 2.07 au. The comet passes though opposition this month as it moves against the stars of Orion (Dec 1-16) and Taurus (16-31).

As always, the Comet Section is happy to receive all comet observations, whether images, drawings or magnitude estimates. Please send your observations via email to < carl.hergenrother @ >.

- Carl Hergenrother (ALPO Comet Section Coordinator)

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