Comet 46P/Wirtanen was discovered in January 1948 by Carl Wirtanen, an astronomer at the Lick Observatory in the Diablo Range just east of San Jose, California. It is a Jupiter Family Comet (i.e. a comet with a period of less than 20 years), with an orbital period of 1987 days (5.44 years) and an estimated nucleus diameter of 1.2-1.4 km. The surface of the comet is considered to be 'hyper-active', meaning that its active surface area is higher than expected. 46P/Wirtanen was the initial intended target of the Rosetta space probe, with the mission being redirected to Comet 67P/Churyumov-Gerasimenko after a launch delay. The comet has periodic close encounters with both Earth and Jupiter, with the most recent close encounter with Jupiter being in 1972 and the most recent close encounter with Earth being in 2018.
As a Near Earth Comet, P46/Wirtanen has the potential to be the source of meteor showers. Furthermore, an optical trail (i.e. a trail of debris derived from the comet, which had remained coherent some time after the comet's passage) associated with the comet was observed in 2019, although no observed meteor shower has ever been connected to P46/Wirtanen.
In a paper posted on the arXiv database at Cornell University on 6 December 2023, and accepted for publication in the journal Astronomy & Astrophysics, Jérémie Vaubaillon of the Observatoire de Paris at Université Paris Science & Letters, Sorbonne Université, and the Université de Lille, Quanzhi Ye of the Department of Astronomy at the University of Maryland, and the Center for Space Physics at Boston University, Auriane Egal, also of the Observatoire de Paris, Université Paris Science & Letters, Sorbonne Université, and the Université de Lille, and of the Planétarium de Montréal, and the Department of Physics and Astronomy at the University of Western Ontario, Mikiya Sato of the National Astronomical Observatory of Japan, and Danielle Moser of the NASA Meteoroid Environment Office at Marshall Space Flight Center, revisit the possibility of a meteor shower being caused by Comet 46P/Wirtanen, and propose a possible event in December 2023.
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Simulations of the orbit of 46P/Wirtanen show that periodic encounters with Jupiter will change its orbit, hampering modelling of the comet's behaviour too far into the past. However, the cumulative effect of these is quite minimal as far back as 1830, enabling a reasonable level of prediction over the past 200 years.
The passage of meteoroid streams generated by comets is notoriously difficult to predict. Vaubaillon et al.'s models show that the trail left by 46P/Wirtanen has intersected the Earth previously, although the density of the dust during these encounters is uncertain. Dust trails left by the comet between 1925 and 1950 were shown to have intersected the Earth in 1912 and 1917, although it is likely that these trails were highly dispersed by this time, and there were no known observations of a meteor show which would have coincided with these encounters. The Earth also encountered a trail made by 46P/Wirtanen in 1974 in both 2007 and 2018, with some records made by dedicated meteor detection networks possibly relating to these encounters.
The relative trajectories of 46P/Wirtanen and the Earth mean that meteoric dust from the comet's trails will enter the Earth's atmosphere at the relatively low speed of 10.2 km per second. At this sort of speed, only millimetre-sized particles would be expected to produce a visible meteor, although radio instruments would be able to detect sub-millimetre sized meteoroids entering the atmosphere. There is precedent for such a method being used to detect meteor showers. The Arid Meteor Shower, derived from Comet 15P/Finlay, was predicted on 7 October 2021, and whilst not actually observed, was detected by the Southern Argentina Agile Meteor Radar-Orbital System and the Canadian Meteor Orbit Radar.
Encouraged by this, Vaubaillon et al. searched the database of the Canadian Meteor Orbit Radar for possible evidence of encounters with the trail of 46P/Wirtanen in 2007, 2012, 2017, and 2018, but were unable to find any conclusive signals.
The path of the 1974 trail of 46P/Wirtanen is thought to have undergone a complex evolution. In 1984 the trail would have had a close encounter with Jupiter, causing a significant change in the path of the affected meteoroids. This means that these meteoroids are no longer on the same path as the comet (which is not thought to be on a collision course with the Earth), and the Earth is likely to directly cross the trail in December 2023. However, the meteoroids are very spatially dispersed, and the Earth is not thought likely to pass through the central part of the stream, with the effect that only sub-millimetre sized particles will enter the Earth's atmosphere, while particles of up to a few centimetres in size may come within 0.05 AU of us (roughly twenty times as distant as the Moon).
Based upon their models, Vaubaillon et al. predict that the peak of meteor activity will happen between 8.00 am and 12.30 pm GMT on Tuesday 12 December 2023, although the peak of activity for larger particles (if they encounter the Earth at all), will be later, in the second half of 12 December or possibly early on 13 December.
As with the Aid Meteor Shower, it is difficult to predict the scale of any meteor shower produced by 46P/Wirtanen. The low velocity at which the Earth will encounter the stream makes it likely that the majority of meteors will be too dim to be visible to be detectable with the naked eye, with only a tiny number of meteors (if any) being visible. However, 46P/Wirtanen is notably more active than other comets, with as much as 40% of the surface of the nucleus actively shedding material, which may lead to more meteoroids being in the trail than predicted based upon studies of other comets.
Vaubaillon et al.'s models all predict that the shower associated with 46P/Wirtanen will be best observed between 8.00 am and 12.30 pm GMT on 12 December 2023, from North and West Australia, Papua New Guinea, New Zealand, and Indonesia. The predicted radiant point of the shower is close to the star λ-Sculptoris, leading Vaubaillon et al. to propose that the meteors should be called the λ-Sculptorids. The predicted small size and low entry speeds of the meteors will make detecting them extremely difficult, nevertheless, Vaubaillon et al. strongly encourage meteor enthusiasts to perform scientific observations and send their reports to the International Meteor Organization.
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