Periodic comet 46P/Wirtanen, 14 days before closest approach

On December 16th, comet 46P will pass Earth at a distance of 0.077 AU (11.5 million km), which is an exceptionally close approach for a periodic comet. It is expected to peak at 3-4 mag, visible to the naked eye from locations without light pollution, and becoming a target for binoculars elsewhere.

The comet is now moving into view from the northern hemisphere. Last night, it reached an altitude of 24° from England, enough to rise above the haze layer, and to reveal some of the structure in its coma.

Because the outer coma already extends beyond the 38 arcminute field of view of our camera, we can only give a lower limit of +6.9 mag for the total brightness (the full comet is probably 1-2 mag brighter than that). We see a short tail, 8 arcminutes long, and two short jets with a length of 1-2 arcminutes. The inner coma is elongated, in the direction of the tail.

We find an Afro value (a measure for the dust production rate of the comet) of 114 cm, up from 105 cm on November 29th. This indicates that activity is increasing as comet 46P approaches perihelion, which it will reach on December 12th.

Photometry (APASS V comparison stars unfiltered, 440″ aperture radius): m1 = 6.9 ± 0.1 mag (underestimated)
Nuclear magnitude (Gaia DR2 G magnitude unfiltered, 6.75″ aperture radius): m2 = 12.2 ± 0.1 mag

Measurement in ICQ format:
46 2018 12 03.92 C 6.9 AQ 25.4Y 7a720 35 8.0m 38 ICQ XX BAMaaI C14.67mQHA GSA KPR 0 U9*0.02 2.2s 2.2

Observations made in support of the PACA_46P campaign.

Northolt Branch Observatories
The PACA Project


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