On December 16th, less than four days after making its closest approach to the sun, Comet 46P/Wirtanen will come within 11.5 million kilometers of the Earth making it one of the ten closest approaching comets since 1950 and the 20th closest approach of a comet dating as far back as the ninth century A.D. While it is a small comet with a nucleus barely 1 km wide, this close proximity should make it appear as bright as third magnitude which is bright enough to be viewed with the unaided eye. In fact, recent photos of the comet indicate that the comet’s gaseous atmosphere is now about as wide as a full moon.
Comet 46P/Wirtanen still has about ten days or so to go before its closest approach to the Earth and it is already as big as a full moon when including its large green gaseous atmosphere. The large green atmosphere is very noticeable in latest photographs and it comes from diatomic carbon (C2) – a gaseous substance common in comet atmospheres that glows green in the near-vacuum of space. Despite its close approach, 46P/Wirtanen will never become a great comet like Hale-Bopp in 1997 as its relatively small core of dirty ice (one-thirtieth the size of Comet Hale-Bopp) simply cannot produce enough gas and dust to create a long tail. The best case scenario is probably a big diffuse cloud of magnitude +3 or +4, barely visible to the unaided eye but an easy target for binoculars and small wide-field telescopes. On the nights of closest approach, 46P/Wirtanen can be found in the constellation Taurus rising in the east at sunset and high in the sky at midnight. Sky watchers in the northern hemisphere may orient themselves using these sky Maps: Dec. 5, Dec. 6, Dec. 7, Dec. 8, Dec. 9, Dec. 10, Dec. 11, Dec. 12, Dec. 13, Dec. 14, Dec. 15, Dec. 16 (courtesy spaceweather.com).
Comet 46P/Wirtanen is a small short-period comet which passes through the inner solar system every 5.4 years. Comet 46P/Wirtanen was discovered by Carl A. Wirtanen in 1948 at the Lick Observatory, California via photographic plate. The plate was exposed on 17 January 1948 during a stellar proper motion survey at the observatory. It took over a year before the object was recognized as a short-period comet due to a lack of observations. 46P belongs to a small family of comets that boast a higher level of activity than expected for their nucleus size and they emit more water vapor than they should. This is one of the reasons for added interest in this regular visitor to the proximity of Earth’s orbit.
Meteorologist Paul Dorian