Star Points: Comet Wirtanen to brighten during December apparition

This month could see the dramatic brightening of a periodic comet peaking at mid-month. Optimistic predictions expect the comet to become visible to the unaided eye. Carroll County residents will have a chance to view the comet at Bear Branch Nature Center, weather permitting. More about that later.

Comet 46P/Wirtanen is an interesting object despite its relatively short observational history.


When discovered photographically in January 1948 at Lick Observatory on California’s Mt. Hamilton, Wirtanen’s calculated orbital period around the sun was 6.7 years. The ‘P’ in its name means that the comet is “periodic” in that it orbits the sun with a period of less than 200 years.

The gravity of the giant gas planet Jupiter has modified the comet’s orbit during close passes near the planet. Such an encounter between the two in 1972 shortened the comet’s orbital period to 5.9 years. Another in 1984 reduced it further to its current 5.5-year period.

These encounters with Jupiter happened to decrease the comet’s minimum distance — or “perihelion” — from the sun as well. This decreased distance leads to an increase in solar heating of the comet’s surface ices and dust, possibly intensifying its brightness.

Currently Wirtanen’s perihelion distance is only 8 percent greater than Earth’s average distance from the sun. This month, Earth and comet will experience a close encounter just four days after the latter reaches its perihelion point. This occurs on the morning of Dec.16 when the comet is only 7.2 million miles from Earth.

This may not sound very close, but as far as comets go it’s going to be nearby. The small separation distance will enhance Wirtanen’s apparent brightness when viewed from Earth. How bright will it get?

I checked the Jet Propulsion Laboratory’s online Horizon’s ephemeris generator. At closest approach to Earth it predicts a maximum magnitude of 9.5 – fainter than stars that can be seen with the unaided eye. But other sources are predicting that the comet could reach magnitude 3, well within the reach of unassisted eyeballs observing the sky from a dark location free of city lights.

As the comet gets closer, the cloud of dust and gas known as the “coma” surrounding the comet’s core or “nucleus” will swell due to increasing solar heating. This is both good news and bad news for observers. The expanding cloud should increase the brightness of the comet, while at the same time diffusing the light by spreading it out. An estimate of magnitude 3 means the comet is as bright as a pin-point star of the same magnitude. But since the comet appears diffused, the light is spread out and thus will have a dimmer surface brightness.

Another close comet, IRAS-Aracki -Alcock, was discovered in 1984 by the Infrared Astronomy Satellite. At its closest point, it passed by Earth at only 3 million miles — less than half the distance that Wirtanen will this month. It was the closest comet in more than 200 years.

That comet appeared as a large bright round fuzzy glow high in the sky, easily visible to the unaided eye. At magnitude 3 or 4 it was about the same brightness that Wirtanen is expected to possibly reach. At its further distance, Wirtanen’s coma will be more condensed and possibly easier to see.

Even if it’s visible to the naked eye, binoculars will be a great way to view Comet Wirtanen and keep track of it from night to night. I recommend Googling the comet online and looking for a finder chart.

A program is scheduled in the Bear Branch Nature Center (BBNC) planetarium at 7:30 p.m. on Dec. 15. Weather permitting, after the show, the comet may be observed through a telescope at the co-located B.F. Roelke Memorial Observatory. Reservations are required for the planetarium. Call 410-386-2103 to reserve your planetarium seat. Observatory observing is free and starts at 8:30 p.m.

On the previous night, Friday, Dec. 14, the Westminster Astronomical Society will have telescopes on hand for a happy hour star party at the Milkhouse Brewery from 5-9 p.m. The telescope observing is free although weather permitting. The brewery is located at 8253 Dollyhyde Road, near Mount Airy though across the county line in Frederick County and not too far from Libertytown.