The year 2018 winds down with the apparition of the modest but easily observable Comet 46/P (Wirtanen). This periodic comet will not rival some of the better “Comets of Christmas Past” such as Comet McNaught in 2006, Comet Hale-Bopp as it brightened towards the end of 1996, or even the relatively disappointing Comet Kohoutek in 1973. But Comet Wirtanen will grow bright enough to see with binoculars and, in dark sky, with the naked eye as it passes through some of the most prominent constellations of the season. It’s a great excuse to dust off your optics and get outside to enjoy the solar system in action and share the view with those around you
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As a periodic comet, Comet Wirtanen is no stranger to the inner solar system. The comet, which was discovered by astronomer Carl Wirtanen in 1948, takes a lap around the Sun every 5.4 years, and it has been visible on every return visit except for 1980 when it was too close to the Sun to observe.
But it’s been around a lot longer than that. Like all comets, Wirtanen is a frozen 4-billion-year-old relic from the formation of the solar system, and it likely spent much of its time in the Kuiper Belt beyond Neptune before it was yanked into the inner solar system. Its elliptical orbit takes it out near the orbit of Jupiter at its farthest point from the Sun, and at its closest approach, it’s about the same distance from the Sun as the Earth.
Some apparitions of the comet are better than others. During its last approach to the Sun in 2013, the comet reached a dismal maximum brightness of magnitude +14.7, hard to see at all even in a big backyard telescope. This time around the comet may reach magnitude +4. That’s a difference in brightness of nearly 20,000 since the last apparition.
Why such a difference? This time, the comet will come much closer to Earth than in 2013, and it will do so as it reaches its maximum intrinsic brightness as it makes its closest approach to the Sun. The closest approach to the Sun happens on December 12, 2018, when the Sun and comet are about 1.08 AU (astronomical units) apart. The closest approach to Earth comes on December 16, 2018 when the comet is just 11.5 million kilometers away, or about 30 Earth-Moon separations. That’s the 10th closest approach of ANY comet in modern times!
A Geminid Meteor captured with Comet 46/P Wirtanen (upper left) on Dec. 10, 2018
Because it’s so close, the coma of Wirtanen, the big glowing head of gas that’s boiled off the small frozen nucleus, will appear nearly as large as the full Moon. So the light from the comet will be spread over a large area. That makes it harder to see, especially in light-polluted sky.
While not especially bright, the comet will be relatively easy to find. The comet starts December southwest of Orion in the constellation Eridanus. Then it quickly moves northwards into Taurus and Auriga, even passing the Pleiades star cluster on Dec. 16. For the whole month, the comet is perfectly located for early-evening observing, so you don’t need to sacrifice your long winter’s nap to enjoy this celestial spectacle.
The location of the comet this year favors northern-hemisphere observers. But deep-southern observers can see the comet until Dec. 18-20, more or less, low over the northeastern horizon.
Here’s a map showing the location of the comet in December 2018 and through early January 2019:
Binoculars are your best choice to help you spot the comet; its low surface brightness makes it hard to see, at least initially, with just your unaided eyes. However, if you live outside of the city, or even in the suburbs, have a look for the comet without optics. You may see it. It will look like a large, diffuse glow. And if you have a telescope, indulge yourself in a closer look at this little cosmic snowball as it’s set aglow by the Sun.
What can you expect to see? Well, you’re not going to see a long tail blazing its way across the sky. Mostly, you’ll see a big fuzzy disk that makes up the coma. The brightness is much higher near the center of the coma around the small nucleus which itself is just 600 meters across. Use averted vision to see maximum detail. And as always, complement your observations with imagination and knowledge of what you’re seeing. It makes for a far better experience.
Larger telescopes and cameras will show the coma with a faint greenish color. This color comes from CN and C2 molecules that emit light at distinctive wavelengths in the green part of the spectrum.
Like many short-period comets, since its first arrival in the inner solar system, Comet Wirtanen has been knocked around a bit by the gravitational influence of Jupiter. Each time the comet gets close to the big planet every few decades as it reaches its farthest point from the Sun, its orbit gets altered slightly. That changes the period and shape of its orbit. Early in the 20th century, the orbital period of Comet Wirtanen was 6.8 years and it came within just 1.62 AU of the Sun. That compares to the 5.4 year period and 1.08 AU perihelion distance today. In fact, as they first enter the inner solar system, most short-period comets like Wirtanen experience multiple gravitational interactions that settle their orbits into their current configurations. That fact that we see such comets at all is a result of the complex gravitation environment of our solar system.
Comet 46/P Wirtanen is getting a great deal of attention at the end of 2018. But it missed its chance at even more fame a few years ago. It was the initial target of reconnaissance of the now-famous Rosetta spacecraft that orbited and then landed a small probe on the surface of a comet. However, the craft missed the launch window for visiting Wirtanen. It was aimed instead at Comet 67/P Churyumov–Gerasimenko instead, and it took some of the most incredible images and video of any spacecraft that was ever launched into the solar system.
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