Just in time for the holidays, our inner solar system is about to receive a visitor that may add a green glow to December's night sky.
46P/Wirtanen, a three-quarter-mile-wide periodic comet that comes our way roughly every 5.4 years, will make its closest approach to Earth on Dec. 16. Categorized as a "hyperactive" comet, it belongs to a small family of comets that appear to emit more water than they should for the size of their nucleus.
Despite a distance of 7.2 million miles, or 30 times the distance from the Earth to the moon, Comet Wirtanen will be the 10th closest comet to graze Earth since 1950. It could also, just maybe, become the brightest of the lot.
A fickle celestial phenomena
At it approaches the sun, Wirtanen is expected to brighten to a magnitude of between 6 to 3.5. (It helps to know that an object's brightness is measured by apparent magnitude. The brighter an object appears, the lower its magnitude, with the brightest objects having negative magnitudes.) While that's within the realm of visibility without the use of a telescope or binoculars, you're still going to have to know where to look to pick out its fuzzy green glow. For comparison, Venus has a magnitude brightness of -4.3 and Jupiter of -2.2. Past comets like Halley's (1986) reached 2.1, while Hale-Bopp (1997) was easy to spot at -1.0.
It's important to remember, however, that comets are notoriously fickle phenomena. Every trip around the sun vaporizes frozen volatiles on the comet's crust that result in the formation of a glowing coma of gas around the nucleus. Solar wind then stretches this into a tail, with some extending for million of miles from the comet's head.
In some instances, comets that scientists expect to brighten end up staying stubbornly immune to the heat of the sun. Others, like Comet 17P/Holmes in 2007, surprise absolutely everyone by temporarily becoming the largest object in the solar system.
From small beginnings
When Comet Holmes made its return approach in October 2007, astronomers weren't expecting much. Discovered in 1892, the 2.2-mile-wide periodic comet had traditionally been a faint object. That all changed over 42 hours from Oct. 23-24 when Holmes suddenly brightened from magnitude 17 to 2.8 — a change of brightness by a factor of about half a million. This not only made it the largest outburst by a comet in recorded history, but its drastically expanded coma temporarily made it the largest object in our solar system. And all from something just a bit larger than New York's Central Park.
Comet 17P/Holmes was briefly the largest object in the solar system. (Photo: Iván Éder/Wikimedia)
At its peak, the coma of Comet Holmes was a sphere wider than the diameter of the moon's orbit around Earth! "This is equivalent to the planet Saturn suddenly becoming as bright as the Full Moon," David Morrison, senior scientist at the NASA Astrobiology Institute, wrote in a newsletter.
While scientists aren't exactly sure as to the source of the outburst, it's likely that it was caused by either a collision with a meteoroid or a build-up of gas within the nucleus that eventually exploded through to the surface.
Looking for Wirtanen
Does Comet Wirtanen have the potential to experience a similar outburst to Comet Holmes? It's certainly possible. It's also possible that you or I could win the lottery one day. The thing about comets is that, beyond gauging their orbits, there's little else to go on with regards to how they'll react to the sun's intense heat. As legendary comet expert Fred L. Whipple once said: "If you must bet, bet on a horse, not a comet!"
Just for fun, consider this: If Comet Wirtanen were to experience even a mild outburst similar to Holmes, it would be quite the show. Holmes was more than 149 million miles from Earth when its coma briefly encompassed an area larger than the sun. Wirtanen will pass by at a fraction of that distance.
Fantasies aside, at worst Wirtanen will be a beautiful sight through telescopes and binoculars and at best an attractive green smudge with the naked eye. According to observations by the University of Maryland, Wirtanen appears to be — so far — living up to expectations.
"Wirtanen is going to be bright. As of early November it had a total brightness around mag ~8 and appeared to be following Yoshida's total magnitude prediction, though we don't know what that will really mean around close approach," they write. "However, even if it does not reach naked eye brightness, it will still be a great object to view with binoculars or a small telescope!"
During its closest approach to Earth in mid-December, seek out dark skies and look for it in the constellation Taurus. According to the One Minute Astronomer, locating this region among the heavens in thankfully relatively straight-forward.
If you can find Orion, finding Taurus is easy. Just extend the line of Orion's belt to the northwest until you find a bright orange star nestled in a V-shaped cluster of stars.
As mentioned above, comets this close to Earth don't happen too often, so it's definitely worth your time to take a moment and gaze up at the celestial wonder that is Wirtanen. Should clouds block your view, don't fret: the comet should be visible for several weeks through the end of the year and into 2019.