It was discovered by the Catalina Sky Survey on the night of Halloween 2013 and, as we enter the final month of 2015, it could become visible to the unaided eye.
I'm referring, of course, to a visitor from the outer parts of our solar system that astronomers have named Comet Catalina.
This comet, like all others, is one of billions of tiny icy remnants of the primordial solar system that tumble silently through the vacuum of space. Occasionally, one of these cosmic nomads drifts inward toward the sun's heat, and its ices disintegrate into a cloud of gas and dust around its nucleus (the "coma"). Sunlight and the solar wind act as a fan and blow this material outward to create one or two tails that always point away from our star.
As compact as a comet's tail may appear to us from Earth, its material is actually distributed over tens of millions of miles; in fact, to achieve the density of the air we breathe, a comet's entire tail would need to be compressed to fit into the size of an average suitcase. In other words, a comet is the closest thing to nothing that's still something!
Just how bright it becomes, however, is anyone's guess. Comets are notoriously fickle; they can flare up at any time, or they can fade and go totally unnoticed by the average stargazer. As noted comet-hunter David Levy likes to say: "Comets are like cats. They both have tails and they both do what they want." Their unpredictable and ghostly nature has led people over the ages — even some today — to interpret them as cosmic harbingers of doom.
Nevertheless, it may be possible to spot this interplanetary nomad in the pre-dawn skies of late November ... if you've got dark skies far from the lights of large cities. Simply use sky maps to find the stars and star patterns in the area, and use binoculars to search for a small fuzzball among them.
On Tuesday, Comet Catalina will appear only about eight degrees above the southeastern horizon an hour or so before sunrise, and by Saturday will have ascended to only about 10 degrees above the horizon. Unfortunately, the moon will also appear in the sky during that period, so it might be best to wait until December arrives and the moon departs from the early morning sky.
One of the dates to mark on your calendar is Dec. 7; on this morning the comet will appear to pass close to the crescent moon and the bright planet Venus.
And, by late December, Comet Catalina will be drifting by about one degree every morning through the stars of the constellation of Virgo, and will be heading almost directly toward Arcturus, the bright yellow-orange star of Bootes. In fact, on the first morning of 2016, the comet will pass just one-half of a degree from this star, and should make a beautiful sight in the low-power eyepiece of a small telescope.
Set your alarms, folks. If this comet becomes visible to the unaided eye you certainly won't want to sleep through it!