A Christmas comet is on the way to our skies. Comet Catalina (C/2013 US 10), which has been slowly brightening in the southern-hemisphere skies all year, has crossed the celestial equator moving north. It will finally become visible to northern-hemisphere observers in the pre-dawn sky from this week through late January, and will remain visible to southern-hemisphere observers until late December. Grab your binoculars and get ready to see this end-of-year visitor from the most distant reaches of the solar system.
Comet Catalina was discovered by the Catalina Sky Survey on October 31, 2013 when it was a very faint 19th magnitude and nearly eight astronomical units (AU) from the Sun (about 1.2 billion kilometers). At first, astronomers suspected it was a short-period asteroid, but further observations revealed its true nature. The comet is a visitor from the Oort Cloud, a vast spherical cloud of comets and icy leftovers from the early days of the solar system that extends far past the orbit of Pluto and perhaps a quarter of the way to the nearest star. A passing star millions of years ago, or perhaps galactic tides, nudged the little comet towards the inner solar system where it fell deeper into the Sun’s gravity well and picked up speed.
Earlier this year, astronomers were hopeful that Comet C/2013 US 10 (Catalina) would become quite bright, perhaps reaching a respectable 3rd magnitude, making it easily visible to the unaided eye. But expectations have been managed downwards recently, and the comet may only reach magnitude 5 or 6. That puts it just at the edge of visibility in dark sky without optical aid. But it will be a fine sight in binoculars or a small telescope. Catalina has been sporting a tail a few degrees long for some time (see image above).
Map showing position of Comet C/2013 US10 (Catalina) from late November 2015 through mid-January 2016. Courtesy of Sky and Telescope magazine.
The comet made a sharp turn around the Sun when it reached perihelion on November 15. Starting on November 24, Catalina will be visible in the pre-dawn sky in the constellation Virgo about 8° above the southeastern horizon in the northern hemisphere (and the northeastern horizon in the southern hemisphere). It will be visible in binoculars in dark sky about 10° (a fist width) east of the bright white star Spica in Virgo late November. Each day brings the comet a little higher above the horizon.
The bright waning Moon gets in the way of good viewing from November 26 through December 3, then gets out of the way again until just before Christmas.
The comet is moving northward about 1º per day through November and December from Virgo and into the constellation Boötes. On January 1, 2016, in the pre-dawn sky, the comet passes within 1/2º of the brilliant star Arcturus, the brightest star north of the celestial equator. It will still be visible to southern-hemisphere stargazers on New Year’s Day but it will quickly move north and out of sight for southern stargazers for the rest of its apparition.
Comet Catalina makes it closest approach to Earth on January 12, 2016 as it speeds by the handle of the Big Dipper from January 14-17. During that time, it will appear to move at 2º per day or about 5′ (arcminutes) per hour. That’s fast enough to show obvious motion relative to the background stars in just 30-60 minutes when observed in binoculars or a small scope. Again, in January, it may become bright enough to see without optics. We’ll have to wait and see– comets from the Oort Cloud are notoriously unpredictable.
This will be Comet Catalina’s first and only visit to the inner solar system. It has picked up sufficient speed to be ejected from the solar system and into interstellar space, never to return.