¡SkyCaramba! Weekly astronomy blog for the week of November 15 to 21, 2015
At the start of this week, a comet named Catalina is at perihelion and whipping around the sun. It will head north of the sun and emerge in the morning sky for a northern hemisphere audience. How well can you see it? That depends on a number of things.
Astronomers with binoculars and small telescopes were able to see the comet before it was closest to the sun. It will probably remain visible that way after perihelion. Of course, everyone hopes for a naked eye comet. Whether that will happen is hard to predict. Scientists have been wrong about such things before. Few are willing to take the risk of making a prediction.
Going close to the sun helps brighten a comet. Not only is it receiving more sunlight to reflect, the solar wind is stronger and tends to loosen up more comet material which, in turn, reflects even more light. Catalina will be 0.83 astronomical units from the sun. That’s 83% of the average distance the earth is from the sun. Many comets have gone closer without achieving naked eye brightness. But there’s still more to the story.
A comet that hasn’t been around the sun very many times has more material to lose as it soars into the solar wind. Astronomers think Catalina is on its first trip ever this close to the sun! So it could have a lot of dust that comes off and makes for a fuzzy head and long tail.
It also is heading closer to Earth until January 12 when it will be 0.72 astronomical units from us. That’s not the closest approach to Earth by a comet, but coming closer to us will still help make it more visible.
Comet Catalina is believed to come from the Oort Cloud. That’s a group of icy bodies theorized to orbit the sun at distances so great they take millions of years to go around just once. The theories say, every once in a while, something nudges one of those ice chunks out of the orbit it was in and it falls into a new orbit. After many years, tens of thousands in one case or millions in another, it’s close enough for observers on Earth to notice it and declare the discovery of a new comet.
What nudges comets like Catalina out of their old orbits? In some cases, two icy rocks orbiting on intersecting courses bump into each other. At least one bounces off the other and comes our way. There could also have been a star or other dense object passing relatively close to our sun that disturbed some of the Oort Cloud objects.
Catalina’s first trip to the close reaches of the solar system is expected to be its last. Astronomers predict it will leave the solar system altogether after we lose sight of it.