A Comet!

Coming Soon to a Sky Near You

Comet PANSTARRS (C/2011 L4), which has been dazzling southern hemisphere skygazers for a couple of weeks now, is making its way around the Sun. It made its closest approach (perihelion) on March 3rd,  and starting the 10th, it should be visible in the post-sunset skies for those of us in the northern hemisphere to enjoy.  Here’s a little gazing chart (courtesy of Astronomy Magazine and Gary A. Becker) to help you find it.  The view should be quite dazzling on March 12th and 13th, when it will appear not too far from a crescent Moon low in the western sky. The comet should be visible through most of the month, although later on it will be competing with the Full Moon, which could wash out the sky a bit. It may likely have two tails — a whitish dust tail and a bluish plasma tail, so it’s worth making the effort to take a look at PANSTARRS.

Back in the day, when I was in graduate school, I spent a lot of time studying images of Comet Halley. We were interested in its plasma tail (also known as the ion tail).  This is a stream of gas molecules that form as the Sun heats the icy nucleus of the comet. The ices start to “sublimate” (similar to how dry ice “melts”) and creates a cloud of gas and dust.  The material flows off the comet, forming a dust tail and the plasma tail. The materials in the plasma tail interact with the solar wind, which causes the plasma tail to glow in a process called ionization. It also creates structures in the plasma tail, and in the right conditions, can cause what is called a “disconnection event”. This occurs when the existing plasma tail encounters changes in the solar wind that are different from the conditions in which it was originally formed. Think of it as forming in one electrical polarity and when it encounters a different polarity, it can’t exist anymore. So, the plasma tail breaks off and a new one forms. This happens over and over again as the comet rounds the Sun. We studied this occurring in Comet Halley as it passed through in 1985 and 1986, and that allowed our team to analyze conditions in the solar wind by looking at the tail, as well as letting us chart what was once thought of as a “pathological” condition in comets.  Turns out it happens to comets with active plasma tails during the inner parts of their orbits around the Sun.  Comet PANSTARRS was showing a pretty active plasma tail as it went to perihelion. I’ll be watching it later this week to see how it fared. So, if your skies are clear later this week and early next, step outside after sunset and check out the comet. It could be quite lovely!