I have heard that comets are like cats. They both have tails and are hard to predict. Comet ISON certainly fits this description.
Early in November, I saw comet ISON as an “easy” binocular object. By easy, I mean that if your binoculars are well focused you can tell you are not looking at a star. There was the faintest hint of a tail. After that viewing, the comet brightened rather suddenly and unexpectedly by a factor of 10 and became visible to the naked eye. Who knows how it will behave next.
Comets are composed of dust and ice and spend most of their time in the outer, colder parts of the solar system, far away from the sun. If a comet’s orbit brings it near the sun, the ice will evaporate, sending a vapor cloud out into space. This cloud of outgassed material is called the coma. When we see a comet, it is the coma, and maybe a tail, that we are seeing.
The dense head of any comet is typically very dark and might be only a few kilometers across, but the bright coma could be many thousands of kilometers across, and the tail could easily be millions of kilometers long.
The tail forms when the solar wind blows this lightweight, evaporated material away from the sun. So a comet’s tail is always pointing away from the sun, even when the comet itself is heading away from the sun. That would be one big difference between comets and cats, as I have never seen a cat running tail first.
ISON reaches perihelion, its closest approach to the sun, on Thursday, and will be running tail first away from the sun after that. That is, if it survives. It gets so close to the sun that it could break up. Keep your fingers crossed for an impressive display.
The reason it is so hard to predict the brightness of any newly discovered comet, is that it is impossible to tell its internal structure and composition. As the comet approaches the sun, the heat of the sun is digging into the comet. Sometimes, a new pocket of ice is found and the comet gets much brighter. But like mining for gold, you don’t know what you will hit until you dig. And sometimes you come up empty.
If you get up to see comet ISON in the east before dawn, you also might try looking for several other objects of interest. On the mornings of the 25th and 26th, Saturn and Mercury are with in 1 degree of each other. A couple of other comets also are bright enough to see with binoculars or a small telescope. Comet 2/P Encke, a regular, but dim visitor every 3.3 years, will be magnitude 5.3, and very close to the horizon. C/2013 R1 Lovejoy was discovered in September, but is now brighter than 8th magnitude.
If you are not an early riser, or prefer to do your observing in the evening, Jupiter is very prominent, rising a couple of hours after Venus sets.
firstname.lastname@example.org. Charles Hakes is an assistant professor in the physics and engineering department at Fort Lewis College and is director of the Fort Lewis Observatory.