Many of the patterns of astronomy are so regular that we quite literally set our clocks by them. Our notions of time are tied to the regular cycles of day and night and of the yearly seasons, all of which have their origins in the rotation of the earth on its axis and in the orbit of the earth around the sun. All of this is comfortably predictable.
Most of the objects we see in our night sky tread equally well-worn and well-mapped paths. So-called periodic comets (those with solar orbital periods less than 200 years) such as Halley’s Comet, which will quite reliably repeat its 1985-1986 apparition in 2061, can be anticipated years in advance. Here is a diagram of Halley’s orbit and it current location; the image was created at this fantastic website.
We know about Halley because we have historical records of its reappearance every 76 years or so going all the way back to 240 BCE. But think for a moment about a long period comet—a really long period comet—perhaps one with an orbital period of 100,000 years, which for official purposes is designated a non-periodic comet. No historical record here! Such comets are presumed to come from the Oort Cloud of comets, a vast swarm of icy objects surrounding our sun at a great distance.
When the orbit of one of these pristine chunks of dusty snow and frozen gases is perturbed, it may wind up falling in a millennia-long plunge toward the sun. As it does, its long-frozen volatile materials—water, carbon dioxide, ammonia, methane—warm up and begin to “boil off”. This amazing image from a flyby of Comet Hartley 2 in 2010 shows jets of volatile materials being ejected from the cometary body.
And when the comet comes really close to the sun, we have the beautiful tail we associate with these heavenly visitors, as in this image of Comet Hale-Bopp in 1995, our last truly spectacular comet.
Notice that there are really two tails, the yellowish dust tail and the bluish gas tail. If you want to follow up on the reasons for this, you can go here.
2013 has two of these non-periodic comets lined up for us, and one of them could be really, really spectacular. Then again, they could both be just ordinary. The first is Comet PANSTARRS, officially designated C/2011 L4. The odd name comes from the telescope by which it was discovered; the official designation has a C for non-periodic comet, followed by the year of discovery, followed by a letter indicating the half-month of discovery, followed by a number indicating the order of discovery in that half-month. This comet was the fourth discovered in the first half of June 2011 (the letter “I” is omitted). Comet PANSTARRS will be closest to the earth on March 5 and closest to the sun five days later.
After some initial excitement about this never-before-seen comet, expectations for how bright it might appear have…ahem…dimmed. You will probably need binoculars to see it, but it is worth a try!
Comet ISON (C/2012 S1) holds out the tantalizing promise of being a “Great Comet”, loosely defined as one that is exceptionally bright. What would make it so?
1. It has probably never before been in the inner solar system. None of its volatile materials have been warmed and lost.
2. If it is physically large, it will naturally release more material as it warms.
3. If it has a lot of volatile material near the surface, it will release more.
4. If it passes very near the sun, it will warm even more.
From its orbit, number 1 is a near-certainty. Numbers 2 and 3 are unknowable at this point. But number 4 is what excites astronomers, for on November 28, 2013 this comet will pass less than one solar radius from the sun’s surface. That’s really, really close! If (big if) it remains intact, it could become as bright as the full moon. Here are some images that assume it all works for our maximum viewing pleasure:
Finally, just so you have some idea of what a Great Comet looks like, here is an image of Comet Ikeya-Seki from 1965, the brightest comet of the last 75 years. Let’s hope Comet ISON is even better!