For hundreds of thousands of years, humans spent their evenings in nearly - complete darkness, surrounded overhead by a display of stars unlike anything most modern people have ever experienced. Thousands of stars visible every clear night, the Milky Way bright enough to cast a shadow, and dozens of intriguing patches of haze that we now recognize as star clusters, nebula and even galaxies.
Man's mind is built to recognize patterns. Through the ages of staring into the night sky, we learned the pattern of the shifts in fixed star patterns - the constellations - that mark the changing seasons. Orion in the winter, Leo in the spring, a summer of Cygnus, Pegasus in the fall.
Man finds comfort in predictable change - the phases of the Moon, annual meteor showers, even the motions of the "wandering stars", the planets, became well known though fascinating to early Man. Indeed it is from the earliest attempts to explain and predict these changes in the sky that the first evidence of pre-scientific thought springs in the most ancient civilizations.
But it is a very different reaction that Man has to sudden and unexplained change. The response is at first a deep rooted fear, followed only later by an unrelenting need to understand and predict. The two most feared and majestic astronomical phenomena in ancient (and even not-so-ancient) civilizations were solar eclipses and bright comets. Though we will not experience a solar eclipse visible from Connecticut this year, 2013 may very well turn out to be the Year of the Comets.
A comet is a body in the solar system consisting of frozen gas and frozen water, combined with bits of rock. A description given by an astronomer from the 1950s is quite accurate - comets are (large) dirty snowballs. Comets orbit the sun in highly elliptical orbits, travelling to great distances from the sun, and then returning to the neighborhood of the sun with periods varying from a few years to millions of years.
When far from the sun, a comet's gases and water remain frozen, and the object is at best barely detectable in large telescopes. As it approaches the sun, its temperature rises, and the frozen gas boils, creating a halo around the comet known as a coma. As it passes the orbit of Mars, roughly, the water is also released. Coming closer to the sun, the released material is pushed away from the sun by the solar wind (a constant stream of gas flowing from the sun) and forms a tail for the comet, stretching up to millions of miles behind the comet.
How bright a comet appears from Earth depends on three factors. The closer the comet gets to the sun, the larger and brighter the tail becomes. The closer to Earth it passes, the brighter it will appear to us. Finally, larger comets create bigger tails. Comets lose material in every approach to the sun. The more times a comet visits the inner solar system, the weaker it becomes.
The origin of comets is believed to be in the Oort cloud, a vast cloud of icy bodies orbiting the sun far, far outside the orbit of Pluto, stretching nearly a light year from the sun. Although no objects have been observed directly in the Oort cloud, the orbits of many of the brightest comets are known to extend to these distances.
It is thought that the Oort objects remain dormant until collisions or near-collisions among them push an object toward the sun, starting it
on a long "fall" through the inner solar system, causing it to become a comet.
These newly-formed comets, on their first few orbits nearing the sun, are the most spectacular, as they are at their largest size, and their material is "fresh". But such new objects are also the most unpredictable, flaring up quite suddenly into unaided sight, growing tails that can stretch over a quarter of the night sky, and then fading over a matter of days back into obscurity. It is no wonder that ancient man (and even rather modern man) attributed these sudden dramatic
objects with portents of doom.
The first major comet of 2013 is already upon us. Comet Pan-STARRS (named after the telescope system that first imaged it) has been visible for over a month in the southern hemisphere, and is now emerging into evening twilight for northern observers. This comet will pass within the orbit of Mercury, and is on its first visit from the Oort cloud. This combination may produce a very bright object in early March, though it is also possible for the comet to explode and disintegrate as it is heated so strongly near the Sun.
Look for the comet just after sunset in the western sky, scanning the sky with binoculars. Though it should be bright enough to be seen with the naked eye, its tail may not be easily seen without assistance in the twilight.
On March 12th in particular, look for Pan-STARRS to appear just to the left of the thin crescent moon after sunset. Throughout the remainder of March, the comet will trace the horizon after sunset, slightly more than an outstretched hand at arms length above the horizon, shifting steadily north as the month progresses, and dimming slowly.
A second comet, Lemmon (named again after the discovering observatory), is also currently visible in the southern hemisphere, though significantly dimmer than Pan-STARRS. This has been a binocular object so far. In late April and early May, this comet should become visible in the pre-dawn skies of the Northern hemisphere, as it recedes from the sun and dims.
But the truly exciting comet of the year will be peaking in mid November. Comet ISON, traveling in an orbit that suggests it is on its maiden voyage to the inner solar system, will come exceedingly close to the Sun at a distance of mere 680,000 miles. Prior to this, it will come within 7 million miles of Mars on October 1st, allowing the Mars Reconnaissance Orbiter to photograph its nucleus as it passes.
Because of its anticipated large size and extremely close approach to the sun, ISON may reach a brightness rivaling the full moon in early November, and remain easily visible in both north and south hemispheres from October through January to the unaided eye. The tail may reach collosal dimensions, filling as much as a quarter of the evening sky.
After it has receded back toward the orbit of Mars, Earth will cross ISON's orbit in late January. This may result in a new annual meteor shower, and the first of these showers may be very dramatic. Analysis of ISON's orbit suggests that it may be related to the Great Comet of 1860, and may come from the same parent body as that behemoth.
Lastly, as I was writing this article, yet another comet has been discovered that will be of interest in 2014. Siding Spring (another observatory name) is of keen interest because current estimates have it coming extremely close to Mars, within 30,000 miles, and there is enough uncertainty in that estimate that a collision is possible (current odds are 700:1). A collision would result in a major impact crater, and the certain death of all of our missions currently at the red planet.
But assuming the less-dramatic outcome, our spacecraft at Mars will give an incredibly detailed view of this comet in 2014. Stay tuned!!