This article originally appeared in The Skeptic, Volume 22, Issue 3, from 2012.
At the time of writing, there is a new comet in the sky – the nattily named Comet C/2009 R McNaught. It’s currently wending its way across the sky from Andromeda, through Perseus, and onwards to Auriga. It may be too close to the sun, or even on its way back from around the sun and not visible to northern hemisphere viewers by the time of publication. I thought a little background on comets in general would be nice.
Since time immemorial, the heavens were seen to be perfect, vast nested crystalline spheres, upon which the planets, stars and other objects moved about. These spheres resonated, each with their own tone, creating the ‘music of the spheres’. The heavens were perfect and unchanging, although, from time to time, an interloper appeared – a new object that wasn’t there before. These apparitions were deemed to signify either great things or disaster (in fact, the word disaster comes from the Greek for “bad star”).
Different comets have a wide range of orbits, ranging from a few years (short-period) to hundreds of thousands of years (long-period). Short-period comets originate in the Kuiper Belt, or its associated scattered disc, which lie beyond the orbit of Neptune. Long-period comets are thought to originate in the Oort Cloud, a cloud of icy bodies in the outer Solar System that were left behind during the condensation of the solar nebula, 5 billion years ago. Long-period comets plunge towards the Sun from the Oort Cloud because of gravitational interactions caused by either the outer planets, or passing stars.
Rare hyperbolic comets pass once through the inner Solar System before being thrown out into interstellar space along hyperbolic trajectories. These may put on a spectacular show before leaving the solar system to traverse the vast distances of interstellar space.
Comets are distinguished from asteroids by the presence of a coma or a tail, this is what most people associate with a bright comet, but they only develop these when they begin to approach the sun and the ice sublimates; for most of their lives, comets do not have tails or coma. However, extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust, and may come to resemble small asteroids. Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System.
The annual periodic meteor showers we see are associated with comets – the debris left behind as the comet passes through the inner solar system leaves a trail of dust in its path. Periodically, the Earth passes through one of these trails, causing meteor showers. If the comet has passed by recently, or we are travelling through a particularly dense region, we can get meteor storms, where the rate of meteors can exceed 100 per hour.
Comets, being one of these types of object that appeared every now and again, were taken to signify either great or terrible things. They were said to predict the coming birth or death of kings, or catastrophes of other sorts. Perhaps one of the most notable comet appearances in history is Halley’s Comet on the Bayeaux Tapestry before the Norman conquest of England in 1066.
The last of these cometary catastrophes to occur was in 1997, with the passage of Comet Hale-Bopp. In 1996, amateur astronomer Chuck Shramek took a photo of the comet, which showed a fuzzy, elongated object nearby. When his computer astronomy program did not identify the star, Shramek called the Art Bell radio program to announce that he had discovered a “Saturn-like object” following Hale-Bopp. UFO enthusiasts soon concluded that there was an alien spacecraft following the comet.
Art Bell even claimed to have obtained an image of the object from an anonymous astrophysicist. However, astronomers Olivier Hainaut and David J. Tholen of the University of Hawaii stated that the alleged photo was an altered copy of one of their own comet images.
In March 1997, the Heaven’s Gate cult, under the leadership of Marshall Applewhite, chose the appearance of the comet as a signal for their mass suicide. They claimed they were leaving their earthly bodies to travel to the spaceship following the comet. They were found with their bags packed, laid out on beds, covered with purple shrouds. They wore running shoes and matching uniforms with “Heaven’s Gate Away Team” patches. This was one of the largest cult suicides in recent years and would come to haunt the legacy of the comet. Thankfully, Hale-Bopp is a long-period comet, so we won’t get to find out if its return will spark similar catastrophe until the year 4385.
As of July 2010, there are almost 4000 known comets of which about 1,500 approach the sun closely and about 500 are short-period. However, this represents only a tiny fraction of the total potential comet population; the reservoir of comet-like bodies in the outer solar system may number up to one trillion. The number visible to the naked eye averages roughly one per year, though many of these are faint and unspectacular. It is fairly rare that we get a spectacular comet like Hale-Bopp and even rarer are comets like Shoemaker-Levy 9, which famously impacted Jupiter in 1994.
The scientific legacy of Comet Hale-Bopp, one the most observed cometary passings in recent years, was a recorded abundance of sodium, deuterium, argon and organic chemicals, meaning that if comets brought water to the early Earth, they could also be responsible for the organic chemistry that could have eventually given rise to life.
