A comet in the night sky always brings excitement that is different from observing any other celestial object or experiencing any other celestial event. The feelings are similar to a total lunar or solar eclipse, but with the bonus that the comet will linger for weeks or months before vanishing, perhaps into eternity. For some reason there is always the urge to see a comet as soon as it appears, even if the comet is predicable and re-appears every few years or decades.
This was not the case hundreds of years ago. The Greek philosopher Aristotle taught that the stars were unchanging and fixed while the Earth was the center of the universe. The cycles of the Sun, Moon, and planets had order and could be predicted accurately. The changeable sky was considered part of the earthly influence. Comets seem to come from nowhere and changed unpredictably, therefore they were thought to be a part of the Earth's atmosphere. Aristotle considered comets a harbinger of wind and drought, which fueled a superstition that comets were an omen of impending doom. Scientific research was almost nil in medieval Europe and the teachings of Aristotle were respected.
Eventually it was proven that comets were a part of the universe instead of the Earth's atmosphere. Nicolas Copernicus, Tycho Brahe, Johannes Kepler, and Galileo Galilei developed the foundation upon which Isaac Newton and Edmond Halley built their theories on the origins of comets. When Edmond Halley correctly predicted the return of a comet in 1752, he helped to dispel the myths surrounding comets. Even though he did not live to see his prediction come true, the comet would forever bare his name as a tribute.
Comets back then were, and sometimes to this very day by some are, incorrectly considered fireballs or something hot careening madly through the Solar System. Fear of comets was hard to dissipate as recently as 1910 when cyanide was detected in the tail of Halley's Comet. The Earth was to pass through the tail that May and people were afraid that the cyanide would poison the air and extinguish all life on Earth. Comet pills were the rage and a great way to make quick cash for those who preyed on others who knew nothing about comets in an era when comets were still a mystery.
It was not until 1950 that Fred Whipple proposed the "dirty snowball" theory that would forever revolutionize the idea of what comets really were. He correctly determined that a comet was nothing more than a mixture of ice and rock and theorized that as a comet approaches the Sun, the increasing heat would vaporize the ice and explosively stir up the dust. This would produce a thin atmosphere called the coma that could grow quite large since gravity is so low. The particles do not readily fall back to the surface and end up being caught up in the solar wind that is constantly blowing throughout the Solar System. This results in a beautiful flowing tail that gives a comet its distinct and unique character among all other celestial objects. The tails can stretch millions of miles into space and yet there is hardly enough material to pack into a snowball! The tail always points away from the Sun regardless of where the comet is in its orbit. Each comet actually has two tails: a dusty and icy tail that is tinted yellowish-white and an ion tail that is tinted bluish-white. The solar wind contains charged particles that can react with the comet particles creating an ion tail. The ion tails are of different density and can appear separate from the dust tail. If the viewing geometry is just right, the two tails can be easily visible, such as was the case for Comet Hale-Bopp and Comet Machholz.
Jan Oort studied the orbits of comets and determined that they originated from far beyond Pluto in a vast cloud of comets that surrounded the Sun. The passage of a distant star could disrupt a portion of the cloud and eject some of the comets out of the Solar System and propel others toward the Sun. Gravitational forces of the planets, especially Jupiter, could further alter the orbits of comets as they approach the Sun.
It is not hard to imagine that comets have struck planets in the past. Theories suggest that most of the water on Earth may have come from the impact of water-rich comets. A comet could have been the cause of the mass extinction of the dinosaurs sixty-five million years ago. We witnessed first hand the impact of Comet Shoemaker-Levy 9 into Jupiter during July 1994. The frightening, multiple black scars in Jupiter's clouds from the impacts of the fragments were a humbling reminder of the destructive power of a cometary impact.
Spacecraft sent to several comets during the past two decades have proved that comets are truly dirty snowballs that consist of ice and dust along with various other compounds. The spacecraft photos show comets with a black crust darker than coal, but loaded with water ice that is emitted from jets deep within the nucleus. Somehow this creates an insulating crust that traps the ice inside the comet. It would be a great idea to break open the crust, expose the ice underneath, and analyze it. A mission called Deep Impact was born.
Launched on January 12, 2005 towards Comet Temple 1, Deep Impact is comprised of two spacecraft: a flyby probe and an impact probe, or impactor. The impactor is actually an 820-pound battery powered spacecraft designed to operate independently for one day. It is about the size of a washing machine and will be released the day before impact. The date of impact is July 4, 2005, a perfect date for cosmic fireworks. The impactor will photograph the comet right up to the moment of impact, similar to the Ranger missions to the Moon. The force of the impact will be a blast as powerful as 10 million tons of dynamite and should create a stadium-size crater possibly up to 14 stories deep along with a large blanket of fresh ejecta and an expanding cloud. The flyby probe will observe and photograph the impact and formation of the crater and analyze the composition of the material that is exposed and ejected. This should give scientists a rare glimpse beneath the surface of a comet. Observations by the spacecraft will begin one month before impact and continue for one month following. Comet Temple 1 was chosen for its favorable geometry with Earth making it easy to reach with a spacecraft and also easy to see from Earth. The size of comet Temple 1 at 9x3x3 miles makes it a large enough target to reduce the chance of a miss by the impactor. It also is not as dusty as most comets, therefore making it less hazardous for a spacecraft to approach, and less likely of being struck by a destructive dust particle.
The estimated time of impact is 2 A.M. EDT on July 4. At that time, Comet Temple 1 will be in Virgo, not far from Spica, shining at a magnitude of about +9.5 and setting in the west. Will we be able to see the effects of the impact? It will pay to observe the comet in the weeks prior to the impact and then see what happens in the days that follow. Perhaps we will witness a temporary brightening of the coma and/or tail, but do not expect fireworks in the commercial sense. We are dealing not with fire, but with ice.
Somehow comets seem to stir the emotions and poetic instincts within us. We eagerly observe them every chance we get; now we are reaching out to touch them. Delicate and beautiful, unpredictable and unique, comets enlighten our souls like spirits drifting eternally in the solar wind.