Short-term comets are the cosmic version of Old Faithful. Although comparing a chunk of ice and dirt streaking through space with a geyser in Yellowstone may seem like a stretch, the comets and the geothermal feature are remarkably predictable, in terms of their functions.
Short-term – or sometimes referred to by the loosely-applied classification of “periodic” – is possibly the most common type of comets observed by the populous. By definition, they have short orbits. Also, they have been observed multiple times during a single perihelion passage (orbit around the sun). These comets pass the Earth up to every 200 years and have a path that stretches as far as Pluto. They travel on an elliptical orbit within the Solar System.
The other classification of comets, the long-term version, refers to those with longer orbits in terms of time and distance. Many of these take more than 200 years to complete an eccentric orbit. Orbital eccentricity is an orbit which deviates from a perfect circle and becomes a nearly open, parabola. This means their orbit can go outside the Solar System (often, to the Oort Clouds which is approximately one light year away from the sun).
Short-term comets are usually named after their discoverers and have been observed and studied by countless astronomers. Possibly the most famous short-term is Halley’s Comet (named after the 18th century English astronomer Edmond Halley), which passes the Earth every 75 to 76 years.
Although officially named in 1705, knowledge of this particular comet had been known since 240 BCE. Ancient Chinese, Babylonians, and medieval Europeans had chronicled its presence over the years.
Also, this particular comet has become an iconic symbol of sorts. During its last visit into the inner Solar System in 1986, amateur astronomers and avid star-gazers – as well as professional astronomers – gathered in areas around the world to get a better look at it as it illuminated the night sky.
Its arrival also created a small cottage industry. For that year, t-shirts, mugs, and other forms of memorabilia dedicated to Halley’s Comet flooded the markets.
The 1986 arrival was more than a mere celestial show. Much of what is known about a comet’s structure came from this event. That year, Haley’s Comet was observed in details by spacecrafts. As a result, data was collected on its nucleus, coma (the nebulous or “light” enveloping the nucleus), and tail formation.
Comets, as a result of this data collection, were found to be made up of a mixture of “volatile ices (water, carbon dioxide, and ammonia) and dust (in Halley’s case, it was made up of dust, non-volatile materials, and a small amount of ice).
There are two types of short-term comets. The Halley family has orbits between 20 and 200 years. The Jupiter family has periods less than 20 years.
Also, many of these comets may have a short life, in cosmic terms. Over thousands or possibly millions of years, many of them will either break up into smaller pieces similar to an asteroid or collide with a large celestial body such as a planet.
The death of one short-term comet had been observed by astronomers worldwide. In July 1994, the Shoemaker-Levy 9 Comet met its demise when its orbit got too close to Jupiter. As a result, it broke up into nine pieces and collided with the planet.
Besides being a wonder of science, short-term comets have also been at the heart of a controversy. Some young-earth creationists believe the life-span of short-term comets proves the earth and the universe is not as old as many scientists claim it is.
One such claim comes from a website Biblical Discipleship Ministries. According to an article entitled, “Support for a Young Universe, God’s Time Clocks,” the site claims that astronomers believe short-term comets have a lifespan of 1,500 to 10,000 years. It also states that “evolutionists” realize “all short-term comets should have died if the universe is 7.3 to 20 billion years.”
The site also claims that scientists have failed to find something that “resupply comets or to form new comets”, thus suggesting that the solar system is a few thousand years at most. How this conclusion was reached – with so little evidence to support it – is unclear.
What is known is that scientific observation of short-term comets is ongoing. Much has been learned about its structure and orbits. Also, there’s much that still needs to be learned about them. There is a possibly these comets are key component to the creation of the universe, as well as Earth and the organic life it holds.
Thus, it is possible, in 2061 – Halley’s Comet’s predicted date of return – more questions about its structure and origin will be answered. And, more questions will be asked.
