What Is a Comet?
Most of us learn very basic information about our sky: the sun, moon, planets, and stars in elementary school. Later on, we may be exposed to more in-depth studies and learn about less often seen aspects, such as comets and meteors.
We've probably all heard the definition of a comet as "a dirty snowball." There is some truth to this, for ice is a large part of the makeup of any comet. This is largely water ice, but also assorted frozen gases.
As the comet approaches the sun, these gases vaporize, and are responsible for the famous long tails that distinguish comets from stars or meteors. According to NASA, the head, or nucleus, is made of rock, dust, some dark organic matter, and those frozen gases.
Comets attract the popular attention due to their relative rarity. Some, called 'long-cycle' comets, such as Halley's Comet, make an appearance only once every 70 -76 years. Seeing such a comet is a once in a lifetime event; twice if you live long enough.
Famed author Mark Twain was born on November 30, 1835 during one visit of Halley's Comet, and famously stated that he would die with its return. He did: on April 21, 1910.
How Do You Tell a Meteor From a Comet?
A meteor streaks across the sky, and you see a 'tail' as it goes. But a meteor is seen for a fraction of a second before it blinks out; it is burned up and fully consumed as it enters our atmosphere. It is this death of the meteor that we see as "shooting stars."
A comet, on the other hand, is of long duration; often days or weeks, (sometimes more), and its tail is larger than the head of the comet by far. Under the right conditions, a comet is an enduring spectacle in the night skies.
Then and Now
In previous centuries, before science and exploration had taken a serious foothold, people feared comets. They were regarded as harbingers of doom and destruction; the proverbial "end of the world."
Such scenarios were common during the Middle ages, into the Dark ages, and lingered even somewhat into the Renaissance era, though by then, education was being valued as it had not been before.
Comets in Modern Times
Comet Kohoutek, shown above, was a spectacular fizzle in 1973. It was much-touted to be a very great bright comet, and thousands stood braving the January chill for a chance to see it--it didn't happen. It was barely visible with binoculars, if you knew just where to look.
Comet West, in 1975, was easily visible, but because astronomers had "learned" not to predict, based on Kohoutek, there was virtually no media notice of West, so it was not seen by many.
Hale-Bopp, in 1995 on the other hand, was widely seen toward the end of July, and it remained visible for a year and a half! This surpased the prior record for "The Great Comet" seen in 1811. Hale-Bopp was a magnitue 2 comet, visible even from big cities with lots of "light pollution." Hale-Bopp is definitely a long-cycle comet; its next predicted approach to the inner solar system won't be until the year 4385!
Halley's Comet, as mentioned above, is on about a 70 - 76 year cycle; it won't be seen again until July of 2061.
Shoemaker-Levy 9 was discovered in March of 1993, and it was near Jupiter. It had been captured into Jupiter's orbit at one point; this resulted in gravitational forces breaking the comet into pieces.
On July 16, 1994, the first of several impacts of this dying comet into Jupiter were captured by several spacecraft in the (relatively) near vicinity. Given the distances across space, "near" can mean a million or more miles! We are "near" to our own sun, for example.
Some comets are only seen once, and are known as "non-periodic" comets. While they may make secondary or tertiary approaches, this may not happen for thousands of years. (Perhaps Hale-Bopp belongs in this category!)
How Bright Is a Comet?
The brightness is ranked on a scale from negative numbers up through about 16. Contrary to what you might expect, the brightness is in inverse ratio to the number assigned. So, a zero or -1 comet is far, far brighter than one ranked at 7 or 12.
This magnitude scale is also applied to planets, so Pluto, ranked at 16, is understandably invisible to all but the large telescopes at observatories, or at least a 10-inch one for amateurs...and you have to have a good sky map to know just where to look.
Wire Gauges Compared to Comet Brightness Scale
This measuring system can be compared to gauges of wire, where 20 gauge (sometimes called 'bell wire'), is very thin and easily broken, up to 14 gauge, the heaviest wire commonly used in residential wiring. Wire comes in much thicker and thinner gauges, but these ranges are the most common.
Will a Comet Come By Again in Our Lifetimes?
Most certainly. There are over 5,000 comets in our solar system. This counts only those discovered as of November of 214; more are discovered all the time. However, in the outer solar system, in an area known as the "Oort cloud," a 'comet nursery,' if you will there are estimated to be over a trillion! (That's 1,000,000,000,000, or a million-million, or 1012)
Comets, in general, arrive about every 5 years, and the "great comets" in approximately 20-year cycles.
We're in luck, in December of 2018; comet 46P/Wirtanen will make its perihelion (closest approach to the Sun) on the12th of December. By the 16th, it should be easlily visible with the naked eye. It will remain visible through early January of 2019. Southern hemisphere comet watchers will lose sight of this one around Christmas.
This is a short-cycle comet, expected to come around every 5 or 6 years, so there will definitely be a comet again in your near future.
1910 - 1911 saw 4 comets within that 2-year span, but that is uncommon.
Just because a comet is famous, like Halley's, for example, does not earn it a classification of "great." That is dependent upon factors as diverse as length of tail; brightness; duration; and how great a span of Earth from which it will be visible.
2019 is predicted to be a disappointing year for comet watchers; it's greatest claim to fame will be P46/Wirtanen, mentnioned above, as a leftover from December 2018.
The next one for 2019 will also be late in the year; no other comets are on the horizon before 2020.
To find comets and other astronomical events any time from 2014, up through 2030, click here.
Enjoy the night skies; comets notwithstanding, there's a lot to see!
© 2018 Liz Elias