Comets: Why the Solar System Rocks!

When we look back, 2013 may be remembered as the Year of the Comets.  As I’m sure you may have heard already we have two potentially immensely wonderful comets heading our direction this year; the first of which will be its closest to the Sun on March 10th.  The more you learn about comets the more you appreciate how amazing the solar system is!  When you consider what comets are, how they get here, and what on Earth makes them shine so beautifully then fade into oblivion for thousands of years you are left with a sense of awe because the answer to all of these questions is…the Sun.

Thanks to Nicolas Copernicus we know that the Sun is the center of our solar system and that everything in the solar system orbits the Sun on regular and predictable paths.  You have the 8 major planets, the asteroids in the asteroid belt, the minor, or dwarf, planets of a region called the Kuiper belt, of which Pluto is a member, then far, far away from the Sun at a distance of almost one light year you is the region known as the Oort Cloud.  The Oort cloud is a massive region of space mostly by tiny chunks of ice and rock left over from the formation of the solar system.  These chunks of ice and rock are so far away from the Sun that they are approximately one-quarter the distance to the nearest star, Proxima Centauri.  The Sun is barely more than a pin point of light out here and its gravity is just strong enough to keep these tiny chunks of ice loosely in orbit.  However, the gravity is so weak that objects in the Oort Cloud are influenced by passing stars and the Milky Way itself.  All it takes is the slightest gravitational nudge from another star to dislodge an ice chunk from its happy orbit and send it drifting slowly towards the inner solar system.

Cutaway drawing of Oort Cloud  Image credit: NASA/JPL

Cutaway drawing of Oort Cloud Image credit: NASA/JPL

This is how we believe most long period comets are born.  Long period comets are comets with highly eccentric (or lopsided) orbits that span between 200 and thousands, or even millions of years.  Comet McNaught that passed through the solar system back in 2007 is a long period comet with an orbit of about 92,600 years.  It’s safe to say that we won’t see that bad boy again in our lifetimes!

Once the ice chunk is dislodged from its orbit in the Oort Cloud it begins its long, slow journey towards the Sun.  The Sun’s gravity begins to pull it in towards itself on an epic tour of the solar system that spans almost an entire light year (one light year is 6 trillion miles).  Comets are typically no bigger than a hundred or so meters across but the Sun causes something to happen on their surface that makes them spectacular sights in the night sky.  Out in the Oort Cloud it is mind-bogglingly cold.  Before they turn into comets the chunks rocks and dust mixed with chunks of frozen water, ammonia, carbon dioxide or methane that are so cold they’re as hard as steel.  But once they get close enough to the Sun they begin to heat up.

Once the comet arrives in the inner solar system the Sun’s heat begins to melt the ice and it begins to evaporate and glow brightly which is caused by solar ionization.  The glowing cloud of evaporating gas is called the coma.  Once the coma is formed the tell-tale…well, tail of the comet begins to form as the solar wind from the Sun blows against the comet.  The comet, tail, and coma steadily brighten as the comet gets closer and closer to the Sun.  They also begin to pick up more speed the closer they get.  By the time a comet is visible on Earth it already has a dazzling coma and tail that can be as bright as the stars and perhaps even the planets!

It is once the comet is within the orbit of Mercury that the fate of the comet is determined.  Most comets slingshot around the Sun at a safe distance that they make it around without a problem and begin their lonely journey back out of the solar system into oblivion.  Other comets called sun grazers get so close to the Sun that they actually pass through the Sun’s upper atmosphere, the photosphere, or even the solar corona where the temperature is millions of degrees Fahrenheit.  Some sun grazers make it out intact while others break apart and disintegrate, much like a frozen coffee mug when boiling water is poured in it.  Others still are known as sun divers which literally plunge right into the Sun and are never heard from again.

Comet Hale Bopp in 1997

Comet Hale Bopp in 1997

Once the point of perihelion, or the comets closest approach to the Sun is reached the comet begins it’s journey back to where it came from.  Depending upon the positioning of the planets on its return journey, some comets stay in orbit around the Sun and will eventually return.  If a planet’s gravity nudges the comet on the way out it could end up being ejected from the solar system entirely and be doomed to roam the void of interstellar space forever.  Whatever the fate of the comet we get to observe the magnificent effects of the Sun on them from the Earth, both visually and scientifically.

This year we have two potentially dazzling and memorable comets heading our way!  The first of which is named comet C/2011 L4 PANSTARRS, or PANSTARRS for short.  With a perihelion of March 10, 2013 it promises to put on a nice show throughout the months of March and April.  Observers in the northern hemisphere won’t be able to see the comet until after its perihelion though.  So be sure to get outside during clear nights in March and April to see this orbiting rocky ice clump.  Currently, PANSTARRS is projected to get as bright as the planet Venus if everything goes according to plan with its passage around the Sun.  PANSTARRS will be bright and low in the sky about 30 minutes after sunset in mid-March.

If you miss PANSTARRS or couldn’t get enough comet viewing action for one year you’re in luck!  Even brighter and more spectacular than PANSTARRS will be comet ISON in the fall months.  ISON is currently close to Jupiter on its voyage towards the Sun but will begin to be visible in binoculars in the beginning of October.  By November 1st ISON will be within the orbit of Earth and should be a spectacular -6 magnitude!  Astronomers measure brightness by magnitude with the lower the number being a brighter object.  The planet Saturn is +1 magnitude and the brightest star in the sky, Sirius, is -1.46.  By the time it reaches its perihelion on November 28th it is expected to reach a -12.6 magnitude which is as bright as the full moon!  That means that as it passes next to the Sun it will be visible during the daytime if you use your hand to cover the Sun!  ISON should put on a show of a lifetime during November and December and will truly be something to tell your kids and grandchildren about because ISON will likely never return.  If you own a telescope or a pair of binoculars make time to get out with your friends and observe this marvelous comet.  This one has the potential to be the brightest comet in recent history, brighter even than the famous Halley’s comet.

Comet McNaught from 2007

Comet McNaught from 2007

It never ceases to amaze me that all the wonderful things we love about comets, their beautiful tails and diamond-like sparkle is all due to the power of the Sun.  We live in an active solar system that is constantly moving and it is all thanks to the Sun’s influence.

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About Tim

My name is Tim Phelan. I am a nerd, amateur astronomer, sports nut, and follower of Jesus. I live in Baltimore, MD where the skies are oh so polluted with light. This is Ravens Country, Birdland, and the City that Reads, or whatever. Follow me on acrosstheuniverseinnotime.com and tphelan.wordpress.com

Posted on January 15, 2013, in Comets, Solar System and tagged , , , , , , . Bookmark the permalink. Leave a comment.

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