Late bloomers, do not despair! On a scale as grand as that of our own solar system, there is precedent for astonishing transformation well into adulthood (how does 4.5 billion years sound).

Trillions of non-descript, rock-encrusted ice boulders encircle the outer solar system even now, patiently awaiting their moment in the Sun; few will get their chance to shine. A tiny percentage will find themselves touring the inner solar system with inspirational news for us all. Their simple message: dare to dream, because late-emerging beauty is not strictly for the birds.

So with renewed hope for that neglected modeling career, and deepest apologies to Han Christian Anderson, let's explore...

Comets: the Ugly Ducklings of our Solar System.

The objects in our solar system can be roughly sorted into a handful of categories. Holding court at the center of it all is our very own star, the Sun, which presides over a cast of nine large, spherical bodies we call planets. Many of these planets have their own natural satellites, called moons. There are asteroids - huge boulders the size of a city; and meteoroids – “space rocks” as big as your house, or as small as a grain of sand. And yes, there are comets.

All of these objects have at least one thing in common: they all formed out of a single cloud of gas and dust. This cloud (all that remained after the star that used to live here blew up) was mostly composed of heavier elements, such as oxygen, silicon, iron, carbon, calcium, and aluminum. These heavier elements were manufactured by the fated star's fusion processes from lighter elements, primarily hydrogen and helium, which dominated the early universe.

Although the available building materials where the same throughout this cloud, the objects that eventually took shape were not all the same. Those differences depended largely upon where they were located in the primordial cloud, and the random collisions that regulated their growth. Fortunately we have a reliable, if not easily interrogated, eyewitness to this period of prehistory in the form of the comet.

The Nature of Comets

Far from the Sun, a comet isn’t much to look at (a bit of an ugly duckling, you might even say). Imagine an enormous black potato the size of a big city. These huge lumps of ice, rock, and dust are called comet nuclei. They have extremely low albedos, about 4 percent in fact, which means they reflect very little of the sunlight that strikes their surface. For this reason, astronomers have difficulty seeing them through telescopes. Their surface terrain is varied by peaks and impact craters. 

Close to the Sun – now that’s a different story. If a comet nucleus finds its way into the inner solar system (inside of Jupiter's orbit), the ugly duckling is slowly transformed into a magnificent swan, with a softly glowing head bigger than the Sun itself, and a gossamer tail longer than the distance from the Earth to the Sun.

Now for our first Comet Quiz! question. Take your time - don'tcha dare cheat.

That’s right, your planetarium needs you. Crowds of people will gather under its glittering dome to hear you describe how energy from the Sun spreads out in waves, called solar wind. Your vivid depiction of a comet nucleus, washed and eroded by these gentle waves of energy like a rock submerged in a lazy mountain stream, will captivate visitors and lead, quite possibly, to a lucrative second career narrating science documentaries for the Discovery Channel.

The Structure of Comets

The transformation from drab duck to splendid swan begins at the moment of the comet's first contact with the warm solar wind. This tepid bath of radiation causes ice just below the surface of the comet nucleus to turn directly into gas  - a process called sublimation. As it approaches the Sun, a big cloud of released gas, called the coma, begins to amass around the nucleus. Compared to the blackened surface of the nucleus, gas is a pretty efficient reflector of sunlight. Once the coma begins to take shape, therefore, the nucleus itself is as good as invisible.

If the shape of the comet’s orbit brings it still closer to the Sun, the energy washing over it may impart enough force to push some of the gas far out into space. Dust trapped within the ice is also set free during sublimation. Since dust has a greater mass than gas, the two often separate to create a pair of distinct tails. Softly glowing in pale blue and white, these tails can extend away from the Sun for distances up to 100 million miles, like the magnificent tail feathers of some colossal interplanetary bird.

Typically, the gas tail is perfectly straight. Gas is very light and willingly goes wherever the solar wind pushes it - directly away from the Sun, as it happens. The solar wind electrically excites the gas in the process, causing it to glow with a softly diffused bluish tint. Electrically excited gas is composed of electrons and ions and often called a plasma, and so the labels gas tail, ion tail, and plasma tail are used by scientists interchangeably.

Dust, on the other hand, is a bit heavier than gas, and a lot less excitable. Its greater mass means a greater gravitational attraction toward the Sun. The dust tail curves slightly away from the perfectly straight path expressed by the gas tail, and is typically shorter. Because it is the mutual gravitational attraction between the tiny grains and the Sun that causes this deviation, the curvature is towards the Sun. Unlike the gas tail, the dust tail's characteristic color, a pale yellowish-white, is simply the result of the sunlight that it reflects.

Although gas tails have been known to extend as far as one AU (astronomical unit, about 93 million miles) away from the Sun, the comet's most voluminous feature, the hydrogen cloud, is larger still. Billowing out into space perhaps several million miles, this colossal feature is so sparse that the full extent of it was not clear until instruments sensitive in the deep ultraviolet wavelength took a closer look in 1970. The hydrogen atoms that form the cloud are liberated from the nucleus in one of two ways: by the sublimation of ices directly beneath the crusty surface of the nucleus; or, from the breakdown of water molecules released into the coma.

The Ancestral Home of Comets

A lot of people believe the solar system ends with the orbit of its outer-most planet, Pluto. Not so. 

There are two primary populations which account for most of solar system's supply of comet nuclei, and you can't get to either using public transportation. Most short-period comets, i.e. those that orbit the Sun once in two-hundred years or less, are believed to originate in the Kuiper Belt. Long-period comets, those with orbital periods of two-hundred years or more, are thought to emanate from the Oort Cloud.

The Kuiper Belt is a ring of sleeping comets that lies just beyond the orbit of Neptune. This immense reservoir of nuclei has given us comets Halley and Encke, and the majority of short-period, regularly-returning comets for that matter. The domain of the Kuiper Belt extends roughly from the edge of Pluto's orbit, about 40 AU, out to a distance of about 500 AU. One estimate suggests a minimum of 200 million comet nuclei reside in the Kuiper Belt. It is shaped rather like a donut, and conforms, more or less, to the plane of the ecliptic.

Comets in the Oort Cloud, on the other hand, have little respect for the ecliptic plane. The Oort Cloud is spherical, meaning that its residents orbit the Sun at any angle that pleases them. The Oort Cloud's territory extends as far out as 50,000 AU, nearly one-fifth the distance to the closest star. The orbit's of nuclei within the Oort Cloud are less eccentric - more circular - than those within the Kuiper Belt. Most non-returning observable comets originate within the Oort Cloud. Their tremendous distance creates the potential for gravitational interference from passing stars  Comets such as Hale-Bopp and Hyukatake are believed to have been tweaked out of their original orbits by a close encounter with a passing star, changing the shape of their orbit in a way that sent them streaking through the inner solar system. Billions of comet nuclei call the Oort Cloud home.

The Origin of Comets

Run a bunch of comet-origin theories up a flagpole and the one most scientists will salute suggests that comet nuclei are piles of left-over building materials from our solar system’s formative years. Just as you couldn't walk around a typical construction site without encountering an occasional heap of discarded lumber and pink fuzzy stuff, so too our solar system is dotted with heaps of material that, for one reason or another, didn't end up in the primary structures. Instead of board remnants and wall insulation, however, careful inventory of a comet nucleus would uncover various combinations of those elements that were most abundant in the pre-solar system nebula. 

Today's best guess puts the age of the solar system at about 4.5 billion years. That makes it about one third as old as the universe itself. Like the nucleus of a comet, the vast molecular cloud from which our life-giving, suburban-galactic neighborhood condensed was itself once a mass of debris. A star from an earlier generation gave its life to form the proto-solar system nebula that eventually became our luxuriously appointed, hardwood-floors-throughout home sweet home (so instead of taking your good fortune for granted maybe you and those Bohemian friends of yours should try showing a little gratitude).

Perhaps set a-spinning by the gravitational influence of a passing star, the nebula began a slow transformation from amorphous cloud to flattened disk. As it flattened, gravitation and collisions between particles resulted in areas of higher and lower density. The denser areas acquired the gravitational advantage early on, and began 'sucking up' any material within their influence. In this way gravity is like capitalism - the rich get richer while the poor just keep getting poorer. Eventually the areas of highest density condensed into the Sun, Jupiter, and the rest of the planets, while most of what was left coalesced to form the moons, asteroids, comets, and meteoroids.

The Composition of Comets

The composition of individual rubble piles at a construction site should vary somewhat, depending upon where within the site they amass. Each comet nucleus is similarly unique. Because they consist mostly of water ice, all comet nuclei probably most likely originated in the outer regions of the early solar system, where the Sun's radiation could not interfere with the accumulation of these volatile substances. Scientists nonetheless expect to find variations in composition based on location within the cloud and distance to the Sun. A nucleus that formed nearer to the Sun's warmth would no doubt contain less ice, for example, with more non-volatile rock and dust per given volume.

During Comet Halley's visit in 1986, unmanned spacecraft analyzed gases within the coma of this, the world's most famous comet. They found a sparse 'atmosphere' predominated by a substance far more valuable than gold or diamonds - good old H₂O. Water vapor accounted for a whopping 80% of the coma's composition. Carbon Monoxide (CO) was the next most common substance, weighing in at about 10%. A further 3½% of the gaseous concoction was attributed to Carbon Dioxide (CO₂), and Formaldehyde ((H₂CO)_n) placed fourth at about two percent. Trace amounts of many other substances were measured as well.

The Contribution of Comets

Why are astronomers, busy as they are, so interested in studying comets? Here are three of many perfectly good reasons. 

First of all, as a comet approaches the Sun, complex molecules within the nucleus are subjected to stress by the thermal energy of the solar wind. This process, called photodissociation, breaks down the 'parent' molecules into a bewildering variety of less complex 'child' molecules, which in turn are free to recombine in new, still more complex ways. Such a process might well produce the sorts of organic compounds necessary for the initiation of life, even if by chance.

Secondly, although the proportion of water measured in Comet Halley is thought to be higher than average, comet nuclei are nonetheless lousy with the stuff. Countless comets are known to have impacted the Earth in its first few million years of evolution, and some scientists wonder if most of the water on its surface may have arrived on the backs of impacting comets.

Thirdly and finally, comet nuclei are indeed eyewitnesses to the formation of our solar system. They are samples of pre-solar system building materials that, unlike most objects in orbit around the Sun these days, are still, to this day, unaltered from their original state. This alone makes them the keystone to any investigation of the solar system's earliest years.

And anyway, isn't it reassuring to know that late-emerging beauty is not strictly an aquatic bird thing.

Many comet science missions are either currently underway, or in advanced stages of preparation.

Check the following links to learn more about these exciting scientific adventures...

Two years after meeting its primary objectives, NASA's DEEP SPACE 1 spacecraft flew past comet Borrelly and captured some of the best images of a comet nucleus to date.

NASA's STARDUST mission is now complete. The spacecraft's prime objective is to collect samples of cometary material, and return them for study back on Earth.

On its 10 year journey to Comet 67 P/Churyumov-Gerasimenko, the ROSETTA spacecraft will pass by two asteroids: 2867 Steins (in 2008) and 21 Lutetia (in 2010).

DEEP IMPACT will explore the deep regions of the nucleus of comet Tempel 1 before, during,
and after it impacts it, and return the observations to Earth.

SKY AND TELESCOPE MAGAZINE's COMET PAGE is full of information and useful links - a terrific jumping-off point for your cometary explorations.

Want to read more about the science and educational philosophy behind the Rendezvous With a Comet Mission Curriculum?

Check out some of the terrific books and other resources used to develop these materials on the BIBLIOGRAPHY page.

Although the theory of the solar system's origin that is presented here is pretty widely accepted, 4 out of 5 historians nonetheless agree that it’s always better to get the story from an eyewitness. The following, therefore, are excerpts from an interview with Mr. Howard "Sully" Sullivan, site supervisor with Gravity Builders, Inc. Gus likes to tell the story this way...

“Sure, I remember that job - about 4.5 billion years ago, wasn’t it?

“The boss pulls me off this black hole project I’m workin’ on and tells me we’ve got this new solar system job out by Alpha Centauri, which is great for me ‘cause I live, like, two solar systems away – so, you know, the commute was better. And hey, it sure beats shoveling (stuff) into a black hole all day, right.”

“So we get to the work site and there’s nothin’ but a great big nebula – you know, one of those giant clouds of gas and dust out in space. Whoa, I says - guess we’re startin’ from scratch on this one.”

“First thing you have to do on these projects is get things movin', so we swing a nearby star passed the site so’s its gravity can start the cloud spinning. See, that way everything starts bumpin’ into everything else, and stuff starts lumping together into larger and larger clumps. Saved me a bundle in overtime. That was my idea, now it's standard practice in the industry. Man, if I had a nickel."

 “Progress was sort of slow at first”, Sully notes, “but since gravity’s ability to attract things gets stronger as objects acquire more mass, we eventually got back on schedule.”

“Gravity is a great tool, but it’s tricky”, continues Sully. “Everything just wants to go to the center of the closest large mass. So before you can say ‘Watch out lady, this is a hard-hat area!’, most of the stuff in your nebula has already formed a gargantuan ball smack dab in the middle of the works.”

Curious about some of the terminology used in the Rendezvous With a Comet Mission Curriculum?

Be sure to browse through the Comet Science GLOSSARY.

JPL explains why they believe cometary exploration is so important in "WHY STUDY COMETS?"

Produce an image of the orbit of any comet with JPL's ORBIT VIEWER web application.

Got a little time to kill? How about browsing through NASA's comprehensive list of comet, asteroid, and meteoroid-related web links.

You'll find fact sheets, mission info, thousands of images, dump trucks full of data, and plenty of other resources on their ASTEROID AND COMET link page.

The NINE PLANETS is a fun and informative online multimedia tour of the solar system from the nice folks at JPL.

The unexpected appearance of comets in the sky has precipitated a lot of irrational behavior over the centuries. Read Gary W. Kronk's article "COMET HYSTERIA AND THE MILLENNIUM: A COMMENTARY" to learn more.

UC at Berkeley has provided some fun and informative web pages that both educators and students will enjoy at THE COMET'S TALE.

More classroom-friendly fun, this time from those clever people who run the Hubble Space Telescope program, otherwise known as the Space Telescope Science Institute (STSCI).

Their site is simply titled COMETS.

The South Florida Sun Sentinel has a nice interactive graphic describing the STRUCTURE OF A COMET on its website. You must have Shockwave installed in your browser  to view it.

THE UNIVERSE - another very good NASA site about comets. This one includes a few excellent animations that illustrate various aspects of a comets orbital behavior.

For best results, print this document in landscape mode.

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