Comets and where they originate - the Oort Cloud and the Kuiper Belt.  Perhaps tiny Pluto isn't a planet at all.
 

Outline:

The painstaking technique used by Clyde Tombaugh to discover the outermost planet Pluto in 1930.  Vital statistics of Pluto and its companion moon Charon - their combined mass (less than one-fifth of our Moon), icy composition, surface features and distance from the Sun.

But is Pluto a planet at all?  Or did it originate in the Kuiper Belt which lies just beyond?  Kuiper abounds with similar icy bodies - planetesimals and comets.  More distant still is the Oort Cloud, another vast storehouse of comets, extending perhaps one-third of the way to the next nearest star.

The structure and nature of comets, including their icy nuclei.  Magnificent two-colour comet tails - yellow for dust and blue for gas.  The behaviour of comets, their highly elongated orbits around the Sun, and the perturbing effects of the Sun and Jupiter.  Sun plungers, Sun-grazers and Shoemaker-Levy 9 which was torn apart by Jupiter's gravity.  One by one, the fragments carpet-bombed the planet, creating shockwaves the size of Earth.

Comet Halley, the most famous comet of all - its appearances in historical times and the epic fly-by of the comet's nucleus in 1986 by the spaceprobe Giotto.  How Halley may have been captured long ago from the Kuiper Belt and locked into its present orbit.  Comet Hale-Bopp, the great cometary visitor of 1997.

The Stardust mission that flies through the coma of Comet Wild Two, collecting samples of dust for dispatch to Earth.
 
 

Sub-chapters:

Pluto and Charon

*  The Lowell Observatory, Flagstaff, Arizona.  An elderly Clyde Tombaugh demonstrates the equipment he used in 1930 to discover Pluto.

*  Pluto, the most remote planet, and its moon Charon.  Orbiting 40 times farther from the Sun than Earth, they are frozen worlds, composed of water and methane, their combined mass less than one-fifth that of our Moon.

*  The Kuiper Belt, a disk of icy fragments just beyond the planets.  Perhaps Pluto originated here.  Pluto may not be a true planet.
 

Icy Vagabonds

*  Farther out is the Oort Cloud, a sphere of freezing debris encircling the Solar System, stretching one-third of the distance to the next nearest star.

*  Pluto's compositional resemblance to the comets from Kuiper and Oort.

*  Comet Hyakutake, viewed in 1996 - nucleus three miles wide, tail stretching for millions of kilometres.

*  Cometary tails - how they form double tails of gas and dust upon approaching the Sun.  What causes the tails to point away from the Sun.

*  The random, highly elongated orbits of comets around the Sun.  Comets that graze the Sun, comets that plunge into it.
 

Jupiter's Pull

*  A graphic illustration of Jupiter's influence on comets - Shoemaker-Levy 9.

*  Fragile S-L9 is torn apart as it's drawn within 50,000 kilometres of Jupiter's cloud tops.

*  July 1994 and, one by one, SL9's fragments strike Jupiter - creating shockwaves as big as Earth and plumes over 1,000 kilometres high.
 

Halley's Comet

*  Comet Halley in history, as Giotto's star of Bethlehem and Hally's appearance in the Bayeux  tapestry depicting the Norman conquest.

*  The Giotto spacecraft's journey to Halley in 1986.  Flying through the inner coma, returning images of the nucleus to Earth.

*  Halley's capture by the Sun, and the resultant 76 year orbit.
 

Visitors from Afar

*  Hale Bopp's fly-by in 1997.

*  How comets only form tails as they approach the Sun.  In the outer Solar System, they become tailless and inert.
 

Collecting Comet Dust

*  The Stardust craft's encounter with Comet Wild Two in 1999.  The process of collecting samples and sending them to Earth.

*  Comets containing particles unchanged since the formation of the Solar System.  Stardust provides an opportunity to analyse these building blocks of the Solar System.
 
 

Background:

The Mysterious Nature of Pluto

Relatively little is known about the frozen ninth planet.  New information will be hard to come by without a space mission.  Nothing has been firmed up for the early 21st century.   So much remains a puzzle, including the exact nature of the planet.  Indeed, is Pluto a planet at all?  It cannot have been formed in the inner Solar System as it contains too much ice.  But the internal composition of this frozen world, with a density 2.02 times that of water, means it contains more rock than the satellites of the gas giants.

If Pluto is a planet, then should it really be called a double planet?  Pluto's moon, Charon, is so closely matched in mass that the two bodies orbit a mutual balance point.  It's the only known case in the Solar System where a moon is massive enough - compared to its planet - to swing its parent body around a point outside the planet.  Pluto itself is tiny - one twenty-fifth the size of Mercury.  Even when combined with its moon Charon, their joint  mass is so small that they could not perturb the motions of a giant planet.

So what exactly is Pluto?  One theory is that Pluto may once have been a satellite of Neptune.  Although containing more rock than the moons of the giant planets, Pluto is similar in mass, size and density.  The same goes for Charon.  Pluto's highly elliptical solar orbit suggests that  it may have escaped from Neptune in some cataclysmic event.  That would also explain the reversed orbit of Neptune's moon Triton and the fact that another moon, Nereid, has been flung into a highly elliptical orbit.

What could have happened?  Possibly, a colossal planetesimal collided with Triton, Nereid and Pluto, radically perturbing their orbital paths.  Tidal forces from the planetesimal then ripped Pluto in two, forming Charon, and catapulted the pair farther out in the outer Solar System.  There exist, however, no other remnants of this planetesimal.  The most widely held theory remains, therefore, that if Pluto is not a planet, then it originated in the Kuiper Belt.
 

The Nature of Comets

A bright comet at dawn or dusk is a magnificent spectacle.  It is hard to believe that that the entire cometary phenomenon originates in a tiny central nucleus, no more than a few kilometres in diameter.  The nucleus is composed of "dirty ice" - a mixture of ices (mainly water ice) and dust.  It is the interaction of the nucleus with the Sun's radiation - particularly the solar wind - that produces the familiar characteristics we see from Earth, especially the spectacular comet tails.

Cometary nuclei are irregular in shape.  They range in size from just a few hundred metres to perhaps 100 kilometres across at most.  Comets, therefore, are comparatively minor bodies on the cosmic scale.  Most orbit the Sun in elongated, elliptical paths.  When the nucleus is a long way from the Sun, it is completely frozen, inactive and invisible from Earth.

As the comet approaches the Sun, the temperature of the surface layers of the nucleus increases sufficiently for certain ices to begin to sublimate - in other words, to turn directly from solid ice into gas.  Jets of gas erupt through cracks in the surface.  They pick up dust particles and drag them away from the nucleus.  This creates a temporary atmosphere around the nucleus - called a coma.  The gases and dust of the coma completely envelop the nucleus and form the fuzzy head of the comet.  The coma, which is more or less spherical, can extend from ten thousand kilometres to as far as one-million kilometres from the nucleus.

When relatively close to the Sun, a comet develops two or more tails, which generally point away from the Sun.  It's these tails - appearing faintly blue and yellow - that make a comet such a awe-inspiring sight from Earth.

The blue tail is formed like this:  Ultra-violet radiation in sunlight reacts with the coma.  Neutral atoms and molecules of gas are split into negatively-charged electrons and positively-charged ions - a process called ionisation.  The ions are caught by the magnetic field of the solar wind - the stream of electrified particles constantly emitted by the Sun.  Thus, ionised gas is carried at high speed from the comet - flourescing a blue trail, straight and narrow, millions of kilometres through space in the opposite direction to the Sun.

The yellow tail, comprising dust, is created by another process.  Sunlight exerts a subtle influence - called radiation pressure - on solid grains of dust in the coma.  This "push" is enough to carry dust from the coma in a shorter, broader tail.  It's also slightly curved.  But, like the blue gas tail, the yellow dust trail streams away from the Sun.  Why yellow?  It's the dust reflecting sunlight.

The blue gas tails stretch from ten-million to one-hundred-million kilometres.  Yellow dust tails range from one-million to ten-million kilometres.

Every time a comet swings around the Sun, it trails huge quantities of fine dust along its orbit.  At its most active in 1997, Comet Hale-Bopp was throwing off nearly 1,000 tonnes of dust every second!  If the Earth passes through one of these trails, particles can be seen  burning up high in the atmosphere.  Such displays are called meteor showers - or shooting stars.
 

Cometary Storehouses - Kuiper Belt and Oort Cloud

In the outer Solar System, beyond the planets, lies the Kuiper Belt - a vast, flattened and diffuse ring containing between ten-million and one-billion icy planetesimals or comets.  The belt is an inner extension of the Oort Cloud - an immense spherical reservoir of comets at an even greater distance.  Kuiper lies roughly in the same plane as the orbits of the planets.

Kuiper Belt objects are primitive, icy remnants from the early phase of Solar System formation.  The belt is probably the source of most short-period comets - that is, those with orbital periods of up to 200 years.  The first Kuiper Belt object was identified in 1992.  Since then many more have been discovered and the number is growing all the time.  Computer simulations have shown that Kuiper Belt objects can jolted by planetary perturbations and sent hurling into orbits closer in to the Sun.

The Oort Cloud is a spherical shell of comets that envelops the Solar System at a gigantic  distance.  Conservative estimates place the inner radius of the Oort Cloud at 20,000 AU, with the outer radius lying near 100,000 AU.  The outer radius, therefore, exists in interstellar space, and extends one-third of the way to the next nearest star.  The Oort Cloud may contain up to between 100-billion and a trillion comets.  Beyond a distance of 50,000 AU, the Sun's gravitational influence is so weak that the pull of nearby stars and giant dust clouds may periodically perturb the Ooor Cloud and send comets tumbling into the Solar System.

The Oort Cloud is generally accepted as the source of long-period comets - those with orbital periods of more than 200 years.  The orbits of some comets can stretch to millions of years.  Occasionally, during their passages through the outer Solar System, long-period comets may be "captured" into shorter orbits by giant planets like Saturn and Jupiter.

Theories about the formation of the Oort Cloud are highly speculative.  One idea is that Oort Cloud comets formed in the outer regions of the primitive solar nebula - perhaps within the Kuiper Belt - and were subsequently ejected into the Oort Cloud by a combination of planetary and stellar perturbations.  It is generally believed that all comets are in some way a by-product of formation of the Solar System.
 

Halley - The Most Famous Comet of All

Halley's Comet is the brightest of the comets whose paths we can predict.  It is also the only comet that has been seen regularly, through history, with the naked eye.  Halley has been observed and recorded for more than 3,000 years - first noted by the Chinese in the winter of 1059/1058 BC.

Halley's Comet travels around the Sun in a highly elliptical orbit.  When farthest away, it lies below the orbit of the planet Neptune, more than 35 times the distance of the Earth from the Sun.  Halley passes our way once every 75 or 76 years - the return period varying because of planetary perturbations.  It was last closest to the Sun on February 9, 1986.  Halley rounds the Sun between the orbits of Mercury and Venus.

Halley is named after the second English Astronomer Royal, Edmond Halley - not because he discovered the comet, but because he was the first person to calculate its path around the Sun.  Halley realised that comets seen in 1531 and 1607 were the same comet that he himself had seen in 1682.  He predicted that the comet would return in 1758 - and it did!  The comet was named in Halley's honour.  Thanks to Halley's acuity, we now can now identify recorded observations of his comet at every return since that 240 BC.  Halley's appearance in 1066, presaged the Norman conquest of England.  The comet is depicted in the Bayeux Tapestry.

On some returns Halley passes too far from Earth to be seen well.  This was so at the last  return in 1985/86.  The next, in 2061, will be no better.  Nevertheless, the last pass was the first of the space age.  A flotilla of spacecraft passed close to Halley.  They sent back a feast of information on the comet's structure and composition.

The only solid part of Halley is a tiny, elongated nucleus.  It is shaped like a peanut, about 16 kilometres long by 9 kilometres wide.  The nucleus is dirty ice, partly insulated from the heat of the Sun by a layer of intensely black dust.  But, in places, the Sun gets through, burning deep fissures that expose the underlying ice.  Jets of gas and dust shoot from the cracks, giving rise to all the activity of the comet, not least the heroic tails.
 

The Giotto Spacecraft

Giotto was the name given to the European Space Agency's first interplanetary spacecraft.  It  flew within 600 kilometres of the sunward side of the icy nucleus of Halley's Comet on March 14, 1986.  Giotto obtained the first ever close-up images of a cometary nucleus.  The  spacecraft was named after the Italian painter Giotto di Bondone who, in 1304, depicted Halley's Comet as the Star of Bethlehem on a fresco in the Scrovegni Chapel at Padua.

Launched by an Ariane 1 rocket from Kourou in French Guiana on July 2, 1985, Giotto was stabilised by spinning the craft at 15 revolutions per minute.  Giotto carried 10 scientific experiments.

Although protected from dust particle impacts by a dual-sheet bumper shield during its breakneck fly-by of Halley's nucleus, Giotto was hit several times by relatively large dust particles - greater than one milligram.  One impact, shortly before closest approach, caused Giotto to wobble, interrupting the communications link with Earth, so that scientific data were lost.  About half of the experiments onboard suffered damage, including the Halley Multicolour Camera.  On July 2, 1990, Giotto became the first spacecraft to use Earth in a "gravity-assist" manoeuvre to rendezvous with Comet Grigg-Skjellerup on July 10, 1992.
 
 

Links for Further Information:

Comprehensive Pluto page.  Internal and surface features, its moon Charon, facts and myths, proposed missions to the planet, news and images.
http://www.windows.umich.edu/cgi-bin/tour.cgi?link=/pluto/pluto.html

Comet Halley page.  Images from ground-based telescopes and satellites.  Spacecraft encounters with the comet, historical records and origins.
http://ceps.nasm.edu:2020/ETP/COMETS/comet_halley.html

ESO's Comet Hale-Bopp page.  Updated to June 1998, with images and data, plus links to other related websites.
http://www.eso.org/outreach/info-events/hale-bopp/

Comet Shoemaker-Levy 9 page.  Reports and images of the impacts, history of SL9, proceedings of the international conference on SL9 held in 1996.  Observations from the Hubble Space Telescope and Galileo spacecraft.
http://nssdc.gsfc.nasa.gov/planetary/comet.html

Stardust and Comet Wild Two page.  History of the comet, details of its composition, overview of the process for imaging both the coma and nucleus.
http://stardust.jpl.nasa/comets/wild2.html
 

Questions and Activities for the Curious:

1.  Describe the technique used by Clyde Tombaugh to discover Pluto at the Lowell Observatory, Arizona, in 1930.

2.  Some people believe that Pluto should not be called a planet at all.  What are the  arguments for and against this viewpoint?

3.  Imagine you are a comet perturbed from the Oort Cloud into the inner Solar System.  Describe your million-year journey towards the Sun, and back again.

4.  Describe the appearance of the two types of cometary tails, explaining the differences between them and how they are formed.

5.  Give an account of the events which led up to the impacts of the 20 or so fragments of Comet Shoemaker-Levy 9 on Jupiter in July, 1994.

6.  Why is Halley's Comet often called "the most famous comet of them all"?

7.  When Halley's Comet returned in 1910, it was photographed for the first time.  In 1986 it was visited by spacecraft for the first time.  How do you think scientists might study the comet when next it returns in 2061?

8.  What are the main discoveries made by the Giotto spacecraft in 1986 of Halley"s Comet?