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ED 347 085 SE 052 915

TITLE To Catch A Comet...Learning From Halley's.INFTITUTION National Aeronautics and Space Administration,

Washington, D.C.REPORT NO EP-252PUB DATE 86NOTE 37p.

PUB TYPE Guides - Classroom Use - Instructional Materials (ForLearner) (051) -- Guides - Classroom Use - TeachingGuides (For Teacher) (052)

EDRS PRICE MF01/PCO2 Plus Postage.DESCRIPTORS *Astronomy; Elementary School Science; Elementary

Secondary Education; Instructional Materials;Integrated Curriculum; *Interdisciplinary Approach;Learning Activities; Resource Materials; *ScienceActivities; Scieime Education; Science Instruction;Secondary School Science; Space Exploration; SpaceSciences; Writing Assignments

IDENTIFIERS Halleys Comet

ABSTRACTComet chronicles and stories extend back over

thousands of years. A common theme has been that comets are a majorcause of catastrophe and tragedy here on earth. In addition, bothAristotle and Ptolemy believed that comets were phenomena within theearth's atmosphere, and it wasn't until the 16th century, when Danishastronomer Tycho Brache carried out his research, that comets werefound to reside well beyond the orbit of the moon. More than acentury later British scientist Edmund Halley placed comets evenfurther out in space by calcu1ati7ig the orbits of a number of them.He also noted that certain comets had been observed to appear in thesame orbital patterns three times at regular 75- to 76-year intervalsand proposed that the explanation was that these were not three, butone comet--the comet that was later to be named Halley's comet in hishonor. Although Comet Halley has receded into ne v..;ter solar system,it still regularly reappears and may still generate excitement forstudents. It is, in effect, a tine capsule, binding grandchildrenwith grandparents, and scientists in one era with scientists ofanother. This booklet provides teachers with information essential tothe study of Halley's comet when this topic is integrated intoexisting lessons plans. Because this booklet is designed as ateaching supplement for the classroom, the instructional materialsare intended for use in the order presented. The first sectionincludes the following chapters: (1) The Ultimate Tine Travelers; (2)Touching Humanity; (3) Halley's History; (4) Where Do Comets ComeFrom; (5) What is a Comet; (6) The Halley Fleet; (7) Through theHalls of Time; (8) Other Heavenly Wayfarers; and (9) Touching theFuture. Section 2, which includes 43 classroom activities, ispresented as a supplement to the first section. Vocabularies listedin section 2 are taken directly from the background informationpresented in section 1. The classroom activities are suggested forinclusion within a broad course of study or as isolated exercises,and are not labeled by discipline, grade, or ability level. Theindividual teacher's creativity and ingenuity can facilitate theirapplication to almost any area of study. (KR)

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To Catch a Comet

Na1iona1 AerormuticS jndSpace Aciminis1ra1ton EP-252

Acknowledgment

Many individu& have contributed to this book.Teacher In Space Hu list Niki Wenger developed theconcept and saw the work through to fruition. With-out her dedication and enthusiasm, this book wouldnot have been possible. Special thanks also go toMarche Ile Canright, at NASA's Lewis Research Cen-ter, for her assistance in the formative stages, andto Malcolm Niedner, at NASAt Goddard Space FlightCenter, for his continuing technical guidance. Pro-duction of T2 Catch A &met was made possible bythe assistance of the NASA Educational Affairs Divi-sion staff. Robert Haynes, Rebecca J. Merck, andMarion Davis edited section 1 of the booklet into itsfinal version, and Pamela Bacon and Muriel M.Thorne provided valuable assistance in compilingthe activities in section 2.

Cover PlittbE Comet HalleyAonl 6. 1986. This oboe:graph wasMen with a 35-mm camera from on board NASM Nicer Air-borne Obseivatory almost 13 kilometers above New Zeeland.(See 'The Halley Fleet')

4 The Ultimate Time Travelers

6 Touching Humanity

8 Halley's History

10 Where Do Comets Come From?

12 What is a Comet?

16 The Halley Fleet

22 Through the Halls of Time

24 Other Heavenly Wayfarers

26 Touching the Future

30 Classroom Activities

Because this booklet Is designed as a teaching supplement for the classroom,the material in it is best used in the onler it is presented. Vocabularkts listed insection 2 are talon directly from background information presented in section 1.Although Comet Halley has receded into the outer solar system, it may still gen-erate excitement for students. It is, in effect, a time capsuki, binding grand-children with grandparents, and scientists of one era with scientists of another.This booklet provides butlers with the information they need to incorporate thestudy of the comet into their existing lessons.

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FOf almost as long as comets have crossed theskies, they have both fascinated and terrified hu-mankind. As we near the end of this century, thefear associated with comets has at long last becomean historical footnote and a new chapter has be-guna Golden Agein which comets are amongthe most important and inttaisively-stutlkd objectsin the universe. These small icy bodies which emitalmost no energy of their own and which populatethe outer reaches of the solar system in unimagin-able numbers, may hold crucial clues to the originof the solar system, and indeed of life on Earth. Itseems almost as if their importance is in inverseproportion to their size. The Golden Age of Cometsresulted from not only this increased appreciation oftheir role in our understanding of the cosmos, butalso from the advent of new observational tech-niques, including measurements from high-flyingairplanes, spacecraft in Earth orbit and interplane-tary space, and even spacecraft traversing thecomet itself!

Blothr's fresco, Motion el the MeV, portrays thffley's Comet as the Star ofBethlehem. MIMI Owl, Padua, Italy.

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Two comets in particular, Giacobini-Tinner (6G-Z")and Hal*, have achieved (or in the case of Halley,addad to its) immortality in the 198tb as a result ofbeing the first such objwts to be visited by space-craft. The findings provided by these missions weretruly unique and could not have been obtained inany other way. There is a class of inquiry in come-tary science which requires °space truth' for an-swers, and these missions provirkd some of them.

But even with all fly data gleaned by theGiambini-Lnner and Halley campaigns, many morequestions remain to be answered by better, moresophisticated missions to Ober comets. If all goeswell, the missions to Halley and G-Z will be seen inhistorical context as just the start of an exciting newage of cometary explorationexploration by 'goingthere.'

Why is the study of comets so imp- Ant? Mostscientists believe that comets were folmed at thesame time and out of the same matter as our solarsystem. Because annets are small and spend mostof their time far from the Sunin 'cold storage' soto speaktheir composition may have remained es-sentially unchanged over the eons. This is certainlynot the case tor Earth, the other planets, and as-teroids, which have basked in the Sun's warmth andundergone substantial change over the billions ofyears the solar system has existed. Comets offer usthe !mist hope of investigating those primordial con-ditions that existed during tlx) solar system's birth.

In addition, some scientists believe cometspfayed a role in shaping life on Earth, as theypassed through the skies leaving behind atoms andvapors that provided our planet with some of life'sessential building blocks.

Comets may also have collided with Earth intimes past, throwing great dust clouds into the at-mosphere and significantly reducing the sunlightthat reached Earth's surtax. The resulting suddendrop in temperature could have caused entire spe-cies of plants and animals to die out, thus allowingother life forms to evolve.

Halley's is without doubt the most intriguing andendearing of all comets. Because its orbit coincideswith the length of an average human liftman, its ar-rival every 76 years bridges generations. Bright asfar as comets go, its appearance in the year 1301 socaptivated the Florentine artist Giotto that he used itas a model for the Star of Bethlehem in his fresco,Adorition of the Magi.

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in 1705, British scientistitstronom Edmund Halleypt.olbhed ftS 'A Synopst of the Astronomy ofCo nets", a monumental work containing the resultfor wNch Is is most remembered today. Using ac-curate observations of cometary positions in the skymade oar the previous several hundred years, Hal-ley vfis able to calculate reasonably accurate orbitsin space for a number of bright comets. His WSwere Kepler's Laws of Planetary Motion and New-tan's Law c4 Universal Gravitation. The former heldthat planetsand prtsumabfy other objects in thesolar systemtravellul in &limes around the Sunwith orbital periods dependent in a prescribtx1 wayon the size of the orbit (a longer period resultingfrom a larger orbit). Newton's theory that the grav-itational force Istween two bodies was proportionalto the product of their masses and inversely propor-*mai to the square of the distance Istween them,provided the theoretical underpinning for Kepler'sLaws.

Halley noticed that three orbit those of cometssighted in 1531, 11X)7, and 1682had nearly iden-tical sizes, shapes, and orientations in space, andthat the appearances of these suppowdly differentcomets were separated by about 75-year intervals.He proposed that they were actually the same object

Hallefe retrograde orbit Is el an Inclination to the orbits el the planets.

returning at regular times, and that this one cometwould reappar sometime in 1758-1759. AlthoughHalky was the first to predict the next visit of acomet, Is unfortunately died before he could see hisprediction fulfillEl. When the comet did return as hesaid it would, first being seen on Christmas Day1758, it was given Halley's flans in his lam%

Comet Halley completes an orbit roughly every 76years. As Halley himself realized, the period variesbecause of gravitational interactions with the planetsin the solar system, especially the giant planetsJupiter and Saturn. The comet has returned as soonas 74.42 years (between 1835 and 1910) and as lateas 79.25 pars (451 to 530). Its arrival in 1986 was4 days later than predicted.

How close Halley's Comet comes to the Sun andEarth also varies. In 837 A.D. Halley's Comet cameto within 4.91 million kilometers of Earth, whichsounds far away but is `at annt length' on the scaleof interplanetary space. In 1986, its closest ap-proach was a remote 62.4 million kihneters?thieved on April 11. At the aphelion point in its or-bitthe farthest distance from the SunHalley'sComet is 5.25 biffion kilometers away (well beyondthe orbit of Notune) and moves at the relativelyslow speed of 3,200 kilometers an hour. In amtrast,at perihelionthe closest appronh to the Sun(achieved on February 9, 1986)the comet is only87.4 millitm kilometers from the Sun and travels atnearly 195,200 kilometers an hour (about 3,200kilometers a minute).

Halley's Comet has a retrograde orbit, whichmeans it circles the Sun in an apposite directionfrom the planets. Its orbital plane ;s inclined 162 de-grees from Earth's orbital plane.

Only several hundred =nets are known whichhave orbits that allow us to view them more thanonce. Those that do, like Halley's, are called "shortpeliod' comets. Halley's has been recorded as tarback as 240 B.C. (and perhaps earlier), and its1985-.86 appearance was its thirtieth definitive re-corded passage. In contrast, long-period cometsmay have calculated orbital periods of many thou-sands of years or more. The traditional demarcation

line between short- and long-period is an orbital pe-riod of 200 years.

Throughout comet chronicles extending overthousands of years, a common time emerged thatcomets are one of the major cats% of tragedy andcatastrophe on Earth. In part, this superstition de-rived from the fact that very often a comet mu (orhad just bEm) in gm sky when something terribleoccurred, and it was, therefore, natural for a peopleto maim this associatkm. Add to th* their almostghostly appearoce in the sky and the near-completelack of knowledge concerning their true nature, andone him a perfect formula for superstition and fear.We strive to prescribe an orderly progression fornatural phenomena and become anxious when ob-jects like comets seem to appar out of nowhere andthen journey to sow unknown destination. Even theword disaster comes from the Latin for evil (dis)star (aster).

Comets were so closely tied to the deaths of lead-ers that when a comet failed to appear at Charle-magne's death, historians made one up and wrote itinto the history books.

Both Aristotle and Ptolemy believed comets werephenomena in the Earth's atmosphere. It wasn't un-til the 16th century, when Danish astronomer TychoBrahe used accurate observations of a comet's posi-tion from two widely separated locations on Earth(the process of Iriangulation*), that comets werefound to reside well beyond the orbit of the MomIn other words, they were bona fide celestial ob-jects, and not atmospheric in nature. Even than,comets were thought of as swords or scimitars bysome because of their curved shapes, whichseemed poised to wield a blow Such a blow wasseen as Intended against those most elevated onEarth-royalty and +he leaders of nations.

Except for Gm irrational actions they lave occa-sionally inspired in hums, comets have done litheelm, to harm hunmMty. For example, such irratiormlactions m4ht indude Nero, who is said to have out-witted Ws aPPearance in 66 A.D. by murtbringMs Whet tvm wives, and most of his family andby setting the cfty of Rome cm fire. In 1531, theIncas thought the comet was a sign of their SunGod's wrath and began a series of human Edificesas appeasenumt. In 1066, William the Conquerer in-terpreted ROO appearance as a sign of his im-pending victory awl so kid me Norman invasion ofSaxon England. When Coms4 1...Stoutet was dis-covered in 1973, sorrm believed it hmaided the endof the world, expacbd on January 31, 1974.

If we were to sludy history in ten= of liallays*rather than years (one Halley being 76 ordinaryEarth years), we could measure the rise and fall ofcivilizations and the ivivance of technology with ascale that coincides with human lifetimes. Each re-turn of the comet would find a different world. In

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1910, the prediction ttmt Earth would pss throUghComet Halley's 'poisonous tail* terrified thousandsof people. Some bought gas masks and comet pillsto protect themselves against the expcted cyanogengas, others stuffed towels under their doors, whilea few even commftted suicide rather than facethe agony.

During 1986, in almost total contrast, we ven-tured to meet it, sending satellites and space probesto its vicinity in space. Scientists from many nationsworked together to make the most of the scientificopportunity Perhaps there am some who, even atthis enlightened point in the scientific study of com-ets, will attempt to link the recent appearance ofHalley to tragic events. Although the future must tellhow the world will react to ths next return of Comet

Itis still challenging to imagine what theworld will be like and how we will greet the return ofComet Halley in 2061.

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The following list covers important world events around the time of each recorded appearance of Comet Halley.

1. 24C B.C. First recorJed apparance in China.Archimedo is in his forties. Con-struction of Great Wall cd China tobegin in 25 years.

2. 164 B.C. Use of gears leads to animal-drivenwater wheel. Last year Rome record-ed 327,032 citizens.

3. 87 B.C. Julius Caesar, at the age of 13, seesHalley's. Athens falls to Rome next

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4. 12 B.C. Virgil began The Aeneid 8 years ago.First time comet is called an omen ofdisaster.

5. 66 A.D The Gospel According to St. Markappeared last year. Jerusulem, at-tacked by Romans, to fall in 4 years.

6. 141 Chinese astronomers recorded acomet in their skies. Ptolemy is de-veloping his theory that Earth is thecenter of the universe.

7. 218 Comet appears over China, then overRome days later. Fearful flaming starprecedes Roman emperor Macrinus'dist.

8. 295

9. 374

10. 451

11, 530

12. 607

13. 684

14. 760

15. 837

Roman persecution 1:* Christians tobegin in 8 years.

Emperor Valentinian to die next year.Books begin to replace scrolls inEurope.

Attiki and his Huns defeated at Battleof Chalons. Indian astronomer de-vises mathematical powers androots.

Saxon invaders found kingdom inwhat is now England. Plague sweepsacross Europe.

First envoy from Japan arrives inChina. Westminster Abbey will befounded in 4 years.

Mount Vesuvius erupts. Plaguesweeps across China.

Bagdad to become seat of Arab Em-pire in 2 years. Turkish Empirefounded.

Algebra is invented in Persia. Closestapproach of Comet Halley to Earth.

Hailers Comet es Noy 13, 11110, show is els photograph with so too fah disoossestlea. Note the meteor is thecomers tali to the left of Woos (bright spot), sod the trolled lights of Hegel* Arizona. Photo: Lowell Observating

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16. 912 Normans become established inFrance.

17. 989 Normans to invade England nextyear. Arabic numbers introduced toE rope.

18. 1066 Battle of Hastings. Frightened bymint, Saxons fall to invading Nor-mans. Earliest representation of thecomet is this appearance, noted inthe Bayeux Tapestry.

19. 1145 Danube River is bridged. In France,music troubadours heard for the firsttime.

20. 1222 Genghis Kahn takes comet as a signhe should conquer the world. Cottonintroduced to Spain.

21. 1301 Flavio to invent mariner's compassnext year. Giotto to use appearanceof comet as inspiration for Star ofBete tem in his painting, Adorationof the Magi. (1303-6).

22. 1378 Chaucer is 35 years old.

23. 1456 Athens falls to the Turks. Columbusis 5 years old.

24. 1531 Copernicus is 58 years old; his theo-ry is yet unpublished. Post officesare established in Engkmd.

25. 1607 Galileo develops an astrommicaltelescope. ,ktmestown is founded.

26. 1682 Philadelp'4a, Pennsylvania, and Nor-folk, 1.firginia, are founded. IsaacNewton explains tides using theoryof gravitation.

27. 1759 James Watt to complete steam en-gine next year. Return of comet pre-dicted by Edmond Halley; cometgiven his name in honor.

28. 1835 Cyrus McCormick invented reaperlast year. Plague sweeps throughEgypt. World population about1 billion.

29. 1910 Gas masks and comet pills sold toprotect people. Boy Scouts of Amer-ica founded. World population about2 billion.

30. 1986 Space probes used to observe CometHalley. World popultion about5 billion.

Great Comet of 1532. Points Venus woodcut.

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WW1 humans first looked up and wondered at thenight sky, they devised newly storiesearly theo-ries, reallyto explain the bright objects abovethem. When Edmund Hidley showed that cometsmove in well-determined orbits under the Whenceof solar gravitation, peo* began to recognize thatcomets moved in wcordance with well-known phys-ical lees, and hence could not be omens of destruc-tion. Halley's work probably marked the *inningof the scientific method as applied to comets. Nearlya century and a half later, when it was dim:Awed byspectroscopy that numy of the *merits and mole-cules in cometary atmospheres and talk are foundon Earth and in the Sun, the theory that conktswere formed during the birth of the solar systemwas first proposed.

Twentieth-century scientists have suggested sev-Ent theories to explain the origin of comets. Onelandmark and still very current idea was !maimed in1950 by Dutch astronomer .11m Van Dort, who stud-ied the orbits of nineteen long-period comets andccmcluded gene,* that the long-period cometswhich visit the ism solar system wId are visiblefrnm Earth come in from a huge comet 'cloud'numbering perhaps a trillion objects. By examiningthe distribution of aphelion distances (the farthestpoiM on the orbit from the Sun), Dort estimated thesize of tht cloud to be 1W,D30 or so astronomicalunits (or `AU') in diameter (one AU is the distancebetween Sun and Earth), a size which is about halfthe distance to the nearest stars and which dwarfsthe 40 AU radius orbit of Pluto, the outermostknown planet. The periods associated with suchhuge elliptical orbits are in the tens of thousands,and even millions, of years.

Those nineteen orbits are not typical of comets inthe 'Dort cloud', however. Dort suggested that theoverwhelming majority of comets in the cloud aremoving in huge, newly-circular orbits which do notcome clow to the inner solar system, and hencethey are not visible from Earth. There must be aforce which converts some of these orbits into high-ly-elongated ellipses with perihelion (closest dis-tance to the Sun) inside 5 AU, where we can dis-cover comets. But what is this force? Dort theorizedthat occasional gravitational enownters of the cometcloud with nearby passing stars in the (Waxy mightshake loose a relativi handful of comets, sendingthem toward the inner solar system. Modern com-puter simulations have shown that such a mocha-

ntm can indeed bring very distant =nets into theinner solar system. lkcent refinements on Dort's pi-wearing idea suggest the presence of a second, orinner Dort Cloud existing beyond the orbit of Nep-tune arel containhig 10 to 100 times more cometsthan ilk outer &loud.

Oort's model told us where the long-period com-ets come from today, but perhaps did not com-pletely answer the question 'where were cometsoriginally formed, and how did they get into the Dortcloud?' There we two general schools d thought,and both derive from the concept that the solar sys-tem was created 4.5 billion years ago from a huge,spinning cloud of interstellar go and dust. After theinitial spherical cloud flattened into a rt.tating disk,the concentrations of gas and dust wo....41 have beenlam) enough for Idlometer-size objects called'planetesimals' to condense out of the disk mate-rial. When a critical number of time had been cre-ated, the planetesimals themseMs would havecome together gravitationally to form the muchlarger planets. The Sun, of course, represented thecollapse and agglomeration of a huge amount ofmaterial near the center of the disk.

In the more prevalent view al comet formation,the Dort cloud of comets was created when kilo-meter-size planetesimals in the Uranus-Neptunezone wsre gravitationally *cted by the newly-formed Uranus and Neptune into the Dort cloud.Thus, comets represent those planetesimals be-tween tier orbits of Uranus and %Mune which didnot condense into those two planets, but were sim-ply ejected by them into near-interstellar spwe.

The other view is that comets were actuallyformed at the distances of the current Dort cloud,out of the very outermost material of the initialspherical cloud of gas and dust (the so-called 'solarnebula"). The major problem with this theory is thatat such vast distances from the Sun, the density ofdust and gas in the solar nebula would have been solow as to virtually preclude condensation into kilo-meter-sized objects.

Any successful model of comets must explain theexistence of comets like Halley's, whose periods arenot long, and whose aphelion distances are not inthe Dort clond, but insirk the riranetary system. Ifall comets originally resided in the cloud (after ejec-tion by Uranus and Neptune, that is), what forceconverted some of their orbits to relatively small el-Hines like Halley's? The answer is gravitational inter-

Tis Dori Cunt 1:1=1. Whol a der ones dose enough (insets al right), semets in the dud lair Me the kw solar system.I:May d &ty and Mew mepoine.

actions with the giant planets Jupiter and Saturn.After stellar encounters convert some of tis circularorbits in the Oort cloud to huge, highly-elongated el-lipsis (with perihelia in the inner solar system).Jupiter and Saturn have enough mass to furtherchange the orbit if the =et passes close to eitherof them. The result of repeated interactions withthese planets can be a very small orbital ellipse (andshort period) such as Halley's. But just the oppositeis also Nssible: comets can be hurled 'slingshotstyle' out of the solar system by a close pass withJupiter if the geometry of the interaction is just right.

Eventually comets die. After thousands of passesaround the Sun, they either disintegrate, evaporate,or become inert. This latter situation probably re-sults from the formation of a thick encnstation ofdust and other nonvolatile material which preventsthe maintenance of an atmosphere and tail. Whenthis happens, the comet continues its orbit aroundthe Sun, but now it resembles a sponge-likeasteroid. Metal= of Nelloy's orbit to planets to our solar system.

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It is a remarkable coincidence that the first modernidea on the structure of comets came in the sameyear as Owl's hypothesis, 1950. The Omer msthe American astronomer Fred L. Whipple, who ad-dressed the fundamental questkm of the uftimatesource of material seen in cometary atnxispheresand tails. As comets near the Sun from the cold out-er reaches 0 the say system, it is im observationalfact that they produce an atmosphere, or 'coma',which is *proximately one hundred thousand kilo-meters across, followed by the formation of a tail ortails which can be tens of millions 0 kilometerslong. Unfortunately, the comas brightness com-pletety hides the central source of its material whenthe comet is close enough to Earth to be studied.The probkim when the comet is disfzet and does notyet possess an obscuring coma is that Zhe centralsource is too faint and small to be studtd in detailfrom the Earth. Is there a tiny solid nucleus whichproduces the coma and tails, or is the source of ma-terial a compact swarm of individual particles?Whipple tried to come to grips with such seeminglysimple questions, and the theory he put forth hassurvived the test of time.

Over the huge majority of its orbital arc, Whipplereasoned that a comet is a small (several kilometerswide), cold, and inert chunk of ice embedded withdust grains and perhaps rocks. He pictured ametsas dirty snowballs. In addition to hydrogen- andoxygen-bearing molecules, spectra of cometary co-mae had shown carbon- and nitrogen-bearing spe-

des, and Whipple therefore knew that there had toIx several Idnds of ice, not just water ice, in his nu-clei. Oder ice might be the dominant comporkult,but carbon monoxide, carbon dioxide, methane, andammonia las, were also possibillits ki smalleramounts. In Whipple's model, the dust grains aretrapped in the icy lattice mid are released by theevaporating ices as the conwt approadies tte Sunand grows warmer. It is this outstreaming 'dustygas" which forms tlur coma and ultimately its tailsof comets. Because of the extremely low pressure ofthe coma gases, the melting la bypases the liquidphase altogethet chiming from a *lid to a gas inone step called 'sublimation.'

Whipples thary explained two observed factsvery WI. Arst, the concentration of ices into solidbait as oppsed to a particle swarm, was consis-tent with the longevity 0 the short-priod comets(such as Halley and Encke, each of which had beenseen for many returns), for the amount of materialsublimated away during each orbital pass shouldonly be meters deep. The second fact had to do withthe observed tendency of some comets to have anorbital period which changed with time. If the nu-cleus of a comet were to rotate, then the maximumsublimation on the sunlit hemisphere should be inthe 'afternoon sector' as a result of thermal inertia,and the resulting uneven reIeas if gas across thefate of the nucleus would create a 'rocket effect'which could, in principle, change the orbital periodever so slightly with time. When Whipple worked

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out how fast the period should chap, he found ex-cellent agreement with the changing penal of cometEncke, the comet with the shortest known orbitalperiod (3.3 years). There have been refinements andimprovements to Mop let theory, but the basicideas have held up through the spacecraft encoun-ters with Halley's Comet.

The coma as traditionally defined consists ofgases seen in the region of the spectrum to whichour eyes are sensitive, and the characteristic diame-ter is 50,000-100,000 kilometers. In 1969, how-ever, a huge cloud of hydrogen, visible only in ultra-violet light, was discovered surrounding two cometsand spreading across millions of kilometers. It is

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wt01 understood that this 'hydrogen cloud* consistsof individual hydrogen atoms which have been dis-sociated from water molecules by ultraviolet sunlightas the aster moktmlis floiv out from the nucleus.

Comets which are large or active enough (i.e.,1 sublimating enough gas and dust from the nucleus)..

will start to develop tails when tlx distame to theSun is 2 AU or so. By tixi time such a colon hasreadied perihelion, which may be signifcantly lessthan 1 AU, tlxi tail lengths nuy exceed 10 millionkilometers. This is a truly astonishing phenomenon

co stops to comtider thA 10 mill* kilo-meters is nearly km times the Sun's &mete; aridthat such an enormous structure derives from an

only kilometers across (the nucleus).There are two basic types of tail and individual

comets toque* manifest both types. They couldbe nvre different in appswance, behavior,

and origin. The first to form is the so-called 'plasmatail.* or "kin tail° is it is equivalently known. Thistail consists of cometary ions spiraling along makmitlt fiekb captured by the comet from the solarwind, and its location in space is approximately

the anti-Sun direction. [An ion is an atom ormok.icule which has been stripped of one or more*crone (ionkee), giving it a net positive charge.Atoms arkl nrilemiles are ionized either by absorp-tion of ultraviolet rays from the Sun, or by impactwith emirgrdic solar-wind pram.] The formation ofplasma tails ki an extremely complex process, butthe most important feature is the 'capture' of mag-mrtic fields from the solar wind. [The solar wind isan extremely hot, supixsonic gas which is producedki the Sun's outer atmosplure and which flowsthroughout the solar system]. This capture occursbecause magnetic fields cannot pass freely throughan ionized gas, and this is exactly what the coma is.The fields in the solar wind Income hung up or

in the coma and get folthld around behind thecomet to form a type of `magnetic windsock.' Be-cause of their net chargi, ions such as CO + (car-bon monoxide ion) spiral along flu: captured fieldlines, fluorescing in the sunlight as they do. It is pri-

nk) filMfescerice Of CO in the blue regionof the spectrum that gives the magnetic tail its

Because the solar wind is anything but steady, aplasma tail once-formed will exhibit considerableclunges from night to night, and even hour to hour.

ps its most incredible property is the occa-sional total disconnectkm from the head, followedby the growth 31 a low attached tail. The discon-nected tail moves rapidly away from the need and isdissipated into intsTplanetary space as the °new' tailgrows. Such disconnection events, or *DE's', havebeen attributed to the interaction of comets with so-

calkid 'tutor lvoundaries" in the solar wind, acrosswhich the dinx:tion of the magnetic NM changes by180 degrees. Haft's Comet had perhign 20 orwe Des In 1985-1986.

ibe second tyre of tail is the dust tail, composedof small (typically micron size) dust grains originallyresident In the connitary nucleus which, upon beingreleased into the COM are pushed by radiationpressure from sunlight behind the comet to form abroad, curving tiA. The particles are liter* travel-ling in their own inikipmdmit orbits. The breadth ofthe dust tall depends in part on the size distributionof the chid grains, because small, light particles areaccelerated by sunlight more easily than large, mas-sive ones.

The kght from dust tails is nothing but scatteredstolight, home this type of tail is yellowish (ormilky) in color, in contrast to the blue plasma tail.Although dust tails are oriented In the general direc-tion away from the Sun, btcause the speds of thedust particles relative to the nucleus are rathersmall, these tails do not congregate very closely ttsthe exact antisolar dirtction, but are Inclined signifi-cantly to it. In this resent, too, they differ markedlyfrom plasma tails. The result when projected on the* can be strildng: the long narrow plasma tailgoing off almost exactly atny from the Sun, and theshorter broader dust tail oriented perhaps 45 oreven 90 degrees away from dui plasma tail. Occa-sionally, under rather unusual viewing conditions,the largest particles in a cemetery dust tail mayproject in the general direction of the Sun. Thisshort structure is called an anti-tail.

Halley's Comet, comets Eknnett, Kohoutek, andMet are four of the recent comets (since 1970)which have displayed prominent dust and plasmatails, but rx:4 all comets have two tails. Some mayhave one idnd of tall but not tlui other, and many ofthe srruill periodic comets have none.As comets near the Sun, they are traveling at thou-sands of kilometers a minute, but they are so farany that we see them as slow-moving ol$ectsagainst the background of the stars that remain inview over Enteral weeks.

Each trip at active comet makes around the Suncausis it to lose several hundred million tons ofdust and vapor. The largest particles shed by ceme-tery dust tails are the source for dazzling meteorshowers when Earth passes through or near a corn-ers orbit. Motes e by passing comets have beenfound on the ocean bottoms and in the atmosphereat high altitudes.

Even before the direct exploration of Halley'sComet by space proba in 1986, it was know; i thatthe sublimation of ices on the cometary nucleus isnot a uniform process acrc 4 the sunlit face of the

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nucleus. Telescopic views and photographs of cem-etery comae have shown tor well over one hundredyears the presence of rotating jets and exixtndinghalos for many aimets. The presumption was ttuitthere are long-lived zones of greater-than-averapactivity on the rotating nucleus which, when illumi-nated by the Sun, emit gas and dust visible as dis-crete structures from Earth. In a somewhat differentcategory are the occiudonal random outbursts of gasand dust which probably represent the sudden ex-posure of fresh ices to the Sun when a portion ofany Insulating surface crust breaks away.

There are moral theories concerning the natureof the jets, sid indeed of cemetery nuclei them-selves. In one, the nucleus is a frozen mass ofhouse-sized chunks of ice or snowballs that areloosely bonded and crusted over Tim jets emergefrom spaces in batmen the snowballs, when achunk breaks away exposing fresh ice, or sunlighthits a thinly crusted region. In another, Meal' cOn-opt. the nucleus is composed of boulders glued to-gether by a dusty ice coating acquired wher thecomet formed. Jets CCM from the ice-filled gapsbetween the boulders.

The Images of Halley's nucleus returned by theVEGA and especially Giotto spacecraft (discussedbelow) will erhaps help us determine which model.if either, is more accurate.

15

The first gececraft to measure a comet's environ-ment by 'going there* was the Interplandary Come-tary Explorer (ICE). Launched by NASA in 1978 asthe third InternatioM Sun-Earth Explorer; or 1SEE-3,the spuecraft's original purpose was to study thesolar wind input to the Earth's magnetosphere froma location just l'upstrean* of the Earth. By 1982, thespacecraft had prformed its primary mission al-most flawlessly for ova four years, and a great dealhad been learned about the interaction of the solarwind with the magnetosphere.

leteseellonel cometery Explores Sweatt ler apparachingBlarebhd-Beest

Its initial objectives essentially whieved, a teamled by Robert Farquhar stip:lied the feasibility ofusing the spacecraft to carry out other missions, in-cluding a flyby of comet Giacobini-hnner (G-Z). G-Zwould be passing perihelion in early-September of1985 at a point in space very netir the Earth's orbitalplane and slightly less than 0.5 AU from Earth, andthere was a chance that by sending ISEE-3 throughthe complex combined gravity field of the Earth andMoon, the spacecraft could be 'sling-shotted° to anencounter with the comet. Fortunately, there wasplenty of hydrazine fuel onboard to perform anyneeded maneuvers, but the problem of finding aproper trajectory through the Earth-Moon systemwhich had G-Z 'at the other end' was another, verymuch more corhplicated matter.

16

Extensive computer studies performed over manymonths shosed that there was indeed a solution,and at this point NASA gave the go-ahead for the G-Z extension of the mbsion. Before it left the Earth-Main system, ISEE-3 made two deep passes in theEarth's mine* WI, returning a harvint of invalu-able data. As tim spitecraft ended its last lunarswingby maneuver in December 1983, and headedfor 6-Z, it received a new name, ICE, for Interna-tiorel Cemetery Explorer.

On September 11 of 1985, ICE flew by the cometand through its pkema tail at a speed of 21 kilo-meters per second. The data returned during theseveral-hour smatter were of the highest qualityand fully justifial the risk d the G-Z tvrtension of thembsion. The pass through the tail itself only20 minutes, but it confirmed tte magnetic windsockmodel of cometary Mama tails and made uniquenmesurements of the ion and Wectron densitiesthere. Just as important, the hours previous to andafter the tail pass provided crucial information onthe interaction of G-Z's atmosphere (or coma) withthe solar OW far from the comet. In particular, theexpected bow wave in front of the comet was ob-served, although some of its charateristics wereimexpected. Perhaps the best terrestrial analogy tothe bow wpm is the wave crated by a ship movingthrugh water. In the cse of comets, the solar windserves is the miter, and the comet is the objectriming through it (the ship).

Emerging from the other side of G-Z without ap-parent damage and with a triumphant mission underits belt, the ICE spacecraft set the stage for theworld to greet the 1985-88 appearance of Halley'sComet. Many nations worked together to send aflotilla of spacecraft to greet Halley, as sell as to ob-serve the comet from the ground, Earth orbit, andinterplanetary space. It was an unprecedented studyof a comet. Activities were coordinated by the inter-national space agencies from Europe, the SovietUnion, Apan, and the United States, and the degreeof cooperation among them was an example of sci-entific planning and international goodwill at theirbest.

The International Halley Watch (IHW) consisted ofmore than 1,000 professional astronomers and 900amateurs from 47 countries, and vms administeredby the National Aeronautics and Space Administra-tion (NASA). The 1HW was responsible for advocat-ing, collecting, and archiving as much of the

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18

wound-based data taken around the world as possi-ble. It is dear that the volume of data iS wormous,and dm archive, due out in 1990, should contain atreasure of &eta for future generations of astrono-mers to study.

Although the United States had no formai encoun-ter mission to Halley, many U.S. spatscraft turnedtheir instruments toward the comet. NASA's interna-tional Uttraviokst Explorer (lUE), an Earth-orbitingtelescope, shrlied the composition of Halley's comaand tail, as well as Ow comet's total gas prodixtionrate, during much of 198546. It was used inten-sively at the time of the international space missionsto the onet M March 1986. The Solar MadmumMission (SMM). a NASA satellite launchal in low-Earth orbit ki 1980 to study the Sun, was tempiyari-ly turned away from the Stm and toward Halley toprovkki unique images in February 1986, whentlxi comet was many behind the Sun at time ofperihelion.

Pioneer Venus Orbiter (or PVO, in orbit aroundVenus) was already situated on the far side of theSun, which placed it in good position to study thecomet near the time of mirlhelion, February 9, 1986.WO measured the size and brightness of the hugehydrogen cloud surrounding Halley, data which re-vealed tint the cornet evaporated 60 tons of waterper second wid that over the total period of evapora-tion its surface sank about 20 to 30 feet. At thatrate, the anal should be alive for another 1,030apparitions or so. At 20 million kilometers in radius,the hydrogen clowl of Hal* was fifteen times largerthan the Sun, making the comet the largest object inthe solar system, if only temporarily.

The week of March 6-14, 1986 vas as eagerly an-ticipated a week in space as scientists are likely toexperience in a long time. Not one, but five space-craft launcluid by the Soviet Union, European, andJapanese space agencies were scheduled to passthrough various regions of Halley's Comet duringthat extraordinary intent And it win here that in-ternational cooperation was most needed in order topromote the success of the last of these individual,yet interconnected space missions (which wereoften collectively referred to as *the Halley Arma-dal. The Glatt; spacecraft of the European SpaceAgency (ESA) was being targeted to make a veryclose approach to the Sun side of Halley's nucleuson March 14 in order to obtain detailed images of itssurface. A minimum distance of 500 kilometers wasdesired, but here there was a problem: ground-based imaging of the very innermost coma couldnot establish the probable position of the invisiblenucleus to an accuracy any better than the desired*miss distance,* i.e., 500 klometers. There wasevery chance, therefore, that unless additional datacould be obtained. Giotto might pass on the night

side where the nucleus would be too dark to photo-graph and the camera would be looking directly intothe Sun! Thus was born the concept, and actualproject, called 'Pathfinder.'

The idea was to make full use of the irnmes ob-tained by the Soviet VEGA-I and VEGA-2 spacecraft,which woukal be making their encounters on March 6and Mardi 9, respectively, to *point the location ofthe nucleus at least ten times more accurately thanwas possible from groural-based photographs. Withthe Halley nucleus twice-observed eight and fivedays before Giotto's close pass, it would In possi-ble to alter Giotto's course extremely la in the mis-sion to ensure the proper tractory on the Sun sir*.Fortunately Us cameras on the VEGA spacecraftworked as hoped, and the result for Gkatto projectwas an historic mission to the heart of Halley'scoma, the memdear region itself. In short, Proj-ect Pathfinder was a brilliant success.

The VEGA spincraft were heavily instrumentedwith experiments to observe Halley, but firstdropped off balloons in the atmosphere of Venus be-fore continuing on to the comet with a gravitationalassist from Venus. Because of Halley's retrognaleorbit and the necessarily prograde orbits of all theArmada spacecraft, the encounter speeds were veryhigh. In the case of the VEGAs, the relative velocity

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between comet and spacecraft was about 80 kilo-meters per second (or about 250 times Ow speed ofsound cur Earth). Such a large encounter velocitymeant that the impact of yam medium size dust par-ticles Oil the scientific instruments could seriouslyimpak or even knock them out of operation, and sothe VEGA spacecraft were equipped with 'bumpershields tor protection.

The VEGAs were both targeted for a minimum en-counter distance between 8000 and WOO kilometerson the sunward side, and the images they returnedshweved not only the existence of a solid nucleus(proving Whipplet 36-year old theory). but also itslocation, size, and brightness. Somewhat surprt-ingly, the nucleus was a blicker-than-coal objectshaped like a potato. It was larger than had been be-lieved, alzsut the size of Manhattan Island, 16 kilo-meters long by 8 kilometers wide, with a mass ofabout 150 billion tons. The nucleus surface washeated to well above freezing, and iet vapors werebreaking through the outer crug from the underly-ing ices. Spectroscopic data showed that water ac-counted for close to percent of the gas comingoff the comet, with the remainder being mostly car-bon dioxide and carbon monoxide. In the case ofHalley's, alvut 16 tons of water, enough to fill asmall swimming pool, was sublimated off the nu-cleus every second during the VEGA-I flyby, andtwice as much during that of VEGA-2 three dayslater.

19

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The oaten al Comet Hanes es sees lasomyselle of 88 Images from Wane adbeim bees cembised tit mmdmire tbe detailsisThis le ell lbe bri01 Old dint arm The$wilstowarda.lsft Tbe animameasures 14.9 by 82 U.Courtesy thy est Erisscope magazine.

Japes first interplanetary spacecraft, named Sul-sea and SA", were sent to Haliey's Comet tostudy the hiterution of comets with the solar windand interplimetary magnetic field, as well as the vasthydrcipm ctud surroinxling Hat*. Because thespacecraft ware to study the con* from relativelysite Ostances, 150,000 kNometers for Suistd onMarch 8 and 7,000,000 kilomebirs for Saiagob onMarch 11, they carrhx1 no dust shields. Still, own atthese large minimum distances, a few dust hitswere recorded by onboard Mstrumentation. The%viable hydrogen production rates oteerved by Sul-sifs ultraviolet camera provided the important factVet Halkiy's nucleus rottas in about 52 hours, andSAVO, was able to monitor the solar wind input tothe conlet not oiVy near the time of the Sidsei en-count% but those of the VEGA and Giotto encoun-Ws as well.

Europe's contribution to the Halley fleet was theGiotto spaceavft (named after the Florentine paint-er), and its mission was to penetrate as close aspossible to the comet's nucleus on March 14 . Thedangle' do to dust impacts was especially heavy forthis armada spacecraft, and it was fitted with a dou-bki bumper shield to protect the instruments andspanciaft subsystem. Many sdentists consideredGiotto to be on a suicide mission with little chancefor survivft the closest approich. indeed, despitethe protection offered by the double shield, Giotto

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was hit by one or several large dust particles atabout 1,200 kilometers from the nucleus, which re-sulted in the telemetry antenna being knocked offthe direction to Earth. Miraculously, howew theantenna realigned itself 34 minutes later and re-sumed transmitting data on the outbound trajectory.Giotto is now being considered for use in studyinganother comet.

Because Ottto came the closest of any of theprobes, it sent back the most detailed imagn ofHaney, which included such features as mesa-likeplateaus, a 1.4-kilometer-diameter crater, and a 430-to 530-meter-high mountain. Giotto's images haveled to the speculation that the comet's dark crustconsists of the dust and complex molecules left overas the comet's ices evaporate. Because vapor ventsmake up only 10 parcel of the surf= and are ac-tive only when facing the Sun, most of the comet'ssurfice remains insulatiti by layers of dusty debristhat encase the underlying ices.

Back on Earth, studies Me made from theground and aboard the Kuiper Airborne Observatory(KAO), NASA's Flying Telescope. For two months,KAO flew in the stratosphere over New Zealand mea-suring the dust, gas, and water that Comet Halley

The Halley Fleet

produced. The KAO is a cargo plane outfitted withan infrared telescope, and flies at 12.7 kilometersaltitude. This distance is above most of Um interfer-ence and infrared alsorbing atmosphere, allowingscientists on board observations impossible to nukefrom the ground. The KAO measurements cd Halley

Ow Fall of 1N35 provided the first detection ofneutral water vapor in any comet. Visible lightcameras onboard the KAO filmed an ion taildisconnection.

Because Halley's volume wfis found to be tentimes greater than expected, and its mass was notmuch different from expectations, its inferred densi-ty is significantly smaller. This has led scitsitists toconclude the nucleus must be quite porous, morelike newly fallen snow rather than compressed ice.

The nucleus of Halley's Comet wobbles and tum-bles OW a badly thrown football, flipping end overend as it spins and flutters. Apparently the rotationalmotion is extremely complicated, and different mo-tion periods of 2.2 and 7.4 days have been deter-mined using different types of data. Probtily a com-bination of both periods is needed to adequatelydescribe the very complicated motion of Halley'snucleus.

21

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History is filled with tales about how comets haveaffectikl the behavior ri humans. But what aboutEarth itself? MVO comets ever changed or alteredour planet? Comets, for exampki, may have playeda role in 'seeding' the early Eartht atmosphere withthe ingredients for life. A number of years no, twoBritish astronomers proposed that comets carryamino acids. Perhaps Earth's primordial soup, fromwhich lite is thought to have sprung, containedsome of the cormlex moleules and amino acidsevaprated off cometary nuclei. Carbonaceous mate-rial in comets may even help explain how the Earthreceived some of its carbon and what form it took.It is estimated that about 100,000 comets the sizeof Halley's could supply all the carbon now in Earth'scrust.

A theory prominent today suggests that a cornutmay have played a role in the disapparance of thedinosaurs. Strong evidence supports a notion that

something struck Earth about 65 million yaws ago,wiping out about three-fourths of the known spe-cies, including the dinosaurs. With these huge landanimals gone, an evitutionary advantage vas givento a relative newcome& the manwnal, whk:h arose tofill the ecological niche.

Althogh Halley's wifi never coNide with Earth be-cause the two orbits do not cross, other combmay, aid °video= that one or more objects kn-pacted Earth comes from dozens of sites where thearcient boundary laws between Cretweous andTertiary rocks show a thin deposit of iridium-en-riched clay. Irklium is a rare ekiment cmEarth, but iscommon in extraterrestrial objects. The rkpositsalso contain grains of quartz that have been de-formed by intense shock waves, possibly caused bya high-impact collision. Carbon in the deposit sug-osts a fire storm may have swept the planet.

Exactly what collided with Earth or how the dino-saurs died is still a controversy but some suggestthe cuttrit was a comet. An apparent conwtion inthe disappearance of life-forms and the impact ofour planet fits with an even broader theory thatsomething disturbs the Clort Cloud every 26 millionyears. This disturbance loom hordes of comets,and sets them on lengthy elliptical orMs. Ideasalxiut what causes this disruption include an invisi-Es tenth planet called Planet X, or a companion starto the Sun, called Nemesis. Such arguments arecausing some scientists to rethink their interpreta-tions of the universe and solar system.

Very close to our own time, it seems plausiblethat a comet may have struck Earth. In 19041, amassive eglosion cxcurred in the air over Siberia,causing a *abaft that felled trees over 50 squarekllometers and knocked down people as far aviay as64 kilometers. No tangible evidence of a physicalprojectile was ever discovered, leading scientists topostulate that an icy comet had entered Earth's at-mosphere, detonated, and vaporized.

Depiction of =net *Key of dinosaur exlInctkm. Paintingby Min Wald, Langley Research Cinder GraphsDepartment.

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24

Aftlxx* Halley% is the most famous comet, othershave been spectular and influential. Barely twoyears before Halley's 1986 visit, Comet Glacobini-&nu becang the first comet to be visited by aswecraft (see albs Way fleet"). The comet wasdiscovered in 1900 and its 1935 visit was its ninthrecorded passage.

Some cornits have exhibited unusual behavior.Comet Biela, with a period of 6.75 years, had nor-mal passives during the 18th and early 1 etli cen-turia. In 1846, it split into two dttinct and com-plete comets, each idternating in comparativebrightness. In 1852, both comets *Rared to beabout 2,400,000 kilometers apart, and then werenever seen again.

Encket Comet has the shottest orbit, 3.3 years,almost no tail, and a very small coma. It has evi-dently lost most of Its Icy mbdure and is turning intoa dark, wow-like asteroid, a process that maytake several thousands of years.

Two comets appeared in 1811, and one remainedvisille for 17 months. The Great Comet of 1843 hada tail 320 million kilometers long and was even visi-

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Presentations of mists kw the pot (cleckwise hem uwww-left):Doodle Comet over Pe* Gem= engraving, "Greet Comet of 1577,"from haw; London Pam*, 11515, "Dbcovery of a comet et Green-wich abservetm"; Gennep eogroving °Grid Comet el 1577."

hie during (Wight. Donatit Cornet, discovered in1858, developed muftis tab, and the coma wascharacterized by the presence of Nimal imtward-moving concentric shells or halos, which suggestedthat whatever process was treating Ow material inttui coma was time-chipendent. In 1861, perhaps thebrightest comet in recorded histim appeared. Be-cause its coma wis huge, and its perpendicular or-bit took it extremity Wu t) the Sun, it was sobright its light cast shalows on Earth.

A relative handful of comets have perihelion dis-tances so small that they almost literally graze theupper abrxisphere of the Sun, and have been giventhe name `Sun gazers.' Their extremely close pusto the Sun results in the vaporizatkon of huge quan-tities of ice, old several of these comets have beenvisible in broad daylight. Some Sun gazers are tornapart by tidal forces exerted on the nucleus by theSun near the time of perihelion. Comet Ensor, dis-ciNered in 1906, and Comet Mstphal, discoveredin 1913, hal been predicted to be sixictacular whenthey passed perihelion, buta they approadvd theSun they became increasingly faint and fuzzy untilthey completely disappeared.

Comets that retreat lack into the depths of thesolar system are not complet* gone; they leavepieces of themselves behind. Every Nomber,around the 14th, we can see dazzling meteor show-ers as Earth passes through the region of CometBella% orbit. Remnants of Halley% provide thesource for mettxx thowers called the Eta Anuarids,which appear in May. and the Orionids, which ap-rear in Octolxir. Other meteor showers occur in Au-gust (the Perseids), December (the Geminids), andin November (the Taurids). The showers are namedafter the constellations in which tlow seem to appearrather than after the comet that left the particlesbehind.

The overwhelming majority of comets are yet tobe discovered as a result of their vast distance fromus in the Dort cloud. Comet Wilson, the newest atthis writing, was discovered by a student. Profes-sionals and amateurs alike discover comets; all ittakes is a knowledge of where to look, a little luck,and a lot of patieme.

Came filsegbini-Ennet courtesy of U.S. Naval Obssivalay.

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28

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NASA's Con* Ibndezvous-Asteroid Flyby mission,or CRAF is current* plammd for bunch sometimein the 1990s. Unlike the Halley flyby rdssions, thegoal of the CRAF mission is to `rendezvous* with acomet, staying with it for hundreds of days in its or-bit. Such a mission would provide significantly morethan the data 'snapshots* returned by Um Haneyprobes. A successful GRAF mission would be ableto map the entire nuclats with high resolution imag-ery, provide a detailed history of the gm producticarate of the comet as it approached the Sun, studythe developing Interaction with the solar wind, andmeasure in a detailed way the composition of the at-mosphere. One posstie experiment would involveimplanting a four-foot-long goff-tee shaped device inthe nucleus to relay information back to Earth.

kers deptelee pi The CRAF speeeend! le MR mend a comet soden(Comet RendenoestAstentid Flyby Ilbeloa).

Although the specific comet with whkth CRAFwould rentbzvous has not yet been selected, mostmission =nabs inclutb a safe flyby of a lame to-teroid before the corm:buy rendezvous.

A second comet flyby is possibb for Gkgto, theEuropean seam probe that pissed within 300 milesof Comet Halty. Giotto survived its encounter withHalley and may now be retargeted to within severalhundral kilometers of the nucleus of Comet Grigg-Sklefferup in 1992 to get color images.

Tim ultimate and most daring mission is for an in-ternational nucleus sampb-return mission that willactually bring back to Earth a piece of a comet nu-cleus for study. Such a mission win be exceedinglyexpensive, will probably require rmw technology,and hence is not likely to occur until the twenty-firstcentury. However, a sample return mission to acomet may spawn as yet undreamed of spin-offsand scientific pplications.

Many questions about comets remain unan-swered, even as new ones hav been raised with therecent gassage of Halley's. With the continual ac-cumubtion of new data, we may be able to answersuch questions as: Why do the gas and dust fetsemerip from some regions on the nucleus awl notothers? Is the nucleus more like an ice-ball or fluffysnow? What is the composition of the surface mate-rial: and How does the comet interact with the salt.wind? We also hope to get a better undastandingabout such fundamental issues as how the solarsystem was formed, and the effect comets rdghtlave had on the evolution of life.

Perhaps the most far-reaching implications fromall the comet studies will result from the muffins-Dona effort initiated by the study of Halley. Coopera-tive space exploration means that participatingcountries will share both costs and benefits. Inter-national science will be enhanced because of thegreater complexity and greater number of missionsmade possible by this continuing cooperation.

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Titts tekn by The Ng* thltlooler Come so limb 13, 111115, et &Moss el MOOS Monsen from the modem et tenet Ilallsy,shows colors censependlag to difterest Ilidd levels. The miens Is lo Ike *per left weer sod eves the dsitest part el It Is elenlyvlstble. Tbe bright eres Is dust jet ertgliellay at the =tit area on tbe modem. Tido lost jot Is IllomIseted soslight. Photo: In PlsockWitte tor Aeresonde.

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Sources

Apfel, Necia H. Astronomy Prolects for YoungScientists. Geneseo, NY.

'A Look Into the Beginning of the Solar System: TheComet-l*ndezvous/Asterold-Plyby Mission.' NASA,Jet Propulsion Laboratory, California Institute ofTechnology, Pasarkna, CA. JPL 400-259 5/85.

' Anatomy of a Comet.' SAy & Telescope 73, (no. 3,March 1E47).

Astro: A New Obwrvatory for Ultraviolet Astronomy.22M985. NASA, Marshall Space Flight Center,Huntsville, Ala.

Branley, Franklyn. Halley: Comet 1996. E.P. Dutton,Inc., 2 Park Ave., New York, NY 10016, 1983.

Chapman, P.D. and Bondurant, R.L. Comet HNleyReturns: A Teacher's Guide, 1985-1996.Washington, DC: U.S. Government Printing Office,EP-197.

Comet Halley, The View From Pioneer Amts.Washington, DC: U.S. Government Printing Office,1986.

Comet Rendezvous Mission. NASA Release 86-150.Washington, DC: NASA Headquarters, Oct. 20,1986.

Encounters with Comet Halley: The First Results.'Nature, International Weekly JOillnal of Science 321,(no. 6067, 15-21 May, 1986).

Exploration of Weft Comet From Space I d Fromthe Ground. The Interagency Consultative Group,The International Halkry Watch, ESA, ESTC,Noordwijk, The Netherlands.

Ford, V.L. and Newell, E.B. Comet Halley OfficialHandbmA. Mt. Stromlo and Siding SpringObservatories, 1985-1986.

G8f8111 P. Kuiper Airborne Observatory. AmesResearch Center: Moffat/ Reld, CA, 1980.

'GiottoESA's Mission to Halley's Comet,' MissionSummary. EM News Retase, 8-10 Rue MarioNikis, 75738 Paris Cudex 15, May 15, 1986.

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Gore, Rick, 'Much More Than Met the Eye; Halley'sComet '86'. National Gsegraphic. (Dec. 1986).pp. 758-85.

Greenleaf, Peter. Everiments in Space Science.Arco Publishing, Inc., 219 Park Ave. S, New York,NY.

' Greeting Halley's Comet,' ME, (December 16,1985).

' International Halley Watch.' JPL Fact Sheet. NASA,Jet Propulsion Laboratory, California Institute ofTeclmotcy, Pasadena, CA.

Lenorovitz, Jeffrey M. `Europeans Propose NewComet Flyby for Giotto Spacecraft.' Avkltion Week &Spice Technolevy 125 (no. 22, Dec. 1, 1986).

Newborn, Ray L. JE and Rabe, Jurgen. °TheInternational Halley Watch.' Journal of the BritishInterplanetary Society, Vol. 37, pp. 28-31, 1984.

Reinhard, R. 'Comet HalleyAfter the HeidelbergSymposium.' ESA Bulletin. No. 49, Feb. 1987.

Schatz, Dennis. The Return of the Comet. PizificScience Center. 1985.

Siegel, Lee. 'Comet Halley Returns!". Daily Press!The Times-Herald, 7505 Warwick Blvd., NewportNews, VA 23607.

NASA. Comet Halley Summary. Jet PropulsionLaboratory, California Institute of Technology,Pasadena, CA.

NASA. The Comet Gkecobini-Zinner Handbook.Jet Propulsion Laboratory, California Institute ofTechnolog:, Pasadena, CA, Feb. 14, 1985.

NASA. The Halley Armada. JPL 40-278, rev. 210/85. Jet Propulsion Laboratory. California Instituteof Technology, Pasadena, CA.

The International Halley Watch. NASA, TM 82181,July 1980.

'What Have We Learned About Halley's Comet?'A Newsletter on Teaching Astronomy. Fall 1986.Astronomical Society of the Pacific. 1290 Ave.,San Francisco, CA.

Meal of Comet Were come sted iv Earth from thePima Venn wow& The coma, or cloud of gases

tite mins, is 20 NOW kilomelms lediametet caress Wig deasesingbrightens from the somet's seder ouhard.

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These activities are prennted to supOement section 1. Activities may be used ;is curriculum ideas or asstand-alone exercises. They are not labeled by discipline, grade, or abillty levO, and the teacher's Indivklualcreativity and ingenuity can make use of them in any area of study. The study of consts offers teachers anopportunity for an interdkciplinary approach in their classrooms. As trew comets are discovined and pre-vices ones return for subsequent visits, comets will continue to fascinate students cr; all aps.

The Ultimate lime ltavelers

Conagit By studying comets, we may learn moreabout the origin and evolution of the solar systemand Um lies between comets and otlxir objects in thecosmos

Wcabulary:

atomscluescometarycomposition

cosmos primordialeons vaporsHalleyinterplanetary

Comets are named after their discoverers, many ofwhom are amateur utronomers.Encourage studentsto become sky- watriters.The sky must be scannedon a routine basis with a good telescope or bin-oculars, in a location that is not affected by lightpollution. By consulting a sky chart, the student cansee if there is a new point of light that moves, rela-tive to the stars, over a period of several nights.This may be a comet.

Create a learning center with a telescopeillustratehow the lentos of the telescop work. Instruct stu-dents on working with a telescope, its uses, and itsimportance in the area of research astronomy.

Have students work in small groups and researchOw question: Why study comets? Have them selecta madkim other than a written report to share theirfindings (e.g., a slide show).

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Provide prompts for narrative, descriptive arxi ex-posit,/ ParegraPhe. Have Peelle write a PeratirePhon the prompt selected for study. Suggestions:

!Welke: Pretend that you have been a piece ofrock in the tail of Halley's Comet since 1910. Write aletter to astronomers telling of your adventuresthrough space since then.

DestriptiviE Pretend that you are the head of acomet. Describe yourself to a newspaper reporter.

hoesttery: You have viand Comet Halley through atelescope. Explain to your classmates how you setup your telescope and found the comet in the sky.

Until the invention of telescopes and cameras, allastronomical observations were recorded in draw-ings. Mari* for drawing can be as simple as apencil and notebook. Take your students out oneclear night to study the sky and make drawings ofwhat they see.

Robs to remember1) make your observations away from any light

pollution;2) allow your eyes Xi minutes to adapt to darkness;3) use a small Ihelftht with muted rix1 light so as

not to diminish night vision;4) give verbal descriptkins to explain features;5) inificata directions of the compass; and6) include latitude, longitude, date, time, type of

instrument used for magnification, and skyconditions.

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Uniting Humanity

Mint Throughout history, Connt Halley's appear-ances have had an impact on human behavior.

Vosabidery:

appeasement heraldedcatastrophe irrationalcelestial orientationchronicles prescribed

scimitarssynopsisvisitation

The 1910 return of Comet Halley caused excitementand fear. Have students investigate local newspaperaccounts of the concerns expressed in their commu-nity at that tin:. The month of May probably carriedthe most stories; why?

a

Have students !smirch news =punts (include ra-dio and TV) of the 1986 appurance of Comet Halleyand cormare them with those of 1910.

Have students discuss, in terms of human behaviorand psychology, why they think kenos meted irra-tonally at the appearmices of comets. Then lovethem write and act out several scenarios that mighthave Mks' pkat during a Comet Halley visitationand whkh show mob psychology and masshytaria.

The Bayeux itpestry, which depicts the Battle ofHastings, include; the 1066 ppearance of CometHalley. What kW of pm ma; it for the Saxons? forthe Normans? Have sturketts research the tapes4

Halley's Comet, a poem by IL Murray, appeared inthe Houston Montle of May 17, 1910. Have stu-dents analyze it

The tall stirs mental MchinpDiaikanous cf doitt:

It might unfttit Hs orNtsAnd knock us fellows out:

One chime WA U mNions,Tin sclettsts dears

But comets are (mowAnd dottts are everywhen

That Hall e y ia a-peepkgSomewlme to ma k e a hit

Our flesh so goes a-cretpkgKt Ono& mat Ma

What mai= that comet tkddishIs headng OM so stnVght;

In d Rs pomp wd glory.And egnlly cd stale.

He sewns to be so charyOf paths he ptmcluates

That he, to all appearancesIs heading for our gets.

Halley's History

IhIcabulary:

frescoParsa

omenplaOue

tapestrytheory

Establish an area of the classroom for a learningcenter dealing wim Comet Halley. Assign a pir ofstudents to a specific time during whitth the cometpassed. In this activity, students become the tele-er. it is the responsibility of each pair to design ma-terial, pictures, activities, etc., rebting to their as-signed time. Biweekly, a different student pair placesmaterial in the "Comet CenteE" Each group must ex-plain the center to the class at the beginning of thetwo-week `expedition." Begin with 240 B.C. and endwith 1987. This activity also can be used with otherfields of study.

Use a long rope or string for a lime line.' On sepa-rate r x 5" index cards, write the events from the'Halley's History.' Give these to the students for ad-ditional research so they can either dramatize orwrite a story about the event. Attach the card to the*time line' at the proper place.

a

Follow the deve:opment of different facets of our so-ciety through the appearano3 of Halley's Comet.Have each student take a different aspect, such asreligion, English h , art, or music, and talkabout its stage of t.; cement at each appearance.Then project the continuing changes of that particu-lar subject into the future for the next severalappearances.

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To help students become more famihar with geogra-phy, enlarge a world map to bulletin board size. Getstick pins of varying colors. Make paper strips ofthe events found in *Halley's History.* Next havestudents select a strip and read it aloud. As a class,discuss the event and its location; then, with thestick pin, attach the event strip at the correct placeon the world map. Discuss the importance of thegeographical location of each event.

Alfred Lord Tennyson's play, Hawkl, which was pub-lished in 1876, is about the Saxon king who died inthe Battle of Hastings. Have students read and dis-cuss this 19th century view of history.

Mark Twain was born In 1835 and died in 1910, twoyears of consecutive Comet Halley appearances.

Have students read A Connecticut Yankee in KingArthurt Court.

Students will find Comet Halley in Giotto's frescoThe Adoration of the Magi, in the Arena Chapel,Padua. Painted in 1303-06, it refers to the 1301appearance of the comet. Have students study thepainting and compare it to pictures of Comet Hal-ley's later appearances created in different media.

The airplane, television, and radio are three moderninventions not available in 1910. Discuss with stu-dents the changes in transportation and communi-cation since then.

Where Do Comets Come From?

Concept In an attempt to explain the origin of com-ets, many theories have been proposed.

Vocabulary:

ellipse interstellar short-periodelongated omen simulationgravitation planetoid spectroscopyinert orbit theory

Assign comet theorists to students to research; in-clude Edmund Halley, Tycho Brahe, Aristotle, FredWhipple, Isaac Newton, and Jan Van Dort. Have stu-dents role-play the person. In a news show sce-nario, each is interviewed by other students andquesticined on his theory and the evidence to sup-port it.

Tycho Brahe, a Danish astronomer, made observa-tions in 1577 that determined the actual distance tocomets. He used the principle of parallax to do this.Students can demonstrate the principle with thissimple experiment:

Hold your finger a few inches in front of your eyes.Close one eye and then the other, noting where yourfinger appears to be compared to an object in thedistance. Do the same thing with your finger held atarm's length. You will note that your finger appearsto shift position by a smaller amount when it is heldat a distance than when held close to your eyes.Your brain mentally compares the view from each of

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your eyes to °see how much nearby objects shiftcompared to what is in the background. In this wayyou can judge the distance to objects in your en-vironment. Challenge the students to relate this testto Brahe's observations of the comet's position.What would his observations have shown?

Theories are not accepted without continuous chal-lenge and testing. Divide the class into small groupsto research nisting theories on comets and assigneach theory to two groups. Using sound logic, onegroup is to argue for the theory the other against.At the end of the debates have tne entire class voteon whether the theory should stand.

Until modem times, all cultures developed mythsand legcnds to explain heavenly happenings. Havestudents research comet stories to compare andcontrast. What are the differences? What were thecultural influences that made the stories vary fromplace to place?

Imagine that you and your class are visitors toEarth. The night sky is totally strange to you and tomake logic out of nonsense you decide to inventconstellations based on myths that you know. Divideinto groups and have each group take a section ofthe sky and make constellations out of the star pat-terns, then explain what the constellation is and why.

3,1

Cocept Ccgnets are described as *dirty snowballs"consisting of a nucleus, coma. dust tail, and ion

ifesetwdery:

&meteernbeddulfluorescencemicron

molecules thermalmdes ultravioletnucleusphenomenon

To help students get a better idea of the size of acomet and how the size of its parts compare, have

them build a model of a cond. Divide the class intoteens and have each team work on a different partof the model. Let the students determine the mate-rials and size to be used. A good mathematics exer-

cise is to let them do the calculations to determinethe appropriate scale to use.

DiSCUS3 the different types of tails on a comet plas-mamagnetic fluorescence; dust tailecatteredsunlight. Draw a diagram of a comet circling theSun. Consider which direction these tails would goat various locations on the cornett orbit.

Have students draw a landscape that represents howthe surface of a comet would look. After discussing'The Halley Fleet,' have them draw a second picturebased on information gained from the Instrumentsof the several spacecraft. Consider the two pictures:how am they similar? different?

The Halley Fleet

Concept: Many nations of the world worked togethersharing space-based technologies to provide an un-precedented study of Halley'S Comet.

Vocabulary:

archive magnetosphere terrestrial

armada orbital plane trajectoryflotilla perihelioninfrared retrograde

Make a list of the instruments built by each countryto study comets. Divide the students into groups,one for each country. Have them research why eachinstrument was chosen. Then have them includetheir own ideas on how and what they would havestudied and the rationale behind each selection.

Based on spacecraft findings, the nucleus of HalleytComet has been compared to a peanut or a potato in

shape. Choose one of those to use as your comet,and design a model of all the parts of the entirecomet in the proper sade. Make the coma, hydro-gen cloud, ion tail, and dust tail, so students canget an understanding about how tiny the nucleusitself is.

Ask students to write a story about Comet Halley,tying fact into fiction; have them tell of their journeythrough space as comets, how different spacecraftlooked to them, what they imagine the spacecraft tobe, their purposes, the beings that sent them. Havesome stories rewritten as a play, film, radio, or TV

script.

Have students research other international space as-tronomy projects. Ask them to project ideas for fu-ture cooperative ventures.

Through the Halls of Time

Concept Comets may have altered the course ofhistory

Vocabalarr

carboncontroversyCretaceous

ecological nicheextraterrestrialingredients

postulateprojectileTertiary

Have students brainstorm a list of exfinct animalsand birds. Divide them into groups and have eachtheorize a possibility for the mass extinctions. Havestudents write scenarios on how life might haveevolved if the extinctions had not occurred.

The geneology of Halley's parallels our own ances-try. An interesting approach to our own ancestral

roots is to trace family histories back to each ap-pearance of Halley's or at different points in its last76-year orbit. For example: where were you (yourparents, your grandparents, etc.) and what wereyou doing when Halley's crossed the orbit of Jupitgr,Saturn, Pluto, etc., on its way to Earth? Where wasyour family during the apparitions of 1910, 1835,1759, and so on? Project the questions into thefuture.

The dating of Halley's appearance, past and future,can be used as a springboard for dating techniquesof all kinds. What are they? How do they work?What kinds of things can wzr date with them? Whenis a particular kind of dating used? Why date thingsat all?

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Other Heavenly Wayfarers

conapt Many comets have appeared to humansthrough the ages. New comets are being discoveredall the time.

Vosalnderr

concentric influential perpendicularcorttellation meteor showers remnant

Take your students out on a night when a meteorshower is predicted to occur. Have them count thenumber of °falling stars' they see trier a period oftime and then find the average. Do this for some ofthe most prominent showers. Keep graphs and ta-bles of the findings.

Every comet is inchvidual. Each travels at a differentspeed and has a unison orbit. Once a comet is dis-covered, scientists can calculate these propertiesand tell us when to expect its return. Have studentsresearch the orbital speeds of known comets. Trans-late these into mach speeds (the speed of sound).Students should aW be able to graph the orbits inkilometers and miles per hour.

Have a poster contest to represent a specific corn-ett aPPearance.

Create a class tapestry on the solar system withspecial emphasis on comets.

Have students view visual art and/or listen to musi-cal compositions that have a celestial or planetary ti-tle and discuss the connection, if ant with the skyand its objects:

Music SymAony No. 41 ('Jupiter) by Mozart(1756-1791) The Planets by Hatt (1874-1934)

Art Saturn Devouring His Children by Goya(1746-1828) Starry Night by Van Gogh(1853-1990)

Have students research the variety of artists' repre-sentations of comets through the ages: manuscriptilluminations (Les Tres Riches Hewes du Duc deBarry). Durer's Melencolla I, Hogarth andRowlandson in Englarkl, Daumier in France, NativeAmericans, decorative arts, 20th century painters.

Comets are often mentioned in literature. Have stu-dents find refermices in lArgil, Shakespeare, JulesVerne, and H.G. Wells.

Touching the Future

conept Other comets will be studied more closelyin the future because of the continuous advance-ments in science and technology.

Vocabulary:

applications implications multinationalenhanced interntion rendezvous

Have students research the lunar landings and theactivities that wire carried out on the Moon by as-tronauts at one-sixth of Earths gravity. Then havethern imagine how a similar spneship would land'on a comet nucleus that has less gravity. Rave themdesign the equipment that would be necessary forsomeone to walk, drive, dig, etc., on an almostgravity-free surface.

Measured ir 'Halleys,* the next appearance of thecomet will be Halley 31 in 2062. Divide the classinto groups and have them descrilm civilization asthey think it will be then. Bring in such componentsas modes of transportation, medical advances,communicattris methods, forms of recreation,businesses such as grocery stores and banks, sothat they all fit logically into an integrated society.

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Have students brainstorm what the sociological is-sues might be in 2062. Have them speculate aboutaverage life span, urban development, role of theUnited States in world affairs, status of minoritiesand women in society, population charzteristics,and major politicid, economic, and/or social issues.

Role play the media in the year 2082. Group stu-dents in small numbers and have each group be aparticular kind of medium. Nave them `cover thereturn of Halley's Comet.

Prepare time capsules. Have the whole class discussand justly how many itms, what kind, where tostore the capsules, the directions to pirme withthem, and Wm to have them opened. In each indi-vidualt capsub3 should be a description of life in thepresent a tape of a favorite song, a picture ofoneself, family, home, school, cig pats, and so on.Whenever a new cornet is discovered, encouragethem to uld new items to the box that are descrip-tive of their Me at that time. Be sum articles aboutthe new cornet are included. Label the boxes andhave the students put them away for safe-keeping toshare with children and grandchildren whenevercomets ppear over the years.

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NASA 'Thacker Resource Centers

Located at the nine NASA research centers, Teacher Resource Centeis (TRCs) have a variety of NASA-relatededucational materials in several formats: videotaps, slides, audiotapes, publications, lesson plans, and ac-tivities. NASA educational materials are available to be copied at the TRCs. Conte the nearest TRC for fur-thet Inftwination.

Abbess Wu and 'WM CenterNASA Thacher Resource RoomTranquility BaseHuntsville, AL 35807Sines Abbama, Arbnsas,Lodsiana, Missouri, and Tennessee.

USA Ames Research CenterTeacher Resource CenterMail Stop 204-7Moffett Field, CA 94035Sines Nash, Arizona, California, Hawaii,ldsho, Montana, Nevada, Oregon, Utah,Washington, and Wyoming.

Gerhierd Spurr Flight CenterTeacher Resource LaboratoryMail Stop 130.3Granbelt, MD 20771Serves Connectiort, Delaviare, District ot Columbia,Main, Maryland, Massachusetts, New Hampshire,New Jersey New York, Pennsylvanla, Rhode Island,and Vermont.

NASA Jet Preinikdon LaboratoryTegeher Resource CenterJPt Educational OutreachMail Stop CS-530Pasadena, CA 91109Senes inquiries related to space exploration andother JPL xtivities.

NASA Lyndon B. Ahnsen Space CenterTeacher Resource CenterMali Stop AP-4Houston, TX 77058Sows Colorado, Kansas, Nebraska, New Mexico,Nord; Dakota, Oklahoma, South Dakota, and Teas.

NASA John F. Kennedy Space CenterEducator Resource LibraryMall Stop ERLKennedy Spice Center, FL 32899Serves Aria Georgia, Puerto Rico, andthe rwgin Iskinds.

NASA Langley Research CenterTeacher Resource CenterMall Stop 146Hampton, VA 23665-5225SIMS Kentudy, North Carolina, South Carolina,VirOme, and West Virginia.

NASA Lewis Research CaboTeacher 1;knource CenterkW Stop 8-1Cleveland, OH 44135Sem Owls, Indiana, Michigan, Minnesota, Ohio,and Wisconsin.

NASA John C. Shunts Space CenterTeacher Resource CenterBuilding 1200Stennis Space Center, MS 39529Serves Mississippi.

Central Operation of Resources for Educators (CORE)Designed for the national and international distribution of aerospace educational materials to enhance theNASA Teicha Resource Center Network. CORE provides educators with another source for NASA education-al audiovisual materials. CORE will process teacher requests by mail for a minimal fee. On school letter-head, educators can write for a catalogue and order form and send to:

Ms. Tina SalyerNASA CORELorain County Joint Vocational School15181 Route 58 SouthOberlin, OH 44074

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