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\f1\fs20\lang1033\langfe1033\cgrid\langnp1033\langfenp1033 {\b\fs36\cf9 Cometa Halley
\par }\pard \s15\qj \li0\ri0\sl360\slmult1\widctlpar\aspalpha\aspnum\faauto\adjustright\rin0\lin0\itap0 {\b 
\par Cometa Halley}{
, este poate cea mai faimoasa dintre toate cometele, care reapare aproximativ la fiecare 76 de ani. Se stie ca a fost vazuta de aproximativ 30 de ori, din 240 i.Hr. pina in 1986. urmatoarea sa aparitie este programata pentru 2061. (Aceste date se refera l
a
 momentul cind cometa a precut la periheliu, punctul unde orbita este cea mai aproape de soare. Chiar daca ultimele aparitii au fost dezamagitoare, datorate unor pozitii nefovorabile ale cometei fata de soare si Terra, ea ramine una dintre cele mai active
 si mai mari comete cunoscute.
\par }\pard\plain \qj \li0\ri0\sl360\slmult1\widctlpar\aspalpha\aspnum\faauto\adjustright\rin0\lin0\itap0 \fs24\lang1033\langfe1033\cgrid\langnp1033\langfenp1033 {\f1\fs20 
\par Poate cea mai buna aparitie, a fost in 837, cind cometa a aparut brusc din directia soarelui si s-a apropiat de Pamint, la o distanta minima de mai putin de 6 milioane de Km. O aparitie faimoasa a fost aceea din 1066, care ch
iar daca a purtat ghinion regelui englez Harold, in batalia de la Hastings, a fost de bun augur, invadatorului normand, Wilhelm. Sosirea cometei in acel an a fost imortalizata pentru posteritate pe tapiseria de la Bayeux.
\par 
\par {\listtext\pard\plain\f14 \loch\af14\dbch\af0\hich\f14 \'76\tab}}\pard \qj \fi-360\li720\ri0\sl360\slmult1\widctlpar\jclisttab\tx720\aspalpha\aspnum\faauto\ls2\adjustright\rin0\lin720\itap0 {\b\f1 Munca lui Halley}{\f1\fs20 
\par }\pard \qj \li0\ri0\sl360\slmult1\widctlpar\aspalpha\aspnum\faauto\adjustright\rin0\lin0\itap0 {\f1\fs20 Cometa a fost 
numita dupa astronomul englez Edmund Halley. Teoria gravitatiei dezvoltata de Issac Newton, a reprezentat un element cheie in descoperirea lui Halley, si anume aceea ca, cometa stralucitoare care a aparut in 1682, avea o orbita care o aducea in interiorul
sistemului solar, la intervale regulate. Mai inainte, fusesera trei supozitii privind traictoria cometelor: ca erau linii drepte, parabole ascutite, sau elipse elongate. Chiar daca in final s-a stabilit o orbita parabolica pentru cele mai multe comete, Ha
lley elocvent in recunoasterea ca alternativa eliptica se potrivea cu observatiile facute asupra cometei din 1682.
\par 
\par Aceasta solutie a fost sustinuta de pozitiile cometei, observate la aparitiile din 1456, 1531 si 1607, iar Halley a fost primul care a sustin
ut ca acele comete aparent diferite, erau defapt uan si aceeasi. Mai mult, aplicind teoria gravitationala a lui Newton, si luind in calcul efectele gravitationale ale lui Jupiter si Saturn, Halley a reusit sa stabileasca cu succes, data aproximativa a urm
atoarei reintoarceri a cometei. Aceasta a fost revazuta la sfirsitul lui 1758.
\par 
\par {\listtext\pard\plain\f14 \loch\af14\dbch\af0\hich\f14 \'76\tab}}\pard \qj \fi-360\li720\ri0\sl360\slmult1\widctlpar\jclisttab\tx720\aspalpha\aspnum\faauto\ls1\adjustright\rin0\lin720\itap0 {\b\f1 Structura cometei}{\f1\fs20 
\par }\pard \qj \li0\ri0\sl360\slmult1\widctlpar\aspalpha\aspnum\faauto\adjustright\rin0\lin0\itap0 {\f1\fs20 
Nucleul central al cometei, format din praf si gheata, care este citeodata descris ca fiind un bulgare de zapada prafuit, este rezervorul activitatii cometar
e si sursa imenselor cantitati de material, injectate in interiorul sistemului solar, la fiecare trecere succesiva pe la periheliu.
\par 
\par Gazele si praful sunt mai intii eliminate in capul cometei, spatiul din jurul nucleului. Apoi ele curg spre spatele cometei
, pentru a forma 2 tipuri de coada: o coada lunga si luminoasa formata prin interactiunea atomilor incarcati si moleculelor, cu particulele ce formeaza vintul solar, si a coada curba si mai difuza (produsa de reactia radiatiilor solare cu particulele mici
 de praf eliminate de nucleu). Comparata cu nucleul cometei, care la Halley este de cca. 10 km, spatiul din jurul acestuia este enorm, ajungind adesea la 1 mil. Km in diametru. Cozile cometelor se intind pe zeci de milioane de kilometri.
\par 
\par }\pard\plain \s15\qj \li0\ri0\sl360\slmult1\widctlpar\aspalpha\aspnum\faauto\adjustright\rin0\lin0\itap0 \f1\fs20\lang1033\langfe1033\cgrid\langnp1033\langfenp1033 {Citeva tari au tri
mis misiuni spatiale le rendez-vous-ul cu cometa Halley din 1986, culminind cu fly-by-ul lui Giotto, o sonda spatiala lansata de Agentia Europeana Spatiala la 2 iulie 1985, care a trecut pe linga cometa cu o viteza de aproximativ 70 km/s la 03:02 GMT, in 
d
imineata lui 14 Martie, 1986. Apropiindu-se de cometa la o distanta minima de 596 de kilometri, sonda a furnizat imagini nemaivazute ale nucleului, prezentindu0l pe acesta ca fiind un obiect intunecat, de 2 ori mai lung decit lat, cu jeturi active, tisnin
d din mai multe locuri de pe fata incalzita a nucleului, dinspre soare. Nucleul este un corp in forma de aluna sau cartof cu dimensiunule de 16 pe 8 pe 8 km.
\par }\pard\plain \qj \li0\ri0\sl360\slmult1\widctlpar\aspalpha\aspnum\faauto\adjustright\rin0\lin0\itap0 \fs24\lang1033\langfe1033\cgrid\langnp1033\langfenp1033 {\f1\fs20 
\par Nucleul cometei se crede a fi la fel de batrin ca si sistemul solar (cca. 4,6 miliarde de ani), si
 a se fi format printr-un proces de acumulare in partea de la exterior a sistemului solar aproximativ in acelasi timp si loc ca si planetele exterioare. Perturbatii ale altor planete au trimis cometa nou-formata pe orbite din ce in ce mai mari, pina cind 
distanta apheliana a devenit asa mare, incit perturbatiile externe au rotunjit orbita si au plasat-o in relativa siguranta a norului Oort \endash 
 un vast roi de comete din apropiere botezat dupa numele astronomului olandez Jan Hendrik Oort, care se intinde de dincolo de cea mai indepartata planeta pina la cea mai apropiata stea.
\par 
\par Aici in ceea ce a fost numit \'93inghetul profund\'94 al soarelui, cometa a incercuit soarele, cu o perioada orbitala de mai multe milioane de ani, pina cind alte perturbatii au inpins-o pe o o
rbita cu distanta de periheliu mica, aducind-o din nou sub influenta gravitationala a lui Jupiter si a celorlalte planete, aceste perturbatii planetare au adus cometa pe actuala ei orbita. Pierzind cam 100 milioane de tone de praf si gheata la fiecare tre
cere pe la periheliu, sansele sale de de supravietuire sunt slabe
\par 
\par Daca cometa va muri cu un strigat sau cu un oftat, asta inca nu se stie. Multe comete aflate pe orbite similare mor in coliziuni cu soarele, in timp ce altele sunt ejectate din sistemul sola
r; citeva se ciocnesc cu planete, citeodata cu Terra. Coliziunile corpurilor cosmice cu planetele au profunde implicatii, in intelegerea formarii vietii pe Terra si a evolutiei acesteia, dar si in formarea si evolutia pe termen lung a celorlalte planete d
in sistemul solar. 
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