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Is-sistema solari

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Sistema Solari tagħna bi Sommarju GlanceInformation
PMS 010-A (JPL)
Ġunju 1991

JPL 410-34-1 6/91
(Updated 5/93)

NASA
Aeronawtika Nazzjonali u l-Ispazju Amministrazzjoni

Laboratorju Propulsjoni Jet
California Institute of Technology
Pasadena, California

Għal kopja stampata ta 'din il-pubblikazzjoni tikkuntattja l-email Pubbliku
uffiċċju fiċ-ċentru NASA fir-reġjun ġeografiku tiegħek.

INTRODUZZJONI

Mill-dinja żgħar tagħna għandna gazed fuq l-oċean kożmika għall
ramel thou untold ta 'snin. Astronomers Ancient osservati punti ta '
Light li dehru li jiċċaqalqu Fost il-Stars. Huma talbu Dawn
oġġetti pjaneti, jiġifieri wanderers, u indikhom wara Ruman
deities - Jupiter, king 'l-allat; Mars, l-alla tal-gwerra;
Merkurju, messaġġier tal-allat; Venus, l-alla tal-imħabba u l-ġmiel,
u Saturnu, missier ta 'Ġove u alla tal-agrikoltura. Il
Wkoll stargazers osservati comets ma dnub frizzanti, u meteors
jew isparar istilel apparentament jaqgħu mis-sema.

Xjenza iffjorixxiet Matul ir-Rinaxximent Ewropea.
Liġijiet fiżiċi fundamentali li jirregolaw mozzjoni planetarju kienu
skoperti, u l-se jduru tal-pjaneti madwar ix-Xemx kienu
ikkalkulat. Fis-seklu 17, Astronomers osservat mezz ġdid
imsejjaħ il-Teleskopju fil-smewwiet u għamel startling
skoperti.

Iżda l-snin mill-1959 kienu jammontaw għal età tad-deheb ta '
Sistema esplorazzjoni solari. Avvanzi fit rocketry wara Gwerra Dinjija
II ppermettiet magni tagħna li jiksru l-qabda tal-gravità tad-Dinja u
ivvjaġġar lejn il-Qamar u biex pjaneti l-oħra.

L-Istati Uniti tard vetturi spazjali awtomatizzati, allura bniedem
expeditions ekwipaġġati, biex tesplora l-Qamar. Magni awtomatiċi tagħna
jkunu orbited u żbarkati fit Venus u Mars; Esplorat il-Sun
ambjent; comets osservati, u magħmula stħarriġ fuq firxa qrib filwaqt li
li jtajru passat Merkurju, Ġove, Saturnu, Uranus u Neptune.

Dawn Vjaġġaturi ġab qabża kbira fl-għarfien tagħna u
fehim tas-sistema solari. Permezz tal-vista elettroniċi
u oħra "sensi" ta 'vettura spazjali awtomatizzati tagħna, kulur u
jbiddel havebeen mogħtija lill Dinjiet Li għal sekli sħaħ deher
għall-għajnejn Dinja marbuta huma diskijiet fuzzy jew punti indistinct ta 'dawl.
U għexieren ta 'oġġetti preċedentement mhux magħrufa ġew skoperti.

Istoriċi futuri x'aktarx se tara Dawn it-titjiriet pijunier
permezz tas-sistema solari bħala wħud mill-aktar notevoli
Kisbiet tas-seklu 20.

Inġenji spazjali AUTOMATED

Il Aeronawtika u l-Ispazju Amministrazzjoni Nazzjonali (s NASA)
vettura spazjali awtomatizzati għall-esplorazzjoni sistema solari jaqgħu fil-ħafna
forom u daqsijiet. Filwaqt li Huma maħsuba biex jissodisfaw separata u
għanijiet speċifikati missjoni approċċ s-sehem inġenju bosta affarijiet komuni.

Kull vettura spazjali komposta minn diversi strumenti xjentifiċi
magħżula għal missjoni partikolari, sostnuta minn subsistemi bażiċi
għall-enerġija elettrika, trajettorja u l-kontroll orjentazzjoni, kif ukoll
kif għall-ipproċessar tad-data u komunikazzjoni mal-Dinja.

Enerġija elettrika hija meħtieġa biex iħaddem il-vettura spazjali
strumenti u sistemi. NASA użi bothering enerġija solari mill-arrays
ta 'ċelluli fotovoltajċi u ġeneraturi nukleari żgħar għall-enerġija tagħha
Sistema ta 'emissjoni solari. Batteriji rikarikabbli huma impjegati għal
backup u kurrent supplimentari.

Immaġina li vettura spazjali HAS journeyed b'suċċess miljuni
ta 'mili permezz ta' spazju itiru, iżda darba wieħed qrib pjaneta, biss biex
għandhom kameras tagħha u strumenti sensing oħra indikaw il ħażina
mod kif veloċitajiet passat il-mira! Biex tgħin evitati Tali inċident, a
subsistema ta 'iskruni żgħar huwa użat biex jikkontrolla vettura spazjali.

Il-iskruni huma marbuta ma 'apparati li jistgħu jżommu
gaze kostanti fil istilel magħżula. Just kif baħħara kmieni tad-Dinja
użati l-istilel biex jinnaviga-oċeani, l-istilel użu vetturi spazjali biex
Iżommu bearings tagħhom fl-ispazju. Mas-subsistema msakkra fuq
punti fissi ta 'referenza, kontrolluri tat-titjira jista' jżomm
strumenti xjentifiċi vetturi spazjali tal mfakkar fil-ġisem fil-mira u
Antenni komunikazzjoni l-Inġenji tal osservat lejn Dinja. Il
Iskruni jistgħu eżaġerat jintużaw biex tirfina l-passaġġ tat-titjira u l-veloċità
tal-vettura spazjali li jiżgura li korp mira wieħed jiltaqa fil
id-distanza ippjanata u fuq il-trajettorja korretta.

Bejn 1959 u 1971, vetturi spazjali NASA ġew mibgħuta
tistudja l-Qamar u l-ambjent tax-xemx; Huma wkoll l-skannjat
pjaneta interni li mhumiex Dinja - Merkurju, Venere u Marte. Dawn
Tliet Dinjiet, u tagħna stess, huma magħrufa bħala l-pjaneti terrestri
Minħabba Huma jaqsmu kompożizzjoni solidu blat.

Għall-missjonijiet kmieni tkixxif planetarju, NASA
impjegati serje ta 'suċċess kbir tal-vettura spazjali imsejjaħ il-
Baħħara. Titjiriet tagħhom ta kontribut fid-ippjanar tal lateralis
missjonijiet. Bejn 1962 u 1975, seba 'missjonijiet Mariner Imwettaq
L-ewwel stħarriġ tal-ġirien tagħna tal-pjaneti fl-ispazju.

Kollha tal-Baħħara użati pannelli solari bħala qawwa primarja tagħhom
sors. L-ewwel u l-aħħar verżjonijiet tal-vettura spazjali kellhom żewġ
ġwienaħ koperti ma 'ċelluli fotovoltajċi. Baħħara oħra kienu
mgħammra ma 'erba' pannelli solari Estensjoni fromtheir ottagonali
bodysuits.

Għalkemm il-Baħħara varjaw mill-Mariner 2 Venus
vettura spazjali, li jiżnu 203 kilogramma gramma (liri 447), il-
Mariner 9 Marzu Orbiter, jiżnu 974 kilogramm gramma (2.147
liri) disinn bażiku tagħhom baqgħet pjuttost simili madwar l-
programmi. Il-vettura spazjali Mariner 5 Venere, per eżempju, kellhom
oriġinarjament kien backup għall-Mariner 4 Mar flyby. Il Mariner
10 vettura spazjali tard għall Venus u Merkurju użat komponenti mħollija
mill-programm ta 'Marzu Orbiter Mariner 9.

Fl-1972, NASA Imniedi Pioneer 10, 'vettura spazjali Jupiter.
Interess kien ċaqliq erbgħa mill-pjaneti barra - Jupiter,
Saturnu, Uranus u Neptune - blalen ġgant ta 'gass dens pjuttost
differenti mill-dinjiet terrestri kellna diġà mistħarrġa.

Erba NASA vettura spazjali kollha - żewġ Pioneers u żewġ Voyagers -
kienu tard fl-1970 li jżuru r-reġjuni ta 'barra tal solari tagħna
sistema. Minħabba d-distanza involuta, Dawn ħa Vjaġġaturi
Kullimkien minn 20-il xahar sa 12-il sena biex jilħaq id-destinazzjoni tagħhom.
Preskrizzjoni vettura spazjali aktar mgħaġġla, dawn se jsiru eventwalment l-ewwel
artifacts umani għall-vjaġġ lejn stilel bogħod. Minħabba li l-Sun
Light Isir hekk ħażin fis-sistema solari barra, dawn il-vjaġġaturi
ma jużawx enerġija solari iżda minflok joperaw fuq l-elettriku
iġġenerat mill-sħana mill-tħassir tar-radjuisotopi.

NASA eżaġerat żviluppati vetturi spazjali speċjalizzata ħafna biex jiġu rriveduti
ġirien tagħna f'Marzu u Venere fl-1970 nofs u tard. Twin
Viking Landers kienu mgħammra biex iservu bħala sismiċi u tat-temp
istazzjonijiet u bħala laboratorji bijoloġija. Żewġ orbiters avvanzati -
dixxendenti ta 'l-inġenju Mariner - l-Viking Landers minn mwettqa
Dinja u mbagħad studjat karatteristiċi Martian minn fuq.

Żewġ forma drum vettura spazjali Pioneer żar Venus fl-1978 Il-
Pioneer Venus Orbiter kienet mgħammra bi strument tar-radar Li
ppermettewlha li "tara" permezz tal-pjaneta dens sħaba kopertura għal
Karatteristiċi tal-wiċċ istudju. Il Pioneer Venus Multiprobe mwettqa ħidma ta 'erba
Li sondi kienu waqgħu permezz ta 'sħab. Il-sondi u l
korp ewlieni - li kollha li kien fiha Xjentifiku Strumenti -
radioed informazzjoni dwar atmosfera tal-pjaneta Matul tagħhom
Inżul lejn il-wiċċ.

Ġenerazzjoni ġdida ta 'vettura spazjali awtomatizzati - inkluż
Magellan, Galileo, Ulysses, Mars Osservatur u Cassini - qed
żviluppati u tard out fis-sistema solari li jagħmlu dettaljata
Eżamijiet thatwill jiżdied il-fehim tagħna ta 'tagħna
viċinat u pjaneta tagħna stess.

IL SUN

A diskussjoni ta 'l-oġġetti fis-sistema solari għandha boot
Bil Il Sun. The Sun dwarfs korpi oħra, li jirrappreżenta
Madwar 99.86 mija ta 'kull massa fis-sistema solari;
Kollha tal-pjaneti, moons, jaħbtu mad, comets, trab u gass jammontaw
biss għal madwar 0:14 fil-mija. Dan jirrappreżenta l--mija 00:14
materjali jibqalek mill formazzjoni tax-xemx. Mija u disgħa
Earths tkun meħtieġa sabiex jiffittjaw mat madwar disk-Xemx, u tiegħu
interior jista 'jkollhom aktar minn 1.3 miljun kienu fuħħar.

Bħala star, l-enerġija Sun generatesHTML permezz tal-proċess ta '
fużjoni. It-temperatura fil-qalba tax-xemx huwa ta '15 miljun grad
Celsius (27 miljun grad Fahrenheit), u l-pressjoni hemm
340 biljun drabi pressjoni ta 'arja Dinja fuq livell tal-baħar. Il Sun
temperatura tal-wiċċ tal 5,500 gradi Celsius (10,000 gradi
Fahrenheit) Jidher kważi chilly Meta mqabbla ma 'temperatura ġewwinija tagħha.
Fil-qalba tax-xemx, l-idroġenu jista fjus fis elju, jipproduċu
Enerġija. The Sun Ukoll tipproduċi kamp manjetiku qawwija u nixxigħat
tal partiċelli ċċarġjati, kemm estensjoni lil hinn mill-pjaneta.

The Sun tidher li kienet attiva għal 4.6 biljun sena u
ADOTTAT biżżejjed fjuwil li jmorru fuq għal ħames biljun snin oħra jew hekk. Fuq
tmiem il-ħajja tiegħu,-Xemx se tibda fjus elju fis
elementi itqal u jibdew jintefħu up, finalment tikber hekk
kbir li wont tibla Dinja. Wara biljun snin bħala "aħmar
ġgant, "f'daqqa waħda se kollass fis-" nanu abjad "- Il-Finali
prodott finali ta 'stilla bħal tagħna. Huwa jista 'jieħu triljun snin sa
jibred off kompletament.

Ħafna vettura spazjali jkunu Analizzati ambjent tax-xemx, iżda ħadd
gotten xi eqreb lejn il-wiċċ tagħha minn vetturi bil Madwar
terzi tad-distanza mill-Dinja għall-Sun. Pioneer 11/05, il-
Pioneer Venus Orbiter, Voyagers 1 u 2 u vetturi spazjali oħra jkollhom
Kollha kampjun l-ambjent tax-xemx. -Vettura spazjali Ulysses,
Imniedi fuq Ottubru 6, 1990, hija missjoni solari konġunta ta 'NASA u
l-Aġenzija Spazjali Ewropea. 8 ta 'Frar, 1992, Ulysses tellgħu qrib
Jupiter u użati gravità Jupiter biex hurl l-isfel taħt il-
pjan tal-pjaneta. Għalkemm xorta se jkun f'distanza kbira
mill-Xemx, Ulysses se jtir fuq reġjuni polari tax-xemx Matul
1994 u l-1995 u se twettaq firxa wiesgħa ta 'studji li jużaw disa
abbord strumenti xjentifiċi.

Aħna fortunati li x-xemx huwa eżattament l-mod li bih. Jekk IT
kienu differenti fi kważi kull mod, il-ħajja kważi ċertament
qatt żviluppaw fid-Dinja.

MERKURJU

Ksib l-ewwel-close-up fehmiet tal Merkurju kien il-primarja
Għan tal-vettura spazjali Mariner 10, Imniedi Novembru 3,
1973, mill-Ispazju Center Kennedy fi Florida. Wara vjaġġ ta '
kważi ħames xhur, li kienet tinkludi flyby ta 'Venere, l-
vettura spazjali għadda fi ħdan 703 kilometru (437 mil) ta 'l-enerġija solari
Pjaneta ġewwanett sistema ta 'Marzu 29, fl-1974.

Sa Mariner 10, ftit kien magħruf dwar il-Merkurju. Anki l-
aqwa opinjonijiet teleskopiku mill-Dinja Wera Merkurju bħala indistinct
Joġġezzjonaw nieqsa kull dettall tal-wiċċ. Il-pjaneta hija tant qrib il-
Sun li huwa Normalment mitlufa fil dija tax-xemx. Meta l-pjan huwa
viżibbli fuq orizzont Dinja eżatt wara l-estinzjoni jew qabel dawn, huwa
mgħottija mill-ċpar u trab fil-atmosfera tagħna. Radar biss
teleskopji taw xi ħjiel ta 'kundizzjonijiet tal-wiċċ Merkurju qabel it
Il Vjaġġ ta 10 Mariner

Ir-ritratti tal-10 Mariner radioed lura lill-Dinja żvelat
qedem, wiċċ ħafna cratered, kompetittiv jixbħu tagħna stess
Moon. L-istampi Wkoll Wera irdum enormi crisscrossing l
pjan. Dawn Meta interior Merkurju apparentament ġew maħluqa
mkessħa u naqas, buckling qoxra tal-pjaneta. Il-irdum huma kif
għoli kif 3 kilometri (2 mili) u sakemm 500 kilometru (310
mil).

Strumenti dwar Mariner 10 skoprew Merkurju li għandha dgħajfa
kamp manjetiku u traċċa ta atmosfera - 1/1000000000000 l
densità tal-atmosfera tad-Dinja u Komposta prinċipalment f'dan argon, neon,
u elju. Meta orbita tal-pjaneta tikkonsidraha eqreb għall-Sun,
Temperaturi tal-wiċċ jvarjaw minn 467 gradi Celsius (872 gradi
Fahrenheit) fuq in-naħa sunlit Merkurju biex -183 gradi Celsius (-298
grad Fahrenheit) fuq in-naħa dlam. Din il-firxa fil-wiċċ
temperatura - 650 gradi Celsius (1,170 grad Fahrenheit) - Silġ
L-akbar għal korp wieħed fis-sistema solari. Merkurju
litteralment bakes u tiffriża fl-Sametime.

Ijiem u ljieli huma twal dwar il-Merkurju. Il-kombinazzjoni ta '
Slow rotazzjoni relattiva għall-Stars (59 ġranet Dinja) u rapidu
rivoluzzjoni madwar ix-Xemx (88 jum tad-Dinja) ifisser li wieħed Merkurju
jum tax-xemx jieħu 176 ġranet Dinja jew sentejn Merkurju - il-ħin li
jieħu l-pjaneta ġewwanett biex jimtlew żewġ orbiti madwar ix-Xemx!

Merkurju Jidher li jkollhom qoxra ta rocklike silikat dawl
Dak ta 'Earth. Xjentisti jemmnu Merkurju Ikollu heavy b'ħafna ħadid
qalba li jiffurmaw ftit inqas minn nofs il-volum tagħha. Li excellant
Qalba raġel helpfull Merkurju, proporzjonalment, minn qalba tal-qamar
Dawk jew ta 'kwalunkwe pjaneti.

Wara l-jiltaqgħu inizjali Merkurju, Mariner 10 magħmula tnejn
Flybys addizzjonali - 21 ta 'Settembru, 1974, u 16 Mar 1975 -
qabel jikkontrolla l-gass użat biex jorjentaw il-vettura spazjali ġiet eżawrita u
Il-missjoni ġie konkluż. Kull flyby seħħet fl-istess lokali
Merkurju żmien meta l-nofs Identiku tal-pjan kien
illuminata; bħala riżultat, aħna xorta ma bbenefikawx nofs tal-
wiċċ tal-pjaneta.

VENUS

Mgħotti minn kopertura tas-sħab dens, Ġove - planetarju eqreb tagħna
proxxmu - kien l-ewwel pjaneta li jiġu esplorati. Il Mariner 2
vettura spazjali, Imniedi fuq Awissu 27, 1962, kienet l-ewwel ta 'aktar
minn tużżana missjonijiet Amerikani u Sovjetika suċċess biex tistudja l-
pjan misterjuża. Kif vettura spazjali tellgħu minn jew orbited Venus, mitfugħin
fl-atmosfera jew bil-mod żbarkati fuq wiċċ Venere ", romantic
miti u l-ispekulazzjonijiet dwar proxxmu tagħna kienu stabbiliti għall-mistrieħ.

14 ta 'Diċembru, 1962, Mariner 2 għadda fi ħdan 34.839
kilometru (21.648 mil) ta 'Venere u sar l-ewwel vettura spazjali
jiġu skennjati pjan ieħor; Strumenti abbord imkejla Venus għal 42
minuti. Mariner 5, Imnedija f'Ġunju 1967, tellgħu ħafna eqreb lejn il-
pjan. Tgħaddi fi ħdan 4.094 kilometru (2.544 mil) ta 'Venere fuq
It-Tieni flyby Amerikana, strumenti Mariner 5 tal mkejla l-
qasam manjetiċi, ionosphere, ċinturini radjazzjoni tal-pjaneta u
ment temperaturi. Fi triqtu lejn Merkurju, Mariner 10 tellgħu mill Venus u
Stampi ultravjola trasmessi lill Dinja li juru l-sħaba
Mudelli ċirkolazzjoni fl-atmosfera Venusian.

Fir-rebbiegħa u fis-sajf tal-1978, żewġ vetturi spazjali kienu
Imniedi Wara jisfaxxa l-misteri tal Venere. Fuq 4 Diċembru
tal-istess sena, il-Pioneer Venus Orbiter sar l-ewwel
vettura spazjali friendliness fl-orbita madwar il-pjaneta.

Ħamest ijiem wara, il-ħames komponenti separati li jiffurmaw il-
tieni vettura spazjali - il-Pioneer Venere Multiprobe - daħlu fil-
Atmosfera Venusian f'postijiet differenti fuq mill-pjaneta. Il
Erba żgħar, sondi indipendenti u l-korp ewlieni radioed
data atmosferiċi lura lill-Dinja Matul dixxendenza tagħhom lejn il-
wiċċ. Għalkemm Iddisinjati biex jeżamina l-atmosfera, wieħed mill-
sondi baqgħu ħajjin l-impatt tagħha mal-wiċċ u Kompliet
jittrasmetti data għal siegħa oħra.

Venere tixbaħ Dinja fid-daqs, il-kompożizzjoni fiżika u
Densità aktar kompetittiv minn kull pjaneta magħrufa oħra. Madankollu,
vettura spazjali skoprew differenzi sinifikanti kif ukoll. Għal
eżempju, rotazzjoni Venere "(Punent għal-Lvant) hu retrograda (b'lura)
Meta mqabbel mal-spin lvant-to-punent tad-Dinja u ħafna mill-oħra
pjaneta.

Madwar 96.5 fil-mija tal-atmosfera Venus "(95 darbiet
dens bħala s Earth) huwa dijossidu tal-karbonju. L-kostitwent prinċipali ta '
Atmosfera tad-Dinja hija nitroġenu. Atmosfera Venus "atti bħal
Serra, li jippermetti radjazzjoni solari biex jilħqu l-wiċċ iżda
insib l-sħana Dan ikun ordinarjament rradjata lura fil
ispazju. Bħala riżultat, silġ fit-temperatura medja tal-wiċċ tal-pjaneta
482 gradi Celsius (900 grad Fahrenheit), hot biżżejjed biex jiddewweb
Ċomb.

A altimetru bir-radjo fuq il-Pioneer Venus Orbiter sakemm il-
ewwel mezz ta 'jaraw permezz ta' kopertura tal-pjaneta sħaba densi u
Determinazzjoni tal-karatteristiċi tal-wiċċ fuq kważi l-pjaneta kollu. Tal NASA
Vettura spazjali Magellan, Imniedi 5 ta 'Mejju, 1989, ĠIET fl-orbita
madwar Venus minn Awwissu 10, 1990. Il-vettura spazjali użat radar
tekniki ta 'mapping sabiex providesprofessional immaġnijiet b'riżoluzzjoni għolja ta 98 ​​fil-mija
tal-wiċċ.

Radar Magellan kixfet pajsaġġ ddominata mill vulkaniċi
karatteristiċi, difetti u craters impatt. Oqsma Enormi tal-wiċċ
juru evidenza ta 'perjodi multipli ta' għargħar bi flussi lava
li tinsab fuq quċċata ta 'dawk ta' qabel. Reġjun elevati jismu Ishtar
Terra hija baċir mimlija lava kbira daqs l-Istati Uniti. Fuq One
aħħar ta 'din sedil plateau Maxwell Montes, muntanji id-daqs ta'
Mount Everest. Ċikatriċi falda il-muntanji huwa ta '100 kilometru
(62-il mil) wiesgħa, 2.5 kilometru (1.5-il mil) Crater Impatt Deep jismu
Cleopatra. (Kważi kollha tal-karatteristiċi fuq Venere huma msemmija għan-nisa;
Maxwell Montes Alpha Regio u Beta Regio huma l-eċċezzjonijiet.)
Craters jgħix fuq Venere għall forsi 400 Miljun snin minħabba
M'hemm l-ebda ilma u l-erożjoni mir-riħ ftit li xejn.

Netwerks estensivi tort line tkopri l-pjaneta, probabbilment l-
riżultat tal-istess tidwir crustal li tipproduċi tectonics pjanċa
fid-Dinja. Imma fuq Venus it-temperatura tal-wiċċ huwa suffiċjenti li
jdgħajfu l-blat, xquq Liema biss dwar kullimkien, Prevenzjoni
Il-formazzjoni tal-pjanċi kbar u difetti kbar terremot bħall-
San Andreas Fault fil-California.

Mudell Venus "temp predominanti huwa ta 'altitudni għolja, għolja
ċirkolazzjoni veloċità ta 'sħab Dan containe aċidu sulfuriku. B'veloċità
Nilħqu għoli daqs 360 kilometru (225 mil) fis-siegħa, il-
sħab ċirku l-pjaneta fil biss erbat ijiem Dinja. Iċ-ċirkolazzjoni
silġ fl-istess direzzjoni - Punent għal-Lvant - it-tidwir bil-mod Venus "
Dinja ta '243 jiem, filwaqt irjieħ tad-dinja blow fiż-żewġ direzzjonijiet -
Punent għal-Lvant u Lvant għall-Punent - f'sitt meded li jalterna. Venus "
atmosfera sservi bħala laboratorju simplifikata għall-istudju ta 'tagħna
temp.

EARTH

Kif jidher mill-ispazju, li jiddistingwi dinja tagħna
karatteristiċi huma ilmijiet blu tagħha, mases art kannella u aħdar
u sħab abjad. Aħna imdaħħla f'envelowp minn oċean ta 'arja Jikkonsisti
ta '78 fil-mija nitroġenu, 21 fil-mija ossiġnu u 1 fil-mija oħra
kostitwenti. L-unika pjaneta fis-sistema solari magħrufa li port
Life, Earth orbiti-Xemx f'distanza medja ta '150 miljun
kilometru (93 mil). miljun Dinja huwa t-tielet pjaneta mill-
Xemx u l-ħames l-akbar fis-sistema solari b'dijametru
Just ftit mijiet ta 'kilometri akbar thanthat ta' Venere.

Spin mgħaġġel pjaneta tagħna u imdewweb nikil-ħadid jagħtu lok qalba
għal kamp manjetiku estensiva, Liema, alongwith-atmosfera,
tarki lilna minn kważi kollha tar-radjazzjoni ta 'ħsara li ġejjin minn
l-Xemx u stilel oħra. Atmosfera tad-Dinja Tħares Us Mill
meteors kif ukoll, li ħafna minnhom burn up qabel ma jkunu jistgħu jsibu l-
wiċċ. Proċessi ġeoloġiċi attivi jkunu ħallew l-ebda evidenza tal-
pelting Dinja kważi ċertament riċevuti malajr wara li ffurmat -
Madwar 4.6 biljun sena ilu. Alongwith l-oħra ffurmata ġdida
pjaneta, kien showered mill debris spazjali fil-jiem bikrija tal-
sistemi solari.

Minn vjaġġi tagħna fl-ispazju, aħna tgħallimna ħafna dwar tagħna
pjaneta dar. L-ewwel American satellita - Explorer 1 - kien
Imniedi minn Cape Canaveral fi Florida fuq Jan 31, 1958, u
skopra żona radjazzjoni intensa, issa msejjaħ il Van Allen
Ċinturini radjazzjoni, madwar Earth.

Minn dakinhar, satelliti oħra ta 'riċerka wrew li tagħna
kamp manjetiku tal-pjaneta hija distorta fi forma tiċrita qatra mill-
l-riħ solari --fluss ta 'partiċelli ċċarġjati jintefgħu' l barra Kontinwament
mill-Xemx Imxejna tgħallmu li l-kamp manjetiku ma fade
off fl-ispazju imma jkollu limiti definiti. U aħna issa knowthat
atmosfera ta 'fuq tagħna wispy, ladarba maħsub kalm u uneventful,
seethes bl-attività - nefħa mill-jum u kuntrattar mill-lejl.
Affettwat minn tibdil fl-attività tax-xemx, l-atmosfera ta 'fuq
Jikkontribwixxu għat-temp u l-klima fid-Dinja.

Minbarra Jaffettwaw temp Dinja, l-attività tax-xemx wassal
li fenomenu viżwali drammatika fil-atmosfera tagħna. Meta mitluba
Partiċelli mill-wind've solari jinqabdu fl tal Earth manjetiċi
Qasam, Huma jikkonfliġġu ma molekuli arja 'l fuq tagħna pjaneta manjetiċi
arbli. Dawn il-molekuli arja mbagħad tibda glow u huma magħrufa bħala l-
auroras jew l-dwal tat-tramuntana u tan-nofsinhar.

Satelliti madwar 35.789 kilometru (22.238 mil) barra fi
ispazju għandhom rwol ewlieni fil kuljum tbassir tat-temp lokali. Dawn
għajnejn elettroniċi attenti Istati Uniti jwissu dwar maltempata perikolużi. Kontinwu
Monitoraġġ Globali tipprovdi ammont vast ta 'data Utli u
Jikkontribwixxi għal fehim aħjar ta 'temp kumpless tad-Dinja
sistema.

Fromtheir punti ta 'żvantaġġ uniku, satelliti jistgħu stħarriġ
Oċeani tad-dinja, użu tal-art u riżorsi, u jissorveljaw l-tać pjaneta
healthfully. Dawn għajnejn fl-ispazju jkunu salvati ħajjiet għadd, sakemm
Latrini murija tremend u l-US li aħna jistgħu jbiddlu jistaqsu tagħna
Planet b'modi perikolużi.

Il-Qamar

Satellita wieħed naturali is-silġ Dinja Moon. L-ewwel bniedem
Passi fuq dinja aljeni saru mill astronawti Amerikani fuq
il-wiċċ fit-trab tal mingħajr arja, ħbieb lifeless tagħna. Fil
Preparazzjoni għall-expeditions Apollo ekwipaġġati umani, NASA
Mibgħuta l-Ranger, Surveyor u Lunar Orbiter awtomatizzat
vettura spazjali biex tistudja l-Qamar bejn l-1964 u fl-1968.

Programm Apollo NASA ħallew wirt kbir ta 'materjal Lunar
u data. Sitt ekwipaġġi żewġ astronawti ġew żbarkati fit-Lunar u esplorati
Wiċċ bejn 1969 u 1972, li jġorru lura ġabra ta 'blat
u l-Ħamrija jiżnu total ta '382 kilogramm gramma (£ 842) u
Tikkonsisti minn aktar minn 2,000 kampjuni separati.

Minn dan il-materjal u studji oħra, xjentisti jkollhom
Mibnija A Storja tal-qamar Dan jinkludi bidu tagħha. Blat
Miġbura mill-artijiet għolja Lunar datata għal madwar 4.0-4300000000
sena. L-ewwel ftit miljun sena ta 'eżistenza tal-qamar
tant kienu vjolenti Li ftit traċċi ta 'dan il-perjodu, il-jibqghu. Bħala imdewweb
Saff ta 'barra Gradwalment mkessħa u solidifikat lejn tipi differenti
tal-blat, l-qamar kien bbumbardjati minn asteroids enormi u iżgħar
oġġetti. Uħud mill-jaħbtu mad kienu kbar daqs Rhode Island jew
Delaware, u kolliżjonijiet tagħhom mal-baċiri Moon maħluqa
mijiet ta 'kilometri madwar.

Dan bumbardament katastrofiċi għax-xejn off Madwar erba
biljun sena ilu, li jħallu l-għoljiet Lunar koperti bil enormi,
craters jikkoinċidu u saff fond ta 'blat għebet u maqsuma.
Sħana prodotta mill-tħassir ta 'elementi radjuattivi Bdiet biex jiddewweb
l-interjuri tal-qamar f'fond ta 'madwar 200 kilometru (125
mili) taħt il-wiċċ. Imbagħad, għall-700 miljun sena li ġejjin -
minn madwar 3.8 sa 3,1 biljun sena ilu - il-lava żdied minn ġewwa
il-Qamar. Il lava Gradwalment mifruxa fuq il-wiċċ, l-għargħar
Il-friskaturi tal-impatt kbar biex jiffurmaw l-oqsma dlam li Galileo
Galilei, astronomu tar-Rinaxximent Taljan, imsejħa maria,
li jfisser ibħra.

Safejn nistgħu tell, ma kien hemm ebda vulkanika sinifikanti
Attività fuq il-Qamar għal aktar minn tliet biljun sena. Peress
Imbagħad, il-wiċċ Lunar ĠIET tinbidel biss jekk jiġi micrometeorites,
mill-partiċelli atomika mill-Xemx u stilel, mill-apparat li jaġġusta
impatti tal meteorites kbar u billi ispazju u astronawti. Jekk
astronawti tagħna kien ġew żbarkati fit-qamar biljun sena ilu, Huma
wouldhave raw pajsaġġ simili ħafna għal dik tal-lum.
Eluf ta 'snin minn issa, il-passi li ħalla l-Apollo
Ekwipaġġi se jibqgħu l-qawwija u ċara.

L-oriġini tal-qamar għadu misteru. Erba Teoriji
tentattiv spjegazzjoni: il-Qamar iffurmat ħdejn id-Dinja bħala separata
Biċċa; kien torri mill-Dinja; huwa ffurmat x'imkien ieħor u kienet
maqbud mill-gravità tal-pjaneta tagħna, jew li kien ir-riżultat ta '
Ħabta bejn Dinja u asteroid dwar id-daqs tal Mars.
L-aħħar teorija għandha xi appoġġ tajjeb iżda hija 'l bogħod milli certainement.

MARZU

Ta 'l-pjaneti, Mars ilha meqjusa l-Suns
kandidat ideali tas-sistema għall-kenn ħajja extraterrestrial.
Astronomers jistudjaw il-pjaneta aħmar permezz teleskopji raw dak
Deher li linji dritti crisscrossing-wiċċ tagħha. Dawn
osservazzjoni - aktar tard Determinati li jistaqsu illusions ottiċi - punt li
L-idea popolari li ħlejjaq intelliġenti kienet mibnija
Sistemi ta 'kanali ta' irrigazzjoni fuq il-pjaneta. Fl-1938, Meta Orson
Welles xandira drama tar-radju bbażata fuq l-fantaxjenza
klassika Gwerra tal-Dinjiet minn HG Wells, biżżejjed nies jemmnu
fil-tale ta 'jinvaduh martians li jikkawżaw paniku qrib.

Raġuni oħra għax-xjenzati li jistennew ħajja fuq Mars kellhom
tagħmel il-bidliet apparenti kulur staġjonali fuq il-pjaneta tal
wiċċ. Dan il-punt il-fenomenu li spekulazzjoni li l-kundizzjonijiet jista
Appoġġ A nwar ta mars veġetazzjoni matul ix-xhur sħan u
kawża pjanti ħajjin biex issir fissi Matul perjodu kesħin.

S'issa, sitt missjonijiet Amerikani biex Mars twettqu.
Erba Mariner vettura spazjali - tliet jtiru bil-pjaneta u waħda
friendliness fl-orbita Martian - mistħarrġa il-pjaneta estensiv qabel
Il-orbiters Viking u Landers waslu.

Mariner 4, Imniedi fl-aħħar 1964, tellgħu passat Marzu 14 ta 'Lulju,
Fl-1965, jidħol fit 9.846 kilometru (6.118 mil) tal-wiċċ.
Tittrasmetti lill Dinja 22 stampi-close-up tal-pjaneta, il-
vettura spazjali sabet ħafna craters u kanali li jseħħu b'mod naturali imma
ebda evidenza ta 'kanali artifiċjali jew ilmijiet flowing. Baħri 6 u
7 Segwit withtheir flybys matul is-sajf tal-1969 u
Rritornati 201 ritratti. Mariner 4, 6 u 7 Wera diversità ta '
Kondizzjonijiet tal-wiċċ kif ukoll rqiqa, kiesaħ, mingħajr umdità atmosferika ta
dijossidu tal-karbonju.

30 ta 'Mejju, 1971, il-Mariner 9 Orbiter tnediet fuq
missjoni li jagħmlu studju sena tul tal-wiċċ Martian. Il
vettura spazjali waslu ħames xhur u nofs wara lift-off, biss biex
Sib Marzu fil-midst ta 'maltempata trab pjaneta wiesgħa Dan
Fotografija impossibbli għal diversi ġimgħat art. Iżda wara l-
maltempata approvati, Mariner 9 Beda jirritorna l-ewwel ta '7.329
stampi; Dawn il-karatteristiċi Martian preċedentement mhux magħrufa żvelat,
inklużi provi li ammonti kbar ta 'ilma darba nixxa madwar
Il-wiċċ, inċiżjoni widien tax-xmajjar u għargħar.

F'Awwissu u Settembru 1975, il-Viking 1 u 2 vetturi spazjali -
kull waħda tikkonsisti minn Orbiter u Lander - tintrefa 'minn
Kennedy Center Ispazju. Il-missjoni kien maħsub biex iwieġeb Diversi
Mistoqsijiet dwar il-pjaneta aħmar, inkluż, Hemm ħajja hemmhekk?
Ħadd ma jistenna li l-vettura spazjali għall-post ibliet Martian, imma kien
ttamat li l-esperimenti bijoloġija fuq il-excellant Viking Landers fil
Inqas issib evidenza ta 'ħajja primittiv - passata jew preżenti.

Viking Lander 1 Saret l-ewwel vettura spazjali b'suċċess
illandjar fuq pjaneta oħra meta hija żbarkati fit Lulju 20, 1976
filwaqt li l-Istati Uniti kien jiċċelebra Bicentennial tagħha. Ritratti
tard lura mill-Planitia Chryse ("Plains ta 'deheb") wera
skoraġġanti, Rusty-aħmar pajsaġġ. Stampi panoramiċi ritornati mill-
Lander żvelat plain rolling, mifruxa ma 'blat u turi
duni tar-ramel rippled. Trab aħmar fin mill-ħamrija Martian tagħti l-
A lewn sema salamun. Meta l-Lander Viking 2 mimsus stabbiliti fuq Utopia
Planitia 3 ta 'Settembru, 1976, huwa jitqies pajsaġġ rolling aktar
minn dak meqjus mill-predeċessur tiegħu - wieħed mingħajr duni viżibbli.

Ir-riżultati tard lura mil-laboratorju fuq kull Lander Viking
ma kinux konklużivi. Kampjuni żgħar tal-ħamrija Martian aħmar kienu
ttestjati fi tliet esperimenti differenti Disinjati biex Jindunaw
Proċessi bijoloġiċi. Filwaqt li wħud mill-riżultati tat-test deher li
jindikaw attività bijoloġika, analiżi aktar tard ikkonfermat Dan it-
attività kienet inorganiċi fin-natura u relatati mal-ħamrija tal-pjaneta
kimika. Hemm ħajja fuq Mars? Ħadd ma jaf għal dubju, iżda l-
Viking missjoni sabet ebda prova li jeżistu molekoli organiċi
hemmhekk.

Il LANDERS Viking Saret stazzjon tat-temp, reġistrazzjoni riħ
veloċità u d-direzzjoni kif ukoll it-temperatura atmosferika u
pressjoni. Kienu osservati bidliet klimatiċi ftit. L-ogħla
temperatura rreġistrata minn xi waħda mill inġenji kien -14 gradi Celsius (7
grad Fahrenheit) fil-Viking Lander 1 sit fil nofs is-sajf.

It-temperatura l-aktar baxxa, -120 gradi Celsius (-184 gradi
Fahrenheit), ġie rreġistrat fl-aktar tat-tramuntana Viking Lander 2
sit matul ix-xitwa. Veloċitajiet tar-riħ kważi uragan kienu mkejjla
L-istazzjon tat-temp Żewġ mars Matul maltempati trab globali, iżda
Minħabba li l-atmosfera hija tant irqiq, il-forza tar-riħ huwa minimu. Viking
Lander 2 fotografat b'irqajja ta ġlata - probabbilment ilma silġ
- Waqt it-tieni xitwa tagħha fuq il-pjaneta.

L-atmosfera Martian, bħal dik ta 'Venere, silġ Primarjament
dijossidu tal-karbonju. Nitroġenu u ossiġenu huma preżenti biss fil-żgħar
Perċentwali. Arja Martian fih biss madwar 1/1000 ilma kemm
AS-arja tagħna, iżda anke dan l-ammont żgħir jista jikkondensaw out, li tifforma
sħab Dan jirkbu għolja fl-atmosfera jew swirl madwar l-għoljiet
tal towering vulkani. Irqajja lokali ta 'ċpar filgħodu kmieni jistgħu jiffurmaw
fis widien.

Hemm evidenza li fil-passat ta 'tendenzi Martian
Atmosfera Mejju ippermettew ilma jiġri fuq il-pjaneta. Fiżika
Karatteristiċi kompetittiv jixbħu xtut, gorges, riverbeds u
gżejjer jissuġġerixxu li x-xmajjar kbar ladarba mmarkat il-pjaneta.

Marzu għandha żewġ moons, Phobos u Deimos. Dawn huma żgħar u
forma irregolari u possessur qedem, cratered uċuħ. Huwa
possibbli l-moons kienu oriġinarjament asteroids Li Schritt wisq
qrib Mars u kienu maqbudin bil-gravità tagħha.

Il-orbiters Viking u Landers qabżu minn marġini kbar
Tul ta 'żmien tad-disinn tagħhom ta' 120 u 90 jum, rispettivament. L-ewwel
li jonqsu kienet l-Viking Orbiter 2, Liema waqfet topera fuq Lulju 24,
1978 Meta nixxija imbattal gass-kontroll attitudni tagħha. Viking Lander
2 operati Sa April 12, 1980, meta kien għeluq Minħabba
deġenerazzjoni batterija. Viking Orbiter 1 nieqaf fuq 7 awissu, 1980,
Meta l-aħħar tal-gass ta 'kontroll attitudni tagħha kienet ġiet użata. Viking
Lander 1 waqaf Funzjonament fit Novembru 13, 1983.

Minkejja r-riżultati inkonklużivi tal-bijoloġija Viking
esperimenti, nafu aktar dwar Marzu minn kwalunkwe pjaneta ieħor ħlief
Dinja. Vettura spazjali Mars Osservatur NASA, Imniedi Settembru 25,
Fl-1992, se jespandu l-għarfien tagħna tal-ambjent Martian u
jgħinu jwasslu għal esplorazzjoni umana tal-Pjaneta l-Aħmar.

Asteroids

Is-sistemi solari għandha numru kbir ta 'blat u metall
Li l-oġġetti huma fl-orbita madwar ix-Xemx iżda huma żgħar wisq li jistaqsu
Pjaneta full-sħiħ meqjus. Dawn l-oġġetti huma magħrufa bħala
asteroids jew minjieri pjaneta. Ħafna, iżda mhux kollha, jinstabu f'medda
jew ċintorin bejn l-se jduru ta Mars u Jupiter. Xi wħud orbiti
Triq li Earth cross, u hemm evidenza li Earth ĠIET
milquta mill asteroids fil-passat. Wieħed mill-inqas jonqosx, l-aħjar
eżempji ppreżervat l Barringer Meteor Crater qrib Winslow,
Arizona.

Asteroids huma materjal mħollija mit-formazzjoni tal-
sistemi solari. Wieħed teorija jissuġġerixxi li dawn huma l-fdalijiet ta '
il-pjaneta li kien meqrud f'ħabta enormi twil ilu. Aktar
probabbli, asteroids huma materjali li qatt coalesced fis-pjaneta.
Fil-fatt, jekk il-massa totali stmat ta 'jaħbtu mad kollha kien Miġbura
fi oġġett wieħed, l-oġġett ikun inqas minn 1500
kilometru (932 mil) madwar - inqas minn nofs id-dijametru ta 'tagħna
Moon.

Eluf ta 'jaħbtu mad havebeen Identifikati mill-Dinja. Huwa
Stmat li 100,000 huma tajbin biżżejjed biex ikunu Eventwalment
fotografat permezz teleskopji bbażati Dinja.

Ħafna mill-għarfien tagħna dwar asteroids ġej minn
jeżamina biċċiet ta 'debris spazjali Din il-kawża għall-wiċċ ta'
Dinja. Li jaħbtu mad huma fuq kors ħabta ma Dinja huma
imsejħa meteoroids. Meta meteoroid strajkijiet atmosfera tagħna fuq għolja
veloċità, frizzjoni li jikkawża dan blokki ta 'materja ispazju jinċeneraw
fi strixxa tad-dawl magħruf bħala Meteor. Jekk in-nota ma meteoroid
burn up kompletament, dak ix-xellug jolqot wiċċ tad-Dinja u silġ
imsejħa meteorite. Wieħed mill-aħjar postijiet biex tfittex meteorites
silġ l kappa tas-silġ ta 'l-Antartika.

Ta 'l-meteorites kollha eżaminati, 92.8 fil-mija huma QORTI TAL
silikat (ġebel), u 5.7 fil-mija huma komposti tal-ħadid u nikil;
il-bqija huma taħlita tal-materjali tliet. Meteorites STONY
huma l-aktar diffiċli biex jiġu identifikati peress li jfittxu simili ħafna
blat terrestri.

Peress asteroids huma materjal mill-solar kmieni ħafna
sistema, xjentisti huma interessati fil-kompożizzjoni tagħhom. Vetturi spazjali
that have flown through the asteroid belt have found that the belt
is really quite empty and that asteroids are separated by very
large distances.

Current and future missions will fly by selected asteroids
for closer examination. The Galileo spacecraft, launched by NASA
in October 1989, investigated the main-belt asteroid Gaspra on
October 29, 1991 and will encounter Ida on August 28, 1993 on its
way to Jupiter. One day, space factories will mine the asteroids
for raw materials.

JUPITER

Beyond Mars and the asteroid belt, in the outer regions of
our solar system, lie the giant planets of Jupiter, Saturn, Uranus
and Neptune. In 1972, NASA dispatched the first of four spacecraft
slated to conduct the initial surveys of these colossal worlds of
gas and their moons of ice and rock. Jupiter was the first port of
call.

Pioneer 10, which lifted off from Kennedy Space Center in
March 1972, was the first spacecraft to penetrate the asteroid
belt and travel to the outer regions of the solar system. In
December 1973, it returned the first close-up images of Jupiter,
flying within 132,252 kilometers (82,178 miles) of the planet's
banded cloud tops. Pioneer 11 followed a year later. Voyagers 1
and 2 were launched in the summer of 1977 and returned spectacular
photographs of Jupiter and its family of satellites during flybys
in 1979.

These travelers found Jupiter to be a whirling ball of liquid
hydrogen and helium, topped with a colorful atmosphere composed
mostly of gaseous hydrogen and helium. Ammonia ice crystals form
white Jovian clouds. Sulfur compounds (and perhaps phosphorus) may
produce the brown and orange hues that characterize Jupiter's
atmosphere.

It is likely that methane, ammonia, water and other gases
react to form organic molecules in the regions between the
planet's frigid cloud tops and the warmer hydrogen ocean lying
below. Because of Jupiter's atmospheric dynamics, however, these
organic compounds — if they exist — are probably short-lived.

The Great Red Spot has been observed for centuries through
telescopes on Earth. This hurricane-like storm in Jupiter's
atmosphere is more than twice the size of our planet. As a high-
pressure region, the Great Red Spot spins in a direction opposite
to that of low-pressure storms on Jupiter; it is surrounded by
swirling currents that rotate around the spot and are sometimes
consumed by it. The Great Red Spot might be a million years old.

Our spacecraft detected lightning in Jupiter's upper
atmosphere and observed auroral emissions similar to Earth's
northern lights at the Jovian polar regions. Voyager 1 returned
the first images of a faint, narrow ring encircling Jupiter.

Largest of the solar system's planets, Jupiter rotates at a
dizzying pace — once every 9 hours 55 minutes 30 seconds. The
massive planet takes almost 12 Earth years to complete a journey
around the Sun. With 16 known moons, Jupiter is something of a
miniature solar system.

A new mission to Jupiter — the Galileo Project — is under
way. On December 7, 1995, after a six- year cruise that takes the
Galileo Orbiter once past Venus, twice past Earth and the Moon and
once past two asteroids, the spacecraft will drop an atmospheric
probe into Jupiter's cloud layers and relay data back to Earth.
The Galileo Orbiter will spend two years circling the planet and
flying close to Jupiter's large moons, exploring in detail what
the two Pioneers and two Voyagers revealed.

GALILEAN SATELLITES

In 1610, Galileo Galilei aimed his telescope at Jupiter and
spotted four points of light orbiting the planet. For the first
time, humans had seen the moons of another world. In honor of
their discoverer, these four bodies would become known as the
Galilean satellites or moons. But Galileo might have happily
traded this honor for one look at the dazzling photographs
returned by the Voyager spacecraft as they flew past these planet-
sized satellites.

One of the most remarkable findings of the Voyager mission
was the presence of active volcanoes on the Galilean moon Io.
Volcanic eruptions had never before been observed on a world other
than Earth. The Voyager cameras identified at least nine active
volcanoes on Io, with plumes of ejected material extending as far
as 280 kilometers (175 miles) above the moon's surface.

Io's pizza-colored terrain, marked by orange and yellow hues,
is probably the result of sulfur-rich materials brought to the
surface by volcanic activity. Volcanic activity on this satellite
is the result of tidal flexing caused by the gravitational tug-of-
war between Io, Jupiter and the other three Galilean moons.

Europa, approximately the same size as our Moon, is the
brightest Galilean satellite. The moon's surface displays a
complex array of streaks, indicating the crust has been fractured.
Caught in a gravitational tug-of-war like Io, Europa has been
heated enough to cause its interior ice to melt — apparently
producing a liquid-water ocean. This ocean is covered by an ice
crust that has formed where water is exposed to the cold of space.
Europa's core is made of rock that sank to its center.

Like Europa, the other two Galilean moons — Ganymede and
Callisto — are worlds of ice and rock. Ganymede is the largest
satellite in the solar system — larger than the planets Mercury
and Pluto. The satellite is composed of about 50 percent ice or
slush and the rest rock. Ganymede's surface has areas of different
brightness, indicating that, in the past, material oozed out of
the moon's interior and was deposited at various locations on the
surface.

Callisto, only slightly smaller than Ganymede, has the lowest
density of any Galilean satellite, suggesting that large amounts
of water are part of its composition. Callisto is the most heavily
cratered object in the solar system; no activity during its
history has erased old craters except more impacts.

Detailed studies of all the Galilean satellites will be
performed by the Galileo Orbiter.

SATURN

No planet in the solar system is adorned like Saturn. Its
exquisite ring system is unrivaled. Like Jupiter, Saturn is
composed mostly of hydrogen. But in contrast to the vivid colors
and wild turbulence found in Jovian clouds, Saturn's atmosphere
has a more subtle, butterscotch hue, and its markings are muted by
high-altitude haze. Given Saturn's somewhat placid-looking
appearance, scientists were surprised at the high-velocity
equatorial jet stream that blows some 1,770 kilometers (1,100
miles) per hour.

Three American spacecraft have visited Saturn. Pioneer 11
sped by the planet and its moon Titan in September 1979, returning
the first close-up images. Voyager 1 followed in November 1980,
sending back breathtaking photographs that revealed for the first
time the complexities of Saturn's ring system and moons. Voyager 2
flew by the planet and its moons in August 1981.

The rings are composed of countless low-density particles
orbiting individually around Saturn's equator at progressive
distances from the cloud tops. Analysis of spacecraft radio waves
passing through the rings showed that the particles vary widely in
size, ranging from dust to house-sized boulders. The rings are
bright because they are mostly ice and frosted rock.

The rings might have resulted when a moon or a passing body
ventured too close to Saturn. The unlucky object would have been
torn apart by great tidal forces on its surface and in its
interior. Or the object may not have been fully formed to begin
with and disintegrated under the influence of Saturn's gravity. A
third possibility is that the object was shattered by collisions
with larger objects orbiting the planet.

Unable either to form into a moon or to drift away from each
other, individual ring particles appear to be held in place by the
gravitational pull of Saturn and its satellites. These complex
gravitational interactions form the thousands of ringlets that
make up the major rings.

Radio emissions quite similar to the static heard on an AM
car radio during an electrical storm were detected by the Voyager
spacecraft. These emissions are typical of lightning but are
believed to be coming from Saturn's ring system rather than its
atmosphere, where no lightning was observed. As they had at
Jupiter, the Voyagers saw a version of Earth's auroras near
Saturn's poles.

The Voyagers discovered new moons and found several
satellites that share the same orbit. We learned that some moons
shepherd ring particles, maintaining Saturn's rings and the gaps
in the rings. Saturn's 18th moon was discovered in 1990 from
images taken by Voyager 2 in 1981.

Voyager 1 determined that Titan has a nitrogen-based
atmosphere with methane and argon — one more like Earth's in
composition than the carbon dioxide atmospheres of Mars and Venus.
Titan's surface temperature of -179 degrees Celsius (-290 degrees
Fahrenheit) implies that there might be water-ice islands rising
above oceans of ethane-methane liquid or sludge. Unfortunately,
Voyager's cameras could not penetrate the moon's dense clouds.

Continuing photochemistry from solar radiation may be
converting Titan's methane to ethane, acetylene and — in
combination with nitrogen — hydrogen cyanide. The latter compound
is a building block of amino acids. These conditions may be
similar to the atmospheric conditions of primeval Earth between
three and four billion years ago. However, Titan's atmospheric
temperature is believed to be too low to permit progress beyond
this stage of organic chemistry.

The exploration of Saturn will continue with the Cassini
mission. Scheduled for launch in the latter part of the 1990s, the
Cassini mission is a collaborative project of NASA, the European
Space Agency and the federal space agencies of Italy and Germany,
as well as the United States Air Force and the Department of
Energy. Cassini will orbit the planet and will also deploy a
probe called Huygens, which will be dropped into Titan's
atmosphere and fall to the surface. Cassini will use radar to peer
through Titan's clouds and will spend years examining the
Saturnian system.

URANUS

In January 1986, four and a half years after visiting Saturn,
Voyager 2 completed the first close-up survey of the Uranian
system. The brief flyby revealed more information about Uranus and
its retinue of icy moons than had been gleaned from ground
observations since the planet's discovery over two centuries ago
by the English astronomer William Herschel.

Uranus, third largest of the planets, is an oddball of the
solar system. Unlike the other planets (with the exception of
Pluto), this giant lies tipped on its side with its north and
south poles alternately facing the sun during an 84-year swing
around the solar system. During Voyager 2′s flyby, the south pole
faced the Sun. Uranus might have been knocked over when an Earth-
sized object collided with it early in the life of the solar
system.

Voyager 2 found that Uranus' magnetic field does not follow
the usual north-south axis found on the other planets. Instead,
the field is tilted 60 degrees and offset from the planet's
center, a phenomenon that on Earth would be like having one
magnetic pole in New York City and the other in the city of
Djakarta, on the island of Java in Indonesia.

Uranus' atmosphere consists mainly of hydrogen, with some 12
percent helium and small amounts of ammonia, methane and water
vapor. The planet's blue color occurs because methane in its
atmosphere absorbs all other colors. Wind speeds range up to 580
kilometers (360 miles) per hour, and temperatures near the cloud
tops average -221 degrees Celsius (-366 degrees Fahrenheit).

Uranus' sunlit south pole is shrouded in a kind of
photochemical “smog” believed to be a combination of acetylene,
ethane and other sunlight-generated chemicals. Surrounding the
planet's atmosphere and extending thousands of kilometers into
space is a mysterious ultraviolet sheen known as “electroglow.”

Approximately 8,000 kilometers (5,000 miles) below Uranus'
cloud tops, there is thought to be a scalding ocean of water and
dissolved ammonia some 10,000 kilometers (6,200 miles) deep.
Beneath this ocean is an Earth-sized core of heavier materials.

Voyager 2 discovered 10 new moons, 16-169 kilometers (10-105
miles) in diameter, orbiting Uranus. The five previously known –
Miranda, Ariel, Umbriel, Titania and Oberon — range in size from
520 to 1,610 kilometers (323 to 1,000 miles) across. Representing
a geological showcase, these five moons are half-ice, half-rock
spheres that are cold and dark and show evidence of past activity,
including faulting and ice flows.

The most remarkable of Uranus' moons is Miranda. Its surface
features high cliffs as well as canyons, crater-pocked plains and
winding valleys. The sharp variations in terrain suggest that,
after the moon formed, it was smashed apart by a collision with
another body — an event not unusual in our solar system, which
contains many objects that have impact craters or are fragments
from large impacts. What is extraordinary is that Miranda
apparently reformed with some of the material that had been in its
interior exposed on its surface.

Uranus was thought to have nine dark rings; Voyager 2 imaged
11. In contrast to Saturn's rings, which are composed of bright
particles, Uranus' rings are primarily made up of dark, boulder-
sized chunks.

NEPTUNE

Voyager 2 completed its 12-year tour of the solar system with
an investigation of Neptune and the planet's moons. On August 25,
1989, the spacecraft swept to within 4,850 kilometers (3,010
miles) of Neptune and then flew on to the moon Triton. During the
Neptune encounter it became clear that the planet's atmosphere was
more active than Uranus'.

Voyager 2 observed the Great Dark Spot, a circular storm the
size of Earth, in Neptune's atmosphere. Resembling Jupiter's Great
Red Spot, the storm spins counterclockwise and moves westward at
almost 1,200 kilometers (745 miles) per hour. Voyager 2 also noted
a smaller dark spot and a fast-moving cloud dubbed the “Scooter,”
as well as high-altitude clouds over the main hydrogen and helium
cloud deck. The highest wind speeds of any planet were observed,
up to 2,400 kilometers (1,500 miles) per hour.

Like the other giant planets, Neptune has a gaseous hydrogen
and helium upper layer over a liquid interior. The planet's core
contains a higher percentage of rock and metal than those of the
other gas giants. Neptune's distinctive blue appearance, like
Uranus' blue color, is due to atmospheric methane.

Neptune's magnetic field is tilted relative to the planet's
spin axis and is not centered at the core. This phenomenon is
similar to Uranus' magnetic field and suggests that the fields of
the two giants are being generated in an area above the cores,
where the pressure is so great that liquid hydrogen assumes the
electrical properties of a metal. Earth's magnetic field, on the
other hand, is produced by its spinning metallic core and is only
slightly tilted and offset relative to its center.

Voyager 2 also shed light on the mystery of Neptune's rings.
Observations from Earth indicated that there were arcs of material
in orbit around the giant planet. It was not clear how Neptune
could have arcs and how these could be kept from spreading out
into even, unclumped rings. Voyager 2 detected these arcs, but
they were, in fact, part of thin, complete rings. A number of
small moons could explain the arcs, but such bodies were not
spotted.

Astronomers had identified the Neptunian moons Triton in 1846
and Nereid in 1949. Voyager 2 found six more. One of the new moons
– Proteus — is actually larger than Nereid, but since Proteus
orbits close to Neptune, it was lost in the planet's glare for
observers on Earth.

Triton circles Neptune in a retrograde orbit in under six
days. Tidal forces on Triton are causing it to spiral slowly
towards the planet. In 10 to 100 million years (a short time in
astronomical terms), the moon will be so close that Neptunian
gravity will tear it apart, forming a spectacular ring to
accompany the planet's modest current rings.

Triton's landscape is as strange and unexpected as those of
Io and Miranda. The moon has more rock than its counterparts at
Saturn and Uranus. Triton's mantle is probably composed of water-
ice, but the moon's crust is a thin veneer of nitrogen and
methane. The moon shows two dramatically different types of
terrain: the so-called “cantaloupe” terrain and a receding ice
cap.

Dark streaks appear on the ice cap. These streaks are the
fallout from geyser-like volcanic vents that shoot nitrogen gas
and dark, fine-grained particles to heights of 2 to 8 kilometers
(1 to 5 miles). Triton's thin atmosphere, only 1/70,000th as thick
as Earth's, has winds that carry the dark particles and deposit
them as streaks on the ice cap — the coldest surface yet found in
the solar system (-235 degrees Celsius, -391 degrees Fahrenheit).
Triton might be more like Pluto than any other object spacecraft
have so far visited.

PLUTO

Pluto is the most distant of the planets, yet the
eccentricity of its orbit periodically carries it inside Neptune's
orbit, where it has been since 1979 and where it will remain until
March 1999. Pluto's orbit is also highly inclined — tilted 17
degrees to the orbital plane of the other planets.

Discovered in 1930, Pluto appears to be little more than a
celestial snowball. The planet's diameter is calculated to be
approximately 2,300 kilometers (1,430 miles), only two-thirds the
size of our Moon. Ground-based observations indicate that Pluto's
surface is covered with methane ice and that there is a thin
atmosphere that may freeze and fall to the surface as the planet
moves away from the Sun. Observations also show that Pluto's spin
axis is tipped by 122 degrees.

The planet has one known satellite, Charon, discovered in
1978. Charon's surface composition is different from Pluto's: the
moon appears to be covered with water-ice rather than methane ice.
Its orbit is gravitationally locked with Pluto, so both bodies
always keep the same hemisphere facing each other. Pluto's and
Charon's rotational period and Charon's period of revolution are
all 6.4 Earth days.

Although no spacecraft have ever visited Pluto, NASA is
currently exploring the possibility of such a mission.

COMETS

The outermost members of the solar system occasionally pay a
visit to the inner planets. As asteroids are the rocky and
metallic remnants of the formation of the solar system, comets are
the icy debris from that dim beginning and can survive only far
from the Sun. Most comet nuclei reside in the Oort Cloud, a loose
swarm of objects in a halo beyond the planets and reaching perhaps
halfway to the nearest star.

Comet nuclei orbit in this frozen abyss until they are
gravitationally perturbed into new orbits that carry them close to
the Sun. As a nucleus falls inside the orbits of the outer
planets, the volatile elements of which it is made gradually warm;
by the time the nucleus enters the region of the inner planets,
these volatile elements are boiling. The nucleus itself is
irregular and only a few miles across, and is made principally of
water-ice with carbon monoxide, carbon dioxide, methane and
ammonia — materials very similar to those composing the moons of
the giant planets.

As these materials boil off of the nucleus, they form a coma
or cloud-like “head” that can measure tens of thousands of
kilometers across. The coma grows as the comet gets closer to the
Sun. Solar charged particles push on gas molecules and the
pressure of sunlight pushes on the cloud of dust particles,
blowing them back like flags in the wind and giving rise to the
comet's “tails.” Gases and ions are blown directly back from the
nucleus, but dust particles are pushed more slowly. As the nucleus
continues in its orbit, the dust particles are left behind in a
curved arc.

Both the gas and dust tails are blown away from the Sun; in
effect, the comet chases its tails as it recedes from the Sun. The
tails can reach 150 million kilometers (93 million miles) in
length, but the total amount of material contained in this
dramatic display would fit in an ordinary suitcase. Comets — from
the Latin cometa, meaning “long-haired” — are essentially dramatic
light shows.

Some comets pass through the solar system only once, but
others have their orbits gravitationally modified by a close
encounter with one of the giant outer planets. These latter
visitors can enter closed elliptical orbits and repeatedly return
to the inner solar system.

Halley's Comet is the most famous example of a relatively
short period comet, returning on an average of once every 76 years
and orbiting from beyond Neptune to within Venus' orbit. Confirmed
sightings of the comet go back to 240 BC This regular visitor to
our solar system is named for Sir Edmond Halley, because he
plotted the comet's orbit and predicted its return, based on
earlier sightings and Newtonian laws of motion. His name became
part of astronomical lore when, in 1759, the comet returned on
schedule. Unfortunately, Sir Edmond did not live to see it.

A comet can be very prominent in the sky if it passes
comparatively close to Earth. Unfortunately, on its most recent
appearance, Halley's Comet passed no closer than 62.4 million
kilometers (38.8 million miles) from our world. The comet was
visible to the naked eye, especially for viewers in the southern
hemisphere, but it was not spectacular. Comets have been so
bright, on rare occasions, that they were visible during daytime.
Historically, comet sightings have been interpreted as bad omens
and have been artistically rendered as daggers in the sky.

Several spacecraft have flown by comets at high speed; the
first was NASA's International Cometary Explorer in 1985. An
armada of five spacecraft (two Japanese, two Soviet and the Giotto
spacecraft from the European Space Agency) flew by Halley's Comet
in 1986. Additional comet missions are being examined in the
United States and abroad.

CONCLUSION

Despite their efforts to peer across the vast distances of
space through an obscuring atmosphere, scientists of the past had
only one body they could study closely — Earth. But since 1959,
spaceflight through the solar system has lifted the veil on our
neighbors in space.

We have learned more about our solar system and its members
than anyone had in the previous thousands of years. Our automated
spacecraft have traveled to the Moon and to all the planets beyond
our world except Pluto; they have observed moons as large as small
planets, flown by comets and sampled the solar environment.
Astronomy books now include detailed pictures of bodies that were
only smudges in the largest telescopes for generations. We are
lucky to be alive now to see these strange and beautiful places
and objects.

The knowledge gained from our journeys through the solar
system has redefined traditional Earth sciences like geology and
meteorology and spawned an entirely new discipline called
comparative planetology. By studying the geology of planets,
moons, asteroids and comets, and comparing differences and
similarities, we are learning more about the origin and history of
these bodies and the solar system as a whole.

We are also gaining insight into Earth's complex weather
systems. By seeing how weather is shaped on other worlds and by
investigating the Sun's activity and its influence throughout the
solar system, we can better understand climatic conditions and
processes on Earth.

We will continue to learn and benefit as our automated
spacecraft explore our neighborhood in space. Missions to each
type of body in the solar system are in flight or under
development or study.

We can also look forward to the time when humans will once
again set foot on an alien world. Although astronauts have not
been back to the Moon since December 1972, plans are being
formulated for our return to the lunar landscape and for the human
exploration of Mars and even the establishment of martian
outposts. One day, taking a holiday may mean spending a week at a
lunar base or a martian colony!

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