命名和語源

彗星以其拖著的長尾巴而得名，「彗」的本意就是帚。《說文》紀載：「彗，埽竹也。」。中國古人把彗星叫做「星孛」，《春秋》記載，魯文公14年（前613年）「秋七月，有星孛入于北斗」。這是世界上關于哈雷彗星的最早記錄。根據董仲舒的說法，「孛者，惡之所生也。謂之孛者，言其孛之有所妨蔽，暗亂不明之貌也」。實際上在中國古代，彗星常與災厄聯繫在一起，劉向也說「孛者，亂臣矣，篡殺之表也」，但也有劉知幾等人早就表示這種說法是無稽之談。

《晉書·天文志》載有：「彗星所謂掃星，本類星，末類彗，小者數寸，長或經天。彗星本無光，傅日而為光，故夕見則東指，晨見則西指。在日南北皆隨日光而指，頓挫其芒，或長或短。」準確的描述了彗星的形態。

西方語言中的「彗星」一詞（如法語：；德語：Komet；英語：comet，古英文：cometa），

源自拉丁文的comēta或comētēs，這是拉丁化的希臘文κομήτης。在牛津英語字典，這個詞是'κομήτης'（' ἀστὴρ '），意思是希臘文的"長髮明星，彗星"。Κομήτης是從κομᾶν（"留著長髮"）轉變過來的，其本身又是從κόμη（意思是"頭上的頭髮"）轉變過來的，而其意思是"彗星的尾巴"。希臘哲學家兼科學家亞里斯多德是第一位使用這個延伸出來的字κόμη, κομήτης，來形容他看見的"長著頭髮的星星"。彗星的天文學符號是 (Unicode ☄ U+2604)，由一個小圓盤和三根如頭髮突起的短線段組成。

研究的歷史

由於彗星無論是形貌或運行規律，都和人類平常觀測的行星和恆星大相逕庭，因此早期彗星始終為不少人所著迷，且由於彗星的出現時間常和不少歷史上大事件的發生時間相近，因此部分民族甚至視彗星為神或災禍的象徵。

早期的觀測和推論

在望遠鏡發明之前，彗星好像無論在何處出現，都會慢慢的消失不見。它們通常都被認為是不好的預兆，會為國王或男性的貴族帶來災難、死亡，甚至被解釋為上天對地球上居民的攻擊。來自古代的資料，例如中國的甲骨文，知道數千年來人類就曾經發現過彗星。烏魯克的國王吉爾伽美什將之解釋為"流星"，而啟示錄、以諾書等則稱之為彗星，或可能是火流星。一個很有名的古老記錄，是出現在貝葉掛毯上的哈雷彗星，這幅掛毯描述的是1066年諾曼征服英格蘭的事蹟。

亞里士多德在他的第一本書，天象論中對彗星看法的論調，主導了西方對彗星的思潮將近兩千年。他否決了幾個早期哲學家認為彗星是行星，或至少是一種與行星有關天象的想法，理由是行星侷限於黃道上，並且是種圓周運動，但彗星可能出現在天空中的任何部分。取而代之的是，他描述彗星是地球大氣層上層的現象，是在炎熱、乾燥的環境下聚集和偶然噴出的火焰。亞里斯多德認為這種機制不僅形成彗星，還包括流星、極光，甚至是銀河。

有幾位後來的哲學家對彗星的看法提出異議。塞內卡延續阿波羅尼奧斯彗星是獨立星體的觀點，在他的天問指出，彗星在天空中有規律的移動，並且不受風影響的性質，這種不受幹擾的行為比較像天體而不是大氣中典型的現象。儘管他認為其它的行星不會出現在黃道之外，但是類似地球的天體沒有理由不能在天空的任何地方出現，人類對天體的認識是非常有限的。然而，亞里斯多德的觀點被證明更有影響力，直到16世紀，彗星還被認為是大氣層內，而不是大氣層之外的現象。

在1577年，一顆明亮的彗星出現了好幾個月。丹麥的天文學家第谷·布拉赫使用他自己和別人在不同地點測量的彗星位置，試圖測量出彗星的視差。但在測量的精確度範圍內，測不出任何視差，這暗示了彗星的距離比月球到地球距離至少還要遠4倍以上。

在《科學的奇蹟》(Marvels of Science)中，作者提到：笛卡爾、歐拉等人相信，整個宇宙充滿在一種難以察覺的介質中，他們稱其為以太。無數的行星和恆星漂浮在以太海洋中。在這種介質中彗星則起到清道夫發作用，以防止以太集結成塊，使其保持穩定適當的稀薄狀態。

軌道的研究

雖然彗星現在已經被証明是天體，但是它們在天空上是如何移動的，卻在下個世紀成為辯論的主題。即使稍後約翰·克卜勒在1609年確定行星是以橢圓軌道環繞著太陽，他認為定律管轄的是行星運動，應該不會影響到其它天體的運動－他相信彗星是在行星之間以直線運動。伽利略雖然堅信哥白尼學說，拒絕第谷的視差測量並且包容亞里斯多德認為彗星是通過大氣層上層直線運動的觀念。

在1610年，威廉·羅耳是第一位建議行星運動的克卜勒定律也適用於彗星的人。在接下來的數十年，其他的天文學家，包括Pierre Petit、Giovanni Borelli、Adrien Auzout、羅伯特·虎克、Johann Baptist Cysat、和喬瓦尼·多梅尼科·卡西尼也都主張彗星是以橢圓或拋物線的曲線路徑繞著太陽；但是，如克里斯蒂安·惠更斯和約翰·赫維留的部分學者依然認為彗星是以直線運動。

這件事經由Gottfried Kirch在1680年11月14日發現的亮彗星得到解決，整個歐洲的天文學家追蹤這顆彗星的位置達數個月。在1681年，薩克遜的牧師進一步的證明這顆彗星是以拋物線運行的天體，並且太陽在其中的一個焦點上。然後艾薩克·牛頓在他1687年發表的數學原理中證明了一個在與距離平方成反比的萬有引力影響下運動的物體，它的軌道所形成的軌跡形狀是圓錐曲線，並且使用1680年的彗星做例子，說明彗星在天球上經過的路徑與拋物線是如何吻合的。

在1705年，愛德蒙·哈雷應用牛頓的方法分析了在1337年至1698年間出現的23顆彗星。他注意到1531年、1607年和1682年的彗星有著非常相似的軌道要素，他進一步考慮到木星和土星的引力攝動對軌道造成的微小差異，更有信心確認這三顆彗星是同一顆彗星的一再出現，他並預測這顆彗星在1758至1759年間會再出現。（稍早些，羅伯特·虎克認定1664年和1618年的彗星是同一顆，同時喬凡尼·卡西尼曾懷疑1577年、1665年、和1680年的，但兩者都不正確。）哈雷預測的回歸日期後來被三位法國數學家的小組：亞歷克西斯·克勞德·克萊羅、約瑟夫·拉朗德和妮可-雷訥·勒波特，再精算過，他們預測這顆彗星的近日點落在1759年，準確在一個月內。當這顆彗星如預測的回來時，它被命名為哈雷彗星（稍後的正式名稱為1P/Halley），下次將於2061年回歸。

在歷史上，彗星的週期不僅要夠短，還要每次都夠明亮，才能夠被記錄好幾次。哈雷彗星是唯一每次都夠亮，在經過太陽系的內側時能以肉眼看見的彗星。自哈雷彗星的週期被確認之後，通過望遠鏡的使用，發現了許多其它的週期彗星。第二顆被發現週期的彗星是恩克彗星（官方正式的名稱是2P/Encke）。德國數學家兼物理學家約翰·弗朗茨·恩克在1819-21年間計算一系列彗星的軌道，他觀察到1786年、1795年、1805年、和1818年的彗星，得出的結論是它們是同一顆彗星，並且成功的預測它在1822年的回歸。到1900年，已經有17顆彗星被觀察到多次通過近日點，並被認定是週期彗星。，已有271顆週期彗星被辨識出來，不過其中有幾顆已經瓦解或是失蹤了。

物理性質的研究

艾薩克·牛頓描述彗星是在傾斜軌道上運動的緊密和持久的固體，它們的尾巴是由核心排放出，被太陽加熱或點燃的稀薄氣體。牛頓懷疑彗星是支援空氣中生命起源的元件，他也相信彗星排放的蒸氣和太陽供應的燃料，可以補充行星的水（經由植物的增長和腐爛還逐漸轉變成行星上的土壤）。

在18世紀初期，一些科學家對彗星的組成已經做了正確的假設。在1755年，伊曼努爾·康德假設彗星是由一些揮發性物質組成，當它們接近近日點時因為汽化而呈現輝煌的亮度。在1836年，德國數學家弗里德里希·威廉·貝塞爾在觀察1835年的哈雷彗星噴發出來的氣流之後，認為噴射力大到足以改變一顆彗星的軌道，。

然而，另一個有關彗星的發現掩蓋了這個想法將近一世紀之久。在1864至1866年間，義大利天文學家喬凡尼·斯基亞帕雷利計算英仙座流星雨的軌道，基於軌道的相似性，它正確的指出該流星雨是斯威夫特－塔特爾彗星的片段。彗星和流星雨之間的聯繫，在1872年被戲劇性的強調，在比拉彗星的軌道上發生了重大的流星雨，而這顆彗星在1846年出現時被觀測到分裂成兩半，並且在1852年後就未曾再見到。"碎石銀行"結構的彗星模型出現了，在模型中，彗星是由鬆散的小岩石堆積而成，並塗上了冰冷的外層。

在20世紀中葉，這種模型呈現出了一些缺點：尤其是，它不能解釋只有少量冰凍物質的物體，可以在經過近日點數次之後，依然可以繼續的蒸發出氣體而持續完美的展現。在1950年，弗雷德·惠普爾提出這一點，認為彗星不是岩石包覆著一些冰，而是冰凍的物質包含了一些塵埃和岩石。這"髒雪球"模型很快的就被接受，並且來自龐大的太空船觀測資料，似乎也支持這樣的見解。這些太空船包括ESO的喬托號探測器和蘇聯的Vega 1和Vega 2，它們在1986年穿越過哈雷彗星的彗髮，拍攝了彗核的影像，和觀察了揮發性物質的彗尾。

近代的發現

關於彗星含有多少冰的辯論仍然持續著。在2001年，NASA的深空一號小組，在NASA的噴射推進實驗室工作，獲得19P/包瑞利彗星表面的高解析影像。他們宣布包瑞利彗星展現出性質不同的噴流，是熱且乾燥的。假設彗星包含水和其他的冰，領導人，美國地質調查局的Laurence Soderblom博士說：光譜顯示表面是熱和乾燥的。令人驚訝的是我們沒有看見水冰的痕跡。然而，他又提出冰可能隱藏在下方，而表面因為太陽的加熱已經乾涸，也或許包表面覆蓋著非常黑的，像煤灰的材料掩蓋了地殼表面任何冰的蹤跡。

在2005年7月，深度撞擊探測器在坦普爾1號彗星上撞出一個坑穴以研究它的內部。這個任務的結果顯示彗星的冰水大部份都是在表面下，這些儲藏的水昇華形成了彗髮，提供了坦普爾1號彗星噴流所需要的蒸發水。之後，它改名為EPOXI，在2010年11月4日飛掠過哈特雷二號彗星。

在1999年2月發射的星塵號太空船，在2004年1月蒐集了維爾特二號彗星來自彗髮的顆粒，並且在2006年1月用莢艙將樣品送回地球。克勞迪雅亞歷山大，在NASA的噴射推進實驗是從事彗星模型建構多年，向space.com報告她對噴流數量的驚訝，它們的外觀在黑暗側和明亮側是一樣的，它們能從彗星的表面舉起大塊的岩石，此一事實表明維爾特二號彗星不是鬆散黏合的瓦礫堆。

更多來自星塵任務的資料顯示來自維爾特二號彗星尾巴物質的結晶可能僅能在火中生成。雖然彗星是在太陽系的外側形成的，但在太陽系早期的形成時間，徑向的物質混合有可能重新分配了原始行星盤的所有物質，所以彗星也包含了在炙熱的太陽系內側形成的結晶顆粒。這在彗星的光譜，以及樣本返回任務都能見到。近來還有更多，取回的物質表明"彗星塵埃類似於小行星的物質"。這些新的結果迫使科學家重新思考彗星和小行星在本質上的區別和差異。

在2011年4月，來自亞歷桑納大學的科學家發現維爾特二號彗星中有液態水存在的證據。他們找到了鐵和必須有水存在下才能形成的硫化銅礦物。此一發現粉碎了彗星從來沒有得到足夠使大量冰塊融化的溫暖環境的現有範例。

即將進行的太空任務將增加能讓我們更清楚認識彗星的組成。歐洲的羅塞塔探測器將前往67P/楚留莫夫－格拉希門克彗星；在2014年，它將進入環繞這顆彗星的軌道和安放一個小登陸艇到它的表面。

物理性質

彗星由彗核、彗髮和彗尾組成。彗核和彗髮構成彗頭。

彗核

彗星的核心固體結構稱為彗核，是由水冰、岩石、和凍結的氣體（如二氧化碳、一氧化碳、甲烷和氨等）融合在一起組成的。1950年代，美國天文學家弗雷德·惠普爾提出「彗星的內核是由含冰的凝聚物組成」的假說，這個「彗星模型」後來令彗星普遍的被大眾（包含惠普爾本人）暱稱為「冰污球」（Icy dirtballs）或「髒雪球」（Dirty Snowballs）。

彗核的表面一般是乾燥、塵土或岩石飛揚的，這暗示冰是隱藏在表面數公尺厚的的地殼之下。除了已經提到的氣體，彗核還包含各種各樣的有機化合物，它們可能包括甲醇、氰化氫、甲醛、乙醇、和乙烷，或許還有更複雜的分子，如長鏈的烴類和氨基酸。在 2009年，從NASA星塵任務帶回的彗星塵埃中發現了氨基酸中的甘氨酸。在2011年8月，NASA一份根據在地球上發現的隕石所做的報告指出，已經發現DNA和RNA的元件（腺嘌呤、鳥嘌呤、及相關的有機分子），可能已經在小行星和彗星上形成。

彗核表面的反照率非常的低，使它們成為太陽系內反照率最低的物體。喬托號太空探測器發現哈雷彗星的彗核只反射了大約4%照射在它上面的光線 ，深空一號發現包瑞利彗星表面反射落在它上面的光線少於3%；相較之下，落在瀝青表面的光都還有7%能被反射。彗核表面黑暗的物質材料可能包括複雜的有機化合物。太陽的熱驅動了較輕的揮發物，留下了較重的有機化合物，往往都是黑色的，像是焦油或是原油。彗星表面相對較低的反照率使它們可以吸收更多需要的熱量，驅動釋氣的程序。

曾經觀察過的彗核直徑有超過的，但是要確定其確實的大小是很困難的。P/2007 R5的彗核直徑大約只有100–200公尺。儘管儀器非常靈敏，但是缺乏較小的彗星可供檢測彗核的大小，使得一些人認為彗核的直徑不會小於。從已知的彗星估計，彗核的平均密度大約是0.6g/cm3，彗核的低質量使彗核不會因為自己的重力造成球形，因此它們的外型是不規則的。

大約6%的近地小行星被認為是熄火彗星，它們的彗核已不再釋放出氣體，包括(14827) Hypnos（睡神星）和(3552) Don Quixote（唐吉訶德）。

彗髮

在彗星的周圍圍繞著的塵埃和氣體形成一個巨大且稀薄的大氣層，稱為彗髮，彗髮受到太陽風和太陽的輻射壓導致背向太陽的巨大尾巴，稱為彗尾。

彗髮通常都由和塵埃構成，其中90%都是當彗星距離太陽就從彗核揮發出來的水。的母分子主要是通過光解和很多規模較小的光電離，還有太陽風扮演光化學的小角色而被摧毀（分解） 。較大的塵埃粉塵粒子沿著彗星軌道的路徑留下，而更小的粒子被光壓推入彗星的尾巴。

雖然固體的彗核一般都小於的直徑，但彗髮可能有數千或數百萬公里的直徑，有時會變得比太陽還要大。。例如，17P/霍姆斯彗星在2007年10月爆發之後大約一個月的短時間，巨大的大氣層就比太陽還要大；1811年大彗星的彗髮也大致與太陽的直徑相當。但即使彗髮再大，在它跨越火星，大約距離太陽，它的大小就會衰減。在這個距離上，太陽風已經足夠強大，可以將氣體和塵埃吹離彗髮，使尾巴增大。

當一顆彗星穿越內太陽系時，彗髮和尾巴都會被太陽照亮而能夠看得見，塵埃會直接反射陽光，而氣體會因為離子化而發光。大多數的彗星因為太暗淡，沒有望遠鏡的協助依然看不見，但每幾十年總會有亮到肉眼足以直接看見的彗星。偶爾，會遇到彗星突然爆發出大量的氣體和塵埃，這時彗髮的大小會增加一段時期。在2007年，17P/霍姆斯彗星就發生這樣的現象。

在1996年，發現彗星輻射出X射線。這使天文學家大為吃驚，因為X射線通常與高溫天體相關聯。X射線是彗星與太陽風的交互作用生成的：當高度電離的太陽風離子飛過彗星的大氣層時，它們與彗星大氣層中的原子和分子撞擊，會從它們獲得一個或多個電子，這個過程稱為」電荷交換」。這種交換或轉讓一個電子給太陽風中的離子讓離子去激發回到基態，導致輻射出X射線和遠紫外線光子 。

彗尾

在太陽系的外緣，彗星依然在冰凍和不活躍的狀態時，由於體積很小，因此很難甚至無法從地球上觀測到。來自哈伯太空望遠鏡的觀測報告，提出在古柏帶內不活躍彗核的統計報告，但是這些檢測不僅受到質疑，並且無法獨立驗證。當彗星接近太陽系的內側時，太陽輻射造成彗核內部揮發性物質蒸發，並且從核心向外噴出，同時會帶走一些塵埃粒子。

氣體和塵埃流會形成指向不同方向，自己獨特的彗尾。塵埃形成彎曲的尾巴會被拋在軌道的後方，通常稱為第二型彗尾。同時，離子尾，或是第一型彗尾總是指向背向太陽的地方，因為它們受到太陽風的作用遠比塵埃更強烈，因此是沿著磁場線而不是軌道的軌跡。在某些場合，如當地球穿越過彗星的軌道平面和我們從側面看見彗星，可能會看見與塵埃尾指向相反的塵埃尾，稱為彗翎（反尾）（在環繞太陽彗星前方的彗尾，與尾端的塵埃尾共線）。

對彗翎的觀察在太陽風的發現上有意義深遠的貢獻。離子尾是彗髮的微粒被太陽紫外線輻射電離後形成的。一旦粒子被電離，它們獲得淨正電核，並反過來在彗星附近引發」誘導磁層」。彗星和它的誘導磁層形成太陽風粒子向外流動的障礙。因為彗星的軌道速度和太陽風的速度都是超音速，弓形震波會在彗星運動和太陽風流動方向的前緣形成。在這些弓形震波，大量的彗星離子（稱為」拾取離子」）被凝聚和集中，並且載入太陽風的磁場和電漿，這樣的場線"披蓋"在彗星的周圍形成了離子尾。

如果離子尾的負載已經足夠了，則磁場線會在那個點上擠在一起，在沿著離子尾的某個距離上會發生磁重聯，這會導致"尾斷離事件"。這種現象已經被觀測到好幾次，在2007年4月20日就有一次值得注意的事件。當恩克彗星通過日冕拋射的物質的時候，它的離子尾就完全的被截斷了。日地關係天文台觀測到了這次的事件。

在2013年，歐洲太空總署的科學家報告金星的電離層向外擴張的方式類似於一顆彗星在類似條件下形成的離子尾。

噴流

不均勻的加熱會導致氣體從彗核表面較薄弱的點，像間歇泉一樣爆發出來。這些氣體和塵埃流會導致彗核旋轉，甚至分裂。在2010年，科學家發現乾冰（凍結的二氧化碳）可以驅動物質從彗核流出成為噴流。紅外線的影像顯示，哈特雷二號彗星的噴流攜帶灰塵顆粒進入彗髮。

與流星雨的關係

由於釋氣的緣故，彗星會留下一些固體的碎片。如果彗星的路徑跨越地球的路徑，當地球經過彗尾碎片的蹤跡，就有可能形成流星雨。例如，每年8月9日至12日，當地球穿越斯威夫特－塔特爾彗星的路徑時，形成的英仙座流星雨；哈雷彗星是10月份的獵戶座流星雨的來源。

軌道特性

大多數彗星都是細長橢圓軌道的太陽系小天體，它們的軌道只有一小部分接近太陽，剩餘的大部分都在深遠的太陽系外緣。彗星通常都以軌道週期的長短來分類：軌道週期越長的橢圓也越細長。

短週期

短週期彗星的定義一般是指週期短於200年的彗星。它們的軌道通常在黃道的上下，並且運行方向與行星相同。它們軌道的遠日點通常在外行星的區域（木星和超越其外）；例如，哈雷彗星的遠日點就在海王星之外不遠處。彗星軌道的遠日點靠近哪一顆行星，它就是該行星的彗星"家庭"這些家庭成員被認為是起因於被行星捕獲到週期較短軌道上的長週期彗星。

週期最短的極端，恩克彗星的軌道不會抵達木星的軌道，並且稱為恩克型彗星。短週期彗星中，週期短於20年和低傾角（不超過30度）的被稱為木星族彗星 。像哈雷彗星的，軌道週期在20至200年之間，軌道傾角從0至超過90度的，稱為哈雷族彗星。，只有72顆哈雷族彗星被觀測過，相較之下木星族彗星則幾乎有470顆。

最近發現的主帶彗星形成一個獨立的類別，不僅軌道在小行星帶內，而且還接近圓形。

因為其橢圓軌道經常會帶它們接近巨大的行星，彗星會受到進一步的重力擾動。短週期彗星的遠日點有趨近於氣體巨星軌道半徑的趨勢。很顯然的，來自歐特雲的彗星在接近巨大行星的時候，經常會受到這些行星強烈的影響。木星是最大的擾動源，因為它的質量是其他行星質量總和的兩倍。這些擾動可以將長週期彗星的軌道轉變成短週期的軌道。

基於其軌道特徵，有些短週期彗星被認為起源于半人馬和古柏帶/離散盤 —一個在海王星外側的盤狀區域—而長週期彗星的來源被認為是更遙遠的一個球形的歐特雲（以提出存在這個假想球殼的何蘭天文學家楊·亨德里克·歐特的名字命名）。一般認為在這個以太陽為中心，大致成球形的遙遠地區內，在大致是圓形的軌道上，存在著許多類似彗星的天體。偶爾，外側行星的影響力（這種情形通常是對古柏帶的天體），或是鄰近的恆星（這種情形通常是對歐特雲的天體）可能會將這些天體中的一顆拋入橢圓形的軌道，將它帶向太陽成為可以看見的彗星。不同於回歸的短週期彗星，沒有之前的觀測資料可以建立它們的軌道，通過這個機制產生的新彗星，其外觀是不可預知的。

長週期

長週期彗星有較高的離心率軌道和範圍從200年至數千乃至百萬年的週期，在近日點附近時，離心率大於1並不完全意味著這顆彗星會逃離太陽系。例如，麥克諾得彗星在2007年1月（曆元）接近近日點時的日心吻切軌道離心率是1.000019，但是它受到太陽的引力約束，週期約為92,600年，因為在它遠離太陽之後離心率已降至1以下。長週期彗星將來的軌道需要再它遠離行星所在的區域以後，再以太陽系的中心計算吻切軌道的曆元，才能確定。依據定義，長週期彗星依然受到太陽引力的約束；這些彗星在接近主要的行星時可能會被彈出太陽系，因此就無須考慮它原本的「週期」是否正確。長週期彗星的軌道會帶它們進入遠離外行星的遠日點，而且它們的軌道平面也不需要躺在黃道面附近。像威斯特彗星和C/1999 F1這些長週期彗星在重心座標系的拱點距離接近70,000天文單位，估計軌道週期大約長達600萬年。

單次出現或非週期彗星都類似長週期彗星，這是因為它們在進入內太陽系接近近日點時，都有拋物線或略呈雙曲線的軌跡。但是，這可能是巨大行星的攝動導致它們的軌道發生改變。單次出現或是有著拋物或雙曲吻切的彗星，會使它們在接近太陽一次之後，就永遠的離開太陽系。太陽的希爾球是一個不穩定的球體，最大的範圍可以達到230,000 AU。只有少數的數百顆彗星在接近近日點的附近時曾被觀測到雙曲線軌道（e > 1），在使用無攝動的日心二體最加擬合才認為它們可能會逃出太陽系。

已經觀測過的彗星，沒有離心率明顯大於1的所以沒有明確的證據可以指出有起源於太陽系外的彗星。C/1980 E1彗星的在1982年通過近日點之前的週期大約是710萬年，但是它在1980年與木星遭遇而被加速，使它成為已知彗星中離心率最大的（1.057）。預測不會再返回內太陽系的彗星包括C/1980 E1、C/2000 U5、C/2001 Q4 (NEAT)、C/2009 R1、C/1956 R1、和C/2007 F1 (LONEOS)。

有些機構使用週期彗星這個術語泛指軌道有週期性的彗星（也就是包括所有的短週期彗星和長週期彗星），而其他人使用它時則完全僅意味著短週期彗星。同樣的，雖然無週期彗星字面的意義是與「僅出現一次的彗星」是相同的，但有些人的意思是所有在有生之年不能看見第二次的彗星（也就是包括週期在200年以上的長週期彗星）。

早期的觀測顯示有幾顆彗星的軌跡真的是雙曲線軌道彗星（也就是無週期彗星），但都未超過被木星攝動而被加速的可能範圍。如果彗星充斥在星際空間內，它們的移動速度應該與臨近太陽的恆星有著相同數量級的相對速度（每秒數十公里的速度）。如果這樣的天體進入太陽系，它們應該有正值的特殊軌道能量，並將真正的觀測到有著雙曲線軌道。粗略的計算顯示，每世紀應該有4顆雙曲線軌道的彗星進入木星軌道的內側，並有著1或2等級的星等。

歐特雲和希爾雲

彗星的死亡

從太陽系排出

如果一顆彗星有足夠快的速度運行，那麼它可以離開太陽系；這就是雙曲線情況的彗星。到目前為止，已知會彈出太陽系的彗星都是曾和太陽系的其它天體，像是木星，發生過交互作用（參見攝動）。所有已知的彗星都起源於太陽系內，而不是以高速度的雙曲線軌道進入太陽系。

耗盡揮發物質

木星族彗星（JFC）和長週期彗星（LPC，參見前述的"軌道特性"）似乎遵循非常不同的衰退法則。木星族彗星的活動大約是10,000年，或是1,000次的公轉，而長週期彗星消失得更快。只有10%的長週期彗星能夠通過短距離的近日點50次依然存活著，而只有1%能超過2,000次。最終，大部分彗星的揮發性材料都會蒸發掉，使得彗星成為小而黑的惰性岩石，或是類似於小行星的廢墟。

瓦解（分裂）

彗星也會碎裂成為碎片，例如：比拉彗星（3D/Biela）于1846年發生分裂，1872年彗核完全分開，結果在1872、1885、1892年都引起非常壯觀的流星暴，每小時流星數達3000∼15000顆左右。73P/Schwassmann–Wachmann從1995年也開始發生這樣的現象。

這些分裂可能是太陽或大行星引力導致的潮汐力造成的，或是由於揮發性物質的"爆炸"，還是其他尚未完全明瞭的原因。

失蹤

許多在數十年前或前個世紀發現的彗星現在已經成為失蹤者了。它們或因為軌道不明確而難以預測未來的出現，或是已經瓦解了。然而，偶爾會發現一顆"新"彗星，但它們的軌道計算顯示，這是舊有的"失蹤"彗星。一個例子是11P/Tempel–Swift–LINEAR，在1869年發現，但在1908年受到木星的攝動就失蹤了，直到2001年才意外的被LINEAR再度發現。

碰撞

有些彗星有著更壯觀的結束－ 要麼落入太陽，或是粉碎後進入另一顆行星或天體。在太陽系的早期，彗星和行星或衛星之間的碰撞是很常見的：例如，地球的衛星表面有許多的撞擊坑，有些可能就是彗星造成的。最近一次彗星與行星的撞擊發生在1994年7月，粉碎了的舒梅克·利維九號彗星與木星相撞。

在早期的階段，有許多彗星和小行星因相撞而進入地球。許多科學家認為彗星的轟擊為年輕的地球（40億年前）帶來了大量的水，形成了目前鋪滿地球的海洋，即使不是全部也是很大的一部分。但也有其它的研究人員對這個理論產生質疑。在彗星上檢測到一些有機分子，使得有人推論彗星或隕石可能為地球帶來了生命的前身－ 甚至就是生物本身。依然有許多彗星是近地彗星，但是地球與小行星撞擊的機率還是高於彗星。

人們懷疑彗星的撞擊，在長時間的尺度上，也能運送大量的水給地球的衛星，所以可能有一些月球冰會留存下來。

彗星和隕石的撞擊被認為是玻璃隕石和澳洲玻璃隕體的成因。

命名規則

在過去的兩個世紀，彗星的命名有幾個不同的規則。在通過有系統的命名約定之前，有許多不同的命名方法。在20世紀的初期之前，大多數的彗星只簡單的依據它們出現的時間命名，特別是明亮的大彗星都只提及年份：像是"1680年大彗星"（C/1680 V1，Kirch's Comet）、"1882年9月大彗星（C/1882 R1）、和"1910年白晝大彗星（1910年1月大彗星）。

在愛德蒙·哈雷表明1531年、1607年、和1682年的彗星是同一顆，並且很成功的預測它在1759年回歸，這顆彗星就被稱為哈雷彗星。相同的，第二顆和第三顆週期彗星恩克彗星和比拉彗星也都是以計算它們軌道的天文學家，而不是最初（原始）發現者的名字命名。之後，週期彗星通常就以發現者的名字命名，但也只有第一次，之後的出現就以通過近日點的年份表示。

在20世紀初期，以發現者的名字為彗星命名變得非常普遍，並且迄今依然是如此，一顆彗星可以使用三位獨立發現者的名字。在最近這些年，許多彗星是由許多天文學家組織的大型團隊機構發現的，就以這個機構的名稱做為彗星的名字。例如，IRAS—荒貴—阿爾科克彗星（Comet IRAS–Araki–Alcock）是紅外線天文衛星（IRAS）、和業餘天文學家玄一荒木與喬治·阿爾科克獨立發現的。在過去，當多顆彗星是由同一個人、獨力的團隊或團隊發現時，會在彗星的名稱之後附加上數字（但限定是週期彗星），用來區別這些彗星；像是舒梅克－李維1至9號。現在，因為一些組織發現的彗星數量眾多，使得這樣的命名變得不切實際，也未能試圖確保每顆彗星有一個唯一的名稱。取而代之的是，使用系統化的彗星型號，藉以避免混淆。

直到1994年，彗星都會先給與一個臨時名稱，這是以發現的年份配合發現的先後順序加上一個小寫的英文字母（例如，1969 i（班尼特彗星）是1969年發現的第9顆彗星）。一旦觀測到這顆彗星通過近日點，並且確定了它的軌道之後，就根據它通過近日點的年份和順序的羅馬數字給與永久性的名稱（這編號通常是該年結束後二年才能編好）。所以彗星1969 i就成為彗星1970 II（它是1970年通過近日點的第二顆彗星），又如舒梅克·利維九號彗星的名稱分別為1993e和1994Ⅹ。

但越來越多的彗星被發現，而且有些是在通過近日點之後才被發現，使這套系統顯得不切實際。於是國際天文學聯合會在1994年推出新的彗星命名系統。從1995年開始，彗星在一年中以每半個月為單位使用一個字母和數字來指示發現的順序（這個系統和用於小行星的類似），所以，例如在2006年2月下半月發現的第4顆彗星，將被命名為2006 D4。此外，還添加前綴字母來顯示彗星的性質：

• P/：確認為週期彗星（目地在定義任何週期短於200年的彗星，或是確認已經觀測通過近日點超過一次以上的彗星）；P前面再加上周期彗星總表編號。所以，哈雷彗星，第一顆被確認週期的彗星，在系統內的名稱是1P/1682 Q1。

• C/ 標示無週期的彗星或週期超過200年的彗星。例如，海爾博普彗星的名稱為C/1995 O1。

• X/ 標示沒有可靠的軌道元素可以計算的彗星（一般來說都是歷史上的彗星）。

• D/ 標示不再回歸或已經消失、分裂或失蹤的彗星。

• A/ 標示被錯誤歸類為彗星，但其實是小行星的天體。

• I/ 標示來自太陽系外部的小天體，如A/2017 U1由於軌道表明來自太陽系外，後被命名為1I/'Omuamua，I表示Interstellar，即星際來客。

最初被當成小行星命名的彗星，在確認後仍然維持原有的名稱，但會加上前綴字母，例如（Catalina–LINEAR）。

在太陽系內，暨是彗星又是小行星的天體已經有五顆，它們分別是：

• 95P/開朗=2060開朗

• 107P/威爾遜-哈靈頓=4015威爾遜-哈靈頓

• 133P/Elst-Pizarro=7968 Elst-Pizarro

• 174P/Echeclus=60558厄開克洛斯

• 176P/LINEAR=118401LINEAR

如果彗星破碎，分裂成數個以上的彗核，則在編號後加上-A、-B..以區分每個彗核。回歸彗星方面，如彗星再次被觀測到回歸時，則在P/（或可能是D/）前加上一個由IAU小行星中心給定的序號，以避免該彗星回歸時重新標記。例如哈雷彗星有以下標記：1P/1682 Q1=1P/1910 A2=1P/1982 U1=1P/Halley=哈雷彗星。

著名的彗星

大彗星

雖然每年都有數以百計的小彗星進入內太陽系，但很少受到一般民眾的注意。大約每十年但不盡如此，會有一顆彗星亮到無須刻意觀察就能看見－ 這種彗星通常被稱為大彗星。在過去的時代，明亮的彗星往往引發一般民眾的恐慌和歇斯底里的反應，被認為是不好的徵兆。最近，在1910年重返的哈雷彗星，因為地球會通過它的彗尾，報紙上錯誤的報導激起民眾對氰化物的恐懼，認為可能會毒害數以萬計的生命，1997年海爾-波普彗星的出現，引起天堂之門教徒大規模的自殺潮。

預測一顆彗星是否能成為大彗星是很困難的，因為有許多因素都會影響到彗星偏離預測的亮度，而不知能否成為大彗星。概括的說，如果彗星有一顆龐大和活躍的核，並且足夠接近太陽，在最亮時沒有被太陽遮掩而能從地球看到，它就有機會成為大彗星。然而，1973年的柯侯德彗星符合前述所有的標準，被預測會成為壯觀的大彗星，但結果並非如此。三年後出現的威斯特彗星，大家對它的期望並不高（或許因為對柯侯德彗星預測的慘敗，使科學家們在預測上趨於保守），但卻成為令人印象深刻的彗星。

在20世紀末期，有很長的一段時間沒有出現大彗星，然後有兩顆大彗星接踵出現。在1996年繼海爾-波普彗星之後，百武彗星隨即現身，並在1997年達到最大亮度。21世紀的第一顆大彗星是C/2006 P1（麥克諾特），它在2007年1月成為肉眼可見的彗星，並且是40年來最亮的彗星。

掠日彗星

掠日彗星是指近日點極為接近太陽的彗星，有時其距離可接近至太陽表面僅數千公里。較小的掠日彗星會在接近太陽時被完全蒸發掉，而較大的彗星則可通過近日點多次。然而，太陽強大的潮汐力通常仍會使它們分裂。

SOHO觀測到的掠日彗星大約90%都是克魯茲族的成員，它們源自一顆在第一次進入內太陽系時就被碎裂成許多小彗星的巨大彗星。其它10%則包含一些零星的彗星，以及4個已經確定有所關聯的群體：分別為科里切特族（Kracht）、科里切特2a族、馬斯登族（Marsden）及邁耶族（Meyer）。馬斯登族和科里切特族或許與96P週期彗星——梅克賀茲一號彗星有所關聯，這顆彗星也可能是象限儀座流星雨和白羊座流星雨的母彗星。

不尋常的彗星

已知的數千顆彗星中，有些是很不尋常的。恩克彗星的軌道從小行星帶的外側進入到行星的水星軌道內側，而29P/施瓦斯曼·瓦茨曼彗星的軌道接近圓形，並且允型在木星和土星軌道之間。在土星和天王星之間的凱龍軌道並不穩定，起出被歸類為小行星，直到注意到它有著暗淡的彗髮，才被認為是彗星。同樣的，137P/舒梅克·利維2號彗星起初也被當成小行星。大約百分之六的近地小行星被認為是不再能排出氣體的熄火彗星。

有些彗星，包括威斯特彗星和池谷關彗星，在通過近日點時被觀察到分裂的現象。3D/比拉彗星是一個值得注意的例子，它在1846年通過近日點時分裂成兩塊，在1852年還觀測到這兩顆分離的彗星，但之後就沒有再看見。取而代之的是在彗星該回歸的1872年和1885年出現了壯觀的流星雨。在每年的11月，當地球跨越過比拉彗星的軌道時，都會出現一個較小的流星雨：仙女座流星雨。

另一顆值得注意的彗星是撞毀的舒梅克－李維九號彗星，它是在1993年被發現的。在發現的時候，這顆彗星的軌道環繞著木星，它是在1992年非常接近木星而被捕獲的。如此靠近的距離使這顆彗星碎裂成數百片，並在1994年7月花費了六天的時間陸續撞擊到木星上。1908年的通古斯事件也被認為可能是類似的事件，有可能是恩克彗星的碎片造成的。

觀測

使用廣視野望遠鏡攝影或雙筒望遠鏡都可能發現新彗星。然而，即使沒有光學設備，業餘天文學家依然可以從線上下載一些衛星的影像，像是SOHO衛星，發現掠日彗星。在2010年12月26日，業餘天文學家Michał Kusiak發現了第2,000顆SOHO的彗星，在可預見的未來，這個數字還會穩定的持續增加。

肉眼可見的彗星是非常罕見的，但業餘的天文望遠鏡（口徑50mm至100mm）就能精細顯示的彗星倒是相當的多－每年都有好幾顆，有時在一個夜晚，甚至同一個時間就能在夜空中看見好多顆。通常可以用天文軟體繪製這些已知彗星的軌道。相較於其它天體，它們會快速的移動，而在望遠鏡的目鏡中，它們的移動通常是很容易察覺的。但是，夜復一夜，它們的移動量也只有幾度，這就是為甚麼觀察者使用星圖就很容易發現它們，就像是在毗鄰的圖示。

彗星顯示的類型取決於其組成和與太陽接近的程度。因為一顆彗星的物質揮發會隨著它與太陽距離的增加而減少，彗星變得越來越難觀測，不只是因為它的距離，還有它的尾巴和用於反射的元素量逐漸的萎縮。

最引人注目的彗星是有著明亮的核心和展示出長長的尾巴，有時需要廣視野的小望遠鏡或雙筒望遠鏡才能獲得最好的景象。因此，大型的業餘儀器（口徑或更大）雖然有更好的集光力，但在觀賞彗星時不一定會有優勢。使用至等級的小口徑儀器能觀賞到的壯觀彗星很頻繁的，但較少受到注意，而其機會遠高於受到媒體關注而非常罕見的大彗星。

彗星被認為也會繞著其它的恆星運轉，但是對目前的系外行星偵測法而言，它們因太遠和太小而難以被檢測到。

對人類文化的影響

彗星奇特的形態，加上偶爾才能看到，古代許多地區的人們都把它視作上天的一種徵兆。在中國古代，人們把它看作災禍降臨的不祥之兆，稱之為「災星」。歐洲曾經把它當作上帝給予的預示。錢鍾書說：「古人每借天變以諫誡帝王」，「以彗星為『天教』、熒惑為『天罰』」，「然君主複即以此道還治臣工，有災異則譴咎公卿」。

大眾文化

在流行文化中，彗星常常被敘述為是預示世界末日和改變世界的預兆，而這個觀點也牢固地根植於西方的傳統看法中。哈雷彗星每次的回歸都會在各種類型的出版物上創造一系列聳動的新聞。其中特別受到注意的是，一些名人的出生和死亡與這顆彗星的回歸，像是馬克·吐溫（他曾預測自己會在彗星於1910年回歸時辭世，而他的預言後來也確實成真），和尤多拉·韋爾蒂（1909年出生），瑪麗·翠萍·卡本特以專曲哈雷來到傑克遜於1987年成名。

在科幻，彗星撞擊被用來描述克服技術困難與威脅的英雄主義（彗星撞地球，1998年的影片），或是用來觸發全球的危機（路西法的錘子，1979年影片），或成批的殭屍（彗星夜，1984年影片）。近期描述撞擊的有儒勒·凡爾納的遠離彗星和zh-cn:图苇·杨松;zh-hk:圖葦·楊松;zh-tw:朵貝·楊笙;的姆米穀的彗星，而大型的載人太空探測有亞瑟·查理斯·克拉克的小說：2061太空漫遊。

日本動畫也常有使用彗星的元素。早期如宇宙戰艦大和號中，來自河外星系的外星人建造的一個機動要塞，以一層類似白矮星的簡並態物質掩飾下高速飛向銀河系侵略時，給遠處的人類以為只是一顆白色的慧星。電視動畫夏洛特中，有一顆和影片標題相同的彗星，接近地球時使一部分青少年擁有了超能力。電影動畫你的名字中，以架空歷史的2013年一顆接近地球的彗星，一個女孩發夢中和男孩交換了靈魂，真相是天賦的預知的超能力。

相關條目

• 彗星酒

• 彗星列表

• 大崩潰 (書)

• 凱撒神廟

• 地球撞擊坑列表

注釋

進階讀物

• .

• Brandt, J.C. and Chapman, R.D.: Introduction to comets, Cambridge University Press 2004

• 《彗星─性質及觀測方法》，香港太空館編製，香港市政局出版，1986年初版，1997年3月再版；ISBN 962-7054-08-8／UC 10641

Etymology

The word comet derives from the Old English from the Latin or . That, in turn, is a romanization of the Greek 'wearing long hair', and the Oxford English Dictionary notes that the term already meant 'long-haired star, comet' in Greek. was derived from 'to wear the hair long', which was itself derived from 'the hair of the head' and was used to mean 'the tail of a comet'.

The astronomical symbol for comets (represented in Unicode) is , consisting of a small disc with three hairlike extensions.

Physical characteristics

Nucleus

The solid, core structure of a comet is known as the nucleus. Cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen carbon dioxide, carbon monoxide, methane, and ammonia. As such, they are popularly described as "dirty snowballs" after Fred Whipple's model. Comets with a higher dust content have been called "icy dirtballs". The term "icy dirtballs" arose after observation of Comet 9P/Tempel 1 collision with an "impactor" probe sent by NASA Deep Impact mission in July 2005. Research conducted in 2014 suggests that comets are like "deep fried ice cream", in that their surfaces are formed of dense crystalline ice mixed with organic compounds, while the interior ice is colder and less dense.

The surface of the nucleus is generally dry, dusty or rocky, suggesting that the ices are hidden beneath a surface crust several metres thick. In addition to the gases already mentioned, the nuclei contain a variety of organic compounds, which may include methanol, hydrogen cyanide, formaldehyde, ethanol, ethane, and perhaps more complex molecules such as long-chain hydrocarbons and amino acids. In 2009, it was confirmed that the amino acid glycine had been found in the comet dust recovered by NASA's Stardust mission. In August 2011, a report, based on NASA studies of meteorites found on Earth, was published suggesting DNA and RNA components (adenine, guanine, and related organic molecules) may have been formed on asteroids and comets.

The outer surfaces of cometary nuclei have a very low albedo, making them among the least reflective objects found in the Solar System. The Giotto space probe found that the nucleus of Halley's Comet (1P/Halley) reflects about four percent of the light that falls on it, and Deep Space 1 discovered that Comet Borrelly's surface reflects less than 3.0%; by comparison, asphalt reflects seven percent. The dark surface material of the nucleus may consist of complex organic compounds. Solar heating drives off lighter volatile compounds, leaving behind larger organic compounds that tend to be very dark, like tar or crude oil. The low reflectivity of cometary surfaces causes them to absorb the heat that drives their outgassing processes.

Comet nuclei with radii of up to have been observed, but ascertaining their exact size is difficult. The nucleus of 322P/SOHO is probably only in diameter. A lack of smaller comets being detected despite the increased sensitivity of instruments has led some to suggest that there is a real lack of comets smaller than across. Known comets have been estimated to have an average density of . Because of their low mass, comet nuclei do not become spherical under their own gravity and therefore have irregular shapes.

Roughly six percent of the near-Earth asteroids are thought to be the extinct nuclei of comets that no longer experience outgassing, including 14827 Hypnos and 3552 Don Quixote.

Results from the Rosetta and Philae spacecraft show that the nucleus of 67P/Churyumov–Gerasimenko has no magnetic field, which suggests that magnetism may not have played a role in the early formation of planetesimals. Further, the ALICE spectrograph on Rosetta determined that electrons (within above the comet nucleus) produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier, are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma. Instruments on the Philae lander found at least sixteen organic compounds at the comet's surface, four of which (acetamide, acetone, methyl isocyanate and propionaldehyde) have been detected for the first time on a comet.

Coma

The streams of dust and gas thus released form a huge and extremely thin atmosphere around the comet called the "coma". The force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous "tail" to form pointing away from the Sun.

The coma is generally made of water and dust, with water making up to 90% of the volatiles that outflow from the nucleus when the comet is within 3 to 4 astronomical units (450,000,000 to 600,000,000 km; 280,000,000 to 370,000,000 mi) of the Sun. The parent molecule is destroyed primarily through photodissociation and to a much smaller extent photoionization, with the solar wind playing a minor role in the destruction of water compared to photochemistry. Larger dust particles are left along the comet's orbital path whereas smaller particles are pushed away from the Sun into the comet's tail by light pressure.

Although the solid nucleus of comets is generally less than across, the coma may be thousands or millions of kilometers across, sometimes becoming larger than the Sun. For example, about a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun. The Great Comet of 1811 also had a coma roughly the diameter of the Sun. Even though the coma can become quite large, its size can decrease about the time it crosses the orbit of Mars around from the Sun. At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, and in doing so enlarging the tail. Ion tails have been observed to extend one astronomical unit (150 million km) or more.

Both the coma and tail are illuminated by the Sun and may become visible when a comet passes through the inner Solar System, the dust reflects sunlight directly while the gases glow from ionisation. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible to the naked eye. Occasionally a comet may experience a huge and sudden outburst of gas and dust, during which the size of the coma greatly increases for a period of time. This happened in 2007 to Comet Holmes.

In 1996, comets were found to emit X-rays. This greatly surprised astronomers because X-ray emission is usually associated with very high-temperature bodies. The X-rays are generated by the interaction between comets and the solar wind: when highly charged solar wind ions fly through a cometary atmosphere, they collide with cometary atoms and molecules, "stealing" one or more electrons from the atom in a process called "charge exchange". This exchange or transfer of an electron to the solar wind ion is followed by its de-excitation into the ground state of the ion by the emission of X-rays and far ultraviolet photons.

Bow shock

Bow shocks form as a result of the interaction between the solar wind and the cometary ionosphere, which is created by the ionization of gases in the coma. As the comet approaches the Sun, increasing outgassing rates cause the coma to expand, and the sunlight ionizes gases in the coma. When the solar wind passes through this ion coma, the bow shock appears.

The first observations were made in the 1980s and 90s as several spacecraft flew by comets 21P/Giacobini–Zinner, 1P/Halley, and 26P/Grigg–Skjellerup. It was then found that the bow shocks at comets are wider and more gradual than the sharp planetary bow shocks seen at, for example, Earth. These observations were all made near perihelion when the bow shocks already were fully developed.

The Rosetta spacecraft observed the bow shock at comet 67P/Churyumov–Gerasimenko at an early stage of bow shock development when the outgassing increased during the comet's journey toward the Sun. This young bow shock was called the "infant bow shock". The infant bow shock is asymmetric and, relative to the distance to the nucleus, wider than fully developed bow shocks.

Tails

In the outer Solar System, comets remain frozen and inactive and are extremely difficult or impossible to detect from Earth due to their small size. Statistical detections of inactive comet nuclei in the Kuiper belt have been reported from observations by the Hubble Space Telescope but these detections have been questioned. As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them.

The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail called the type II or dust tail. At the same time, the ion or type I tail, made of gases, always points directly away from the Sun because this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory. On occasions—such as when Earth passes through a comet's orbital plane, the antitail, pointing in the opposite direction to the ion and dust tails, may be seen.

The observation of antitails contributed significantly to the discovery of solar wind. The ion tail is formed as a result of the ionization by solar ultra-violet radiation of particles in the coma. Once the particles have been ionized, they attain a net positive electrical charge, which in turn gives rise to an "induced magnetosphere" around the comet. The comet and its induced magnetic field form an obstacle to outward flowing solar wind particles. Because the relative orbital speed of the comet and the solar wind is supersonic, a bow shock is formed upstream of the comet in the flow direction of the solar wind. In this bow shock, large concentrations of cometary ions (called "pick-up ions") congregate and act to "load" the solar magnetic field with plasma, such that the field lines "drape" around the comet forming the ion tail.

If the ion tail loading is sufficient, the magnetic field lines are squeezed together to the point where, at some distance along the ion tail, magnetic reconnection occurs. This leads to a "tail disconnection event". This has been observed on a number of occasions, one notable event being recorded on 20 April 2007, when the ion tail of Encke's Comet was completely severed while the comet passed through a coronal mass ejection. This event was observed by the STEREO space probe.

In 2013, ESA scientists reported that the ionosphere of the planet Venus streams outwards in a manner similar to the ion tail seen streaming from a comet under similar conditions."

Jets

Uneven heating can cause newly generated gases to break out of a weak spot on the surface of comet's nucleus, like a geyser. These streams of gas and dust can cause the nucleus to spin, and even split apart. In 2010 it was revealed dry ice (frozen carbon dioxide) can power jets of material flowing out of a comet nucleus. Infrared imaging of Hartley 2 shows such jets exiting and carrying with it dust grains into the coma.

Orbital characteristics

Most comets are small Solar System bodies with elongated elliptical orbits that take them close to the Sun for a part of their orbit and then out into the further reaches of the Solar System for the remainder. Comets are often classified according to the length of their orbital periods: The longer the period the more elongated the ellipse.

Short period

Periodic comets or short-period comets are generally defined as those having orbital periods of less than 200 years. They usually orbit more-or-less in the ecliptic plane in the same direction as the planets. Their orbits typically take them out to the region of the outer planets (Jupiter and beyond) at aphelion; for example, the aphelion of Halley's Comet is a little beyond the orbit of Neptune. Comets whose aphelia are near a major planet's orbit are called its "family". Such families are thought to arise from the planet capturing formerly long-period comets into shorter orbits.

At the shorter orbital period extreme, Encke's Comet has an orbit that does not reach the orbit of Jupiter, and is known as an Encke-type comet. Short-period comets with orbital periods less than 20 years and low inclinations (up to 30 degrees) to the ecliptic are called traditional Jupiter-family comets (JFCs). Those like Halley, with orbital periods of between 20 and 200 years and inclinations extending from zero to more than 90 degrees, are called Halley-type comets (HTCs). , 94 HTCs have been observed, compared with 725 identified JFCs.

Recently discovered main-belt comets form a distinct class, orbiting in more circular orbits within the asteroid belt.

Because their elliptical orbits frequently take them close to the giant planets, comets are subject to further gravitational perturbations. Short-period comets have a tendency for their aphelia to coincide with a giant planet's semi-major axis, with the JFCs being the largest group. It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined. These perturbations can deflect long-period comets into shorter orbital periods.

Based on their orbital characteristics, short-period comets are thought to originate from the centaurs and the Kuiper belt/scattered disc —a disk of objects in the trans-Neptunian region—whereas the source of long-period comets is thought to be the far more distant spherical Oort cloud (after the Dutch astronomer Jan Hendrik Oort who hypothesized its existence). Vast swarms of comet-like bodies are thought to orbit the Sun in these distant regions in roughly circular orbits. Occasionally the gravitational influence of the outer planets (in the case of Kuiper belt objects) or nearby stars (in the case of Oort cloud objects) may throw one of these bodies into an elliptical orbit that takes it inwards toward the Sun to form a visible comet. Unlike the return of periodic comets, whose orbits have been established by previous observations, the appearance of new comets by this mechanism is unpredictable. When flung into the orbit of the sun, and being continuously dragged towards it, tons of matter are stripped from the comets which greatly influence their lifetime; the more stripped, the shorter they live and vice versa.

Long period

Long-period comets have highly eccentric orbits and periods ranging from 200 years to thousands or even millions of years. An eccentricity greater than 1 when near perihelion does not necessarily mean that a comet will leave the Solar System. For example, Comet McNaught had a heliocentric osculating eccentricity of 1.000019 near its perihelion passage epoch in January 2007 but is bound to the Sun with roughly a 92,600-year orbit because the eccentricity drops below 1 as it moves farther from the Sun. The future orbit of a long-period comet is properly obtained when the osculating orbit is computed at an epoch after leaving the planetary region and is calculated with respect to the center of mass of the Solar System. By definition long-period comets remain gravitationally bound to the Sun; those comets that are ejected from the Solar System due to close passes by major planets are no longer properly considered as having "periods". The orbits of long-period comets take them far beyond the outer planets at aphelia, and the plane of their orbits need not lie near the ecliptic. Long-period comets such as C/1999 F1 and C/2017 T2 (PANSTARRS) can have aphelion distances of nearly with orbital periods estimated around 6 million years.

Single-apparition or non-periodic comets are similar to long-period comets because they also have parabolic or slightly hyperbolic trajectories when near perihelion in the inner Solar System. However, gravitational perturbations from giant planets cause their orbits to change. Single-apparition comets have a hyperbolic or parabolic osculating orbit which allows them to permanently exit the Solar System after a single pass of the Sun. The Sun's Hill sphere has an unstable maximum boundary of . Only a few hundred comets have been seen to reach a hyperbolic orbit (e > 1) when near perihelion that using a heliocentric unperturbed two-body best-fit suggests they may escape the Solar System.

, only two objects have been discovered with an eccentricity significantly greater than one: 1I/ʻOumuamua and 2I/Borisov, indicating an origin outside the Solar System. While ʻOumuamua, with an eccentricity of about 1.2, showed no optical signs of cometary activity during its passage through the inner Solar System in October 2017, changes to its trajectory—which suggests outgassing—indicate that it is probably a comet. On the other hand, 2I/Borisov, with an estimated eccentricity of about 3.36, has been observed to have the coma feature of comets, and is considered the first detected interstellar comet. Comet C/1980 E1 had an orbital period of roughly 7.1 million years before the 1982 perihelion passage, but a 1980 encounter with Jupiter accelerated the comet giving it the largest eccentricity (1.057) of any known solar comet with a reasonable observation arc. Comets not expected to return to the inner Solar System include C/1980 E1, C/2000 U5, C/2001 Q4 (NEAT), C/2009 R1, C/1956 R1, and C/2007 F1 (LONEOS).

Some authorities use the term "periodic comet" to refer to any comet with a periodic orbit (that is, all short-period comets plus all long-period comets), whereas others use it to mean exclusively short-period comets. Similarly, although the literal meaning of "non-periodic comet" is the same as "single-apparition comet", some use it to mean all comets that are not "periodic" in the second sense (that is, to also include all comets with a period greater than 200 years).

Early observations have revealed a few genuinely hyperbolic (i.e. non-periodic) trajectories, but no more than could be accounted for by perturbations from Jupiter. Comets from interstellar space are moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of km per second). When such objects enter the Solar System, they have a positive specific orbital energy resulting in a positive velocity at infinity (v_{\infty}\!) and have notably hyperbolic trajectories. A rough calculation shows that there might be four hyperbolic comets per century within Jupiter's orbit, give or take one and perhaps two orders of magnitude.

Oort cloud and Hills cloud

The Oort cloud is thought to occupy a vast space starting from between to as far as from the Sun. This cloud encases the celestial bodies that start at the middle of our solar system—the sun, all the way to outer limits of the Kuiper Belt. The Oort cloud consists of viable materials necessary for the creation of celestial bodies. The planets we have today, exist only because of the planetesimals (chunks of leftover space that assisted in the creation of planets) that were condensed and formed by the gravity of the sun. The eccentric made from these trapped planetesimals is why the Oort Cloud even exists. Some estimates place the outer edge at between . The region can be subdivided into a spherical outer Oort cloud of , and a doughnut-shaped inner cloud, the Hills cloud, of . The outer cloud is only weakly bound to the Sun and supplies the long-period (and possibly Halley-type) comets that fall to inside the orbit of Neptune. The inner Oort cloud is also known as the Hills cloud, named after J. G. Hills, who proposed its existence in 1981. Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo; it is seen as a possible source of new comets that resupply the relatively tenuous outer cloud as the latter's numbers are gradually depleted. The Hills cloud explains the continued existence of the Oort cloud after billions of years.

Exocomets

Exocomets beyond the Solar System have also been detected and may be common in the Milky Way. The first exocomet system detected was around Beta Pictoris, a very young A-type main-sequence star, in 1987. A total of 11 such exocomet systems have been identified , using the absorption spectrum caused by the large clouds of gas emitted by comets when passing close to their star. For ten years the Kepler space telescope was responsible for searching for planets and other forms outside of the solar system. The first transiting exocomets were found in February 2018 by a group consisting of professional astronomers and citizen scientists in light curves recorded by the Kepler Space Telescope. After Kepler Space Telescope retired in October 2018, a new telescope called TESS Telescope has taken over Kepler's mission. Since the launch of TESS, astronomers have discovered the transits of comets around the star Beta Pictoris using a light curve from TESS. Since TESS has taken over, astronomers have since been able to better distinguish exocomets with the spectroscopic method. New planets are detected by the white light curve method which is viewed as a symmetrical dip in the charts readings when a planet overshadows its parent star. However, after further evaluation of these light curves, it has been discovered that the asymmetrical patterns of the dips presented are caused by the tail of a comet or of hundreds of comets.

Effects of comets

Connection to meteor showers

As a comet is heated during close passes to the Sun, outgassing of its icy components also releases solid debris too large to be swept away by radiation pressure and the solar wind. If Earth's orbit sends it through that trail of debris, which is composed mostly of fine grains of rocky material, there is likely to be a meteor shower as Earth passes through. Denser trails of debris produce quick but intense meteor showers and less dense trails create longer but less intense showers. Typically, the density of the debris trail is related to how long ago the parent comet released the material. The Perseid meteor shower, for example, occurs every year between 9 and 13 August, when Earth passes through the orbit of Comet Swift–Tuttle. Halley's Comet is the source of the Orionid shower in October.

Comets and impact on life

Many comets and asteroids collided with Earth in its early stages. Many scientists think that comets bombarding the young Earth about 4 billion years ago brought the vast quantities of water that now fill Earth's oceans, or at least a significant portion of it. Others have cast doubt on this idea. The detection of organic molecules, including polycyclic aromatic hydrocarbons, in significant quantities in comets has led to speculation that comets or meteorites may have brought the precursors of life—or even life itself—to Earth. In 2013 it was suggested that impacts between rocky and icy surfaces, such as comets, had the potential to create the amino acids that make up proteins through shock synthesis. The speed at which the comets entered the atmosphere, combined with the magnitude of energy created after initial contact, allowed smaller molecules to condense into the larger macro-molecules that served as the foundation for life. In 2015, scientists found significant amounts of molecular oxygen in the outgassings of comet 67P, suggesting that the molecule may occur more often than had been thought, and thus less an indicator of life as has been supposed.

It is suspected that comet impacts have, over long timescales, also delivered significant quantities of water to Earth's Moon, some of which may have survived as lunar ice. Comet and meteoroid impacts are also thought to be responsible for the existence of tektites and australites.

Fear of comets

Fear of comets as acts of God and signs of impending doom was highest in Europe from AD 1200 to 1650. The year after the Great Comet of 1618, for example, Gotthard Arthusius published a pamphlet stating that it was a sign that the Day of Judgment was near. He listed ten pages of comet-related disasters, including "earthquakes, floods, changes in river courses, hail storms, hot and dry weather, poor harvests, epidemics, war and treason and high prices".

By 1700 most scholars concluded that such events occurred whether a comet was seen or not. Using Edmond Halley's records of comet sightings, however, William Whiston in 1711 wrote that the Great Comet of 1680 had a periodicity of 574 years and was responsible for the worldwide flood in the Book of Genesis, by pouring water on Earth. His announcement revived for another century fear of comets, now as direct threats to the world instead of signs of disasters. Spectroscopic analysis in 1910 found the toxic gas cyanogen in the tail of Halley's Comet, causing panicked buying of gas masks and quack "anti-comet pills" and "anti-comet umbrellas" by the public.

Fate of comets

Departure (ejection) from Solar System

If a comet is traveling fast enough, it may leave the Solar System. Such comets follow the open path of a hyperbola, and as such, they are called hyperbolic comets. Solar comets are only known to be ejected by interacting with another object in the Solar System, such as Jupiter. An example of this is Comet C/1980 E1, which was shifted from an orbit of 7.1 million years around the Sun, to a hyperbolic trajectory, after a 1980 close pass by the planet Jupiter. Interstellar comets such as 1I/ʻOumuamua and 2I/Borisov never orbited the Sun and therefore do not require a 3rd-body interaction to be ejected from the Solar System.

Volatiles exhausted

Jupiter-family comets and long-period comets appear to follow very different fading laws. The JFCs are active over a lifetime of about 10,000 years or ~1,000 orbits whereas long-period comets fade much faster. Only 10% of the long-period comets survive more than 50 passages to small perihelion and only 1% of them survive more than 2,000 passages. Eventually most of the volatile material contained in a comet nucleus evaporates, and the comet becomes a small, dark, inert lump of rock or rubble that can resemble an asteroid. Some asteroids in elliptical orbits are now identified as extinct comets. Roughly six percent of the near-Earth asteroids are thought to be extinct comet nuclei.

Breakup and collisions

The nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart. A significant cometary disruption was that of Comet Shoemaker–Levy 9, which was discovered in 1993. A close encounter in July 1992 had broken it into pieces, and over a period of six days in July 1994, these pieces fell into Jupiter's atmosphere—the first time astronomers had observed a collision between two objects in the Solar System. Other splitting comets include 3D/Biela in 1846 and 73P/Schwassmann–Wachmann from 1995 to 2006. Greek historian Ephorus reported that a comet split apart as far back as the winter of 372–373 BC. Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact.

Comets 42P/Neujmin and 53P/Van Biesbroeck appear to be fragments of a parent comet. Numerical integrations have shown that both comets had a rather close approach to Jupiter in January 1850, and that, before 1850, the two orbits were nearly identical.

Some comets have been observed to break up during their perihelion passage, including great comets West and Ikeya–Seki. Biela's Comet was one significant example when it broke into two pieces during its passage through the perihelion in 1846. These two comets were seen separately in 1852, but never again afterward. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A minor meteor shower, the Andromedids, occurs annually in November, and it is caused when Earth crosses the orbit of Biela's Comet.

Some comets meet a more spectacular end – either falling into the Sun or smashing into a planet or other body. Collisions between comets and planets or moons were common in the early Solar System: some of the many craters on the Moon, for example, may have been caused by comets. A recent collision of a comet with a planet occurred in July 1994 when Comet Shoemaker–Levy 9 broke up into pieces and collided with Jupiter.

Nomenclature

The names given to comets have followed several different conventions over the past two centuries. Prior to the early 20th century, most comets were simply referred to by the year when they appeared, sometimes with additional adjectives for particularly bright comets; thus, the "Great Comet of 1680", the "Great Comet of 1882", and the "Great January Comet of 1910".

After Edmond Halley demonstrated that the comets of 1531, 1607, and 1682 were the same body and successfully predicted its return in 1759 by calculating its orbit, that comet became known as Halley's Comet. Similarly, the second and third known periodic comets, Encke's Comet and Biela's Comet, were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred to by the year of their appearance.

In the early 20th century, the convention of naming comets after their discoverers became common, and this remains so today. A comet can be named after its discoverers or an instrument or program that helped to find it. For example, in 2019, astronomer Gennady Borisov observed a comet that appeared to have originated outside of the solar system; the comet was named C/2019 Q4 (Borisov) after him.

History of study

Early observations and thought

From ancient sources, such as Chinese oracle bones, it is known that comets have been noticed by humans for millennia. Until the sixteenth century, comets were usually considered bad omens of deaths of kings or noble men, or coming catastrophes, or even interpreted as attacks by heavenly beings against terrestrial inhabitants.

Aristotle (384–322 BC) was the first known scientist to utilize various theories and observational facts to employ a consistent, structured cosmological theory of comets. He believed that comets were atmospheric phenomena, due to the fact that they could appear outside of the zodiac and vary in brightness over the course of a few days. Aristotle's cometary theory arose from his observations and cosmological theory that everything in the cosmos is arranged in a distinct configuration. Part of this configuration was a clear separation between the celestial and terrestrial, believing comets to be strictly associated with the latter. According to Aristotle, comets must be within the sphere of the moon and clearly separated from the heavens. Also in the 4th century BC, Apollonius of Myndus supported the idea that comets moved like the planets. Aristotelian theory on comets continued to be widely accepted throughout the Middle Ages, despite several discoveries from various individuals challenging aspects of it.

In the 1st century AD, Seneca the Younger questioned Aristotle's logic concerning comets. Because of their regular movement and imperviousness to wind, they cannot be atmospheric, and are more permanent than suggested by their brief flashes across the sky. He pointed out that only the tails are transparent and thus cloudlike, and argued that there is no reason to confine their orbits to the zodiac. In criticizing Apollonius of Myndus, Seneca argues, "A comet cuts through the upper regions of the universe and then finally becomes visible when it reaches the lowest point of its orbit." While Seneca did not author a substantial theory of his own, his arguments would spark much debate among Aristotle's critics in the 16th and 17th centuries.

Also in the 1st century, Pliny the Elder believed that comets were connected with political unrest and death. Pliny observed comets as "human like", often describing their tails with "long hair" or "long beard". His system for classifying comets according to their color and shape was used for centuries.

In India, by the 6th century astronomers believed that comets were celestial bodies that re-appeared periodically. This was the view expressed in the 6th century by the astronomers Varāhamihira and Bhadrabahu, and the 10th-century astronomer Bhaṭṭotpala listed the names and estimated periods of certain comets, but it is not known how these figures were calculated or how accurate they were.

In the 11th century Bayeux Tapestry, Halley's Comet is depicted portending the death of Harold and the triumph of the Normans at the Battle of Hastings.

According to Norse mythology, comets were actually a part of the Giant Ymir's skull. According to the tale, Odin and his brothers slew Ymir and set about constructing the world (Earth) from his corpse. They fashioned the oceans from his blood, the soil from his skin and muscles, vegetation from his hair, clouds from his brains, and the sky from his skull. Four dwarves, corresponding to the four cardinal points, held Ymir's skull aloft above the earth. Following this tale, comets in the sky, as believed by the Norse, were flakes of Ymir's skull falling from the sky and then disintegrating.

In 1301, the Italian painter Giotto was the first person to accurately and anatomically portray a comet. In his work Adoration of the Magi, Giotto's depiction of Halley's Comet in the place of the Star of Bethlehem would go unmatched in accuracy until the 19th century and be bested only with the invention of photography.

Astrological interpretations of comets proceeded to take precedence clear into the 15th century, despite the presence of modern scientific astronomy beginning to take root. Comets continued to forewarn of disaster, as seen in the Luzerner Schilling chronicles and in the warnings of Pope Callixtus III. In 1578, German Lutheran bishop Andreas Celichius defined comets as "the thick smoke of human sins ... kindled by the hot and fiery anger of the Supreme Heavenly Judge". The next year, Andreas Dudith stated that "If comets were caused by the sins of mortals, they would never be absent from the sky."

Scientific approach

Crude attempts at a parallax measurement of Halley's Comet were made in 1456, but were erroneous. Regiomontanus was the first to attempt to calculate diurnal parallax by observing the great comet of 1472. His predictions were not very accurate, but they were conducted in the hopes of estimating the distance of a comet from the Earth.

In the 16th century, Tycho Brahe and Michael Maestlin demonstrated that comets must exist outside of Earth's atmosphere by measuring the parallax of the Great Comet of 1577. Within the precision of the measurements, this implied the comet must be at least four times more distant than from Earth to the Moon. Based on observations in 1664, Giovanni Borelli recorded the longitudes and latitudes of comets that he observed, and suggested that cometary orbits may be parabolic. Galileo Galilei, one of the most renowned astronomers to date, even attempted writings on comets in The Assayer. He rejected Brahe's theories on the parallax of comets and claimed that they may be a mere optical illusion. Intrigued as early scientists were about the nature of comets, Galileo could not help but throw about his own theories despite little personal observation. Maestlin's student Johannes Kepler responded to these unjust criticisms in his work Hyperaspistes. Jakob Bernoulli published another attempt to explain comets (Conamen Novi Systematis Cometarum) in 1682.

Also occurring in the early modern period was the study of comets and their astrological significance in medical disciplines. Many healers of this time considered medicine and astronomy to be inter-disciplinary and employed their knowledge of comets and other astrological signs for diagnosing and treating patients.

Isaac Newton, in his Principia Mathematica of 1687, proved that an object moving under the influence of gravity by an inverse square law must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the comet of 1680 as an example.

He describes comets as compact and durable solid bodies moving in oblique orbit and their tails as thin streams of vapor emitted by their nuclei, ignited or heated by the Sun. He suspected that comets were the origin of the life-supporting component of air. He also pointed out that comets usually appear near the Sun, and therefore most likely orbit it. On their luminosity, he stated, "The comets shine by the Sun's light, which they reflect," with their tails illuminated by "the Sun's light reflected by a smoke arising from coma".

In 1705, Edmond Halley (1656–1742) applied Newton's method to 23 cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar orbital elements, and he was further able to account for the slight differences in their orbits in terms of gravitational perturbation caused by Jupiter and Saturn. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 1758–9. Halley's predicted return date was later refined by a team of three French mathematicians: Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, who predicted the date of the comet's 1759 perihelion to within one month's accuracy. When the comet returned as predicted, it became known as Halley's Comet.

As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized in his Universal Natural History that comets were condensed from "primitive matter" beyond the known planets, which is "feebly moved" by gravity, then orbit at arbitrary inclinations, and are partially vaporized by the Sun's heat as they near perihelion. In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor during the appearance of Halley's Comet in 1835, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit, and he argued that the non-gravitational movements of Encke's Comet resulted from this phenomenon.

In the 19th century, the Astronomical Observatory of Padova was an epicenter in the observational study of comets. Led by Giovanni Santini (1787–1877) and followed by Giuseppe Lorenzoni (1843–1914), this observatory was devoted to classical astronomy, mainly to the new comets and planets orbit calculation, with the goal of compiling a catalog of almost ten thousand stars. Situated in the Northern portion of Italy, observations from this observatory were key in establishing important geodetic, geographic, and astronomical calculations, such as the difference of longitude between Milan and Padua as well as Padua to Fiume. In addition to these geographic observations, correspondence within the observatory, particularly between Santini and another astronomer Giuseppe Toaldo, about the importance of comet and planetary orbital observations.

In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock. This "dirty snowball" model soon became accepted and appeared to be supported by the observations of an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) that flew through the coma of Halley's Comet in 1986, photographed the nucleus, and observed jets of evaporating material.

On 22 January 2014, ESA scientists reported the detection, for the first definitive time, of water vapor on the dwarf planet Ceres, the largest object in the asteroid belt. The detection was made by using the far-infrared abilities of the Herschel Space Observatory. The finding is unexpected because comets, not asteroids, are typically considered to "sprout jets and plumes". According to one of the scientists, "The lines are becoming more and more blurred between comets and asteroids." On 11 August 2014, astronomers released studies, using the Atacama Large Millimeter/Submillimeter Array (ALMA) for the first time, that detailed the distribution of HCN, HNC, , and dust inside the comae of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON).

Spacecraft missions

• The Halley Armada describes the collection of spacecraft missions that visited and/or made observations of Halley's Comet 1980s perihelion. The space shuttle Challenger was intended to do a study of Halley's Comet in 1986, but exploded shortly after being launched.

• Deep Impact. Debate continues about how much ice is in a comet. In 2001, the Deep Space 1 spacecraft obtained high-resolution images of the surface of Comet Borrelly. It was found that the surface of comet Borrelly is hot and dry, with a temperature of between , and extremely dark, suggesting that the ice has been removed by solar heating and maturation, or is hidden by the soot-like material that covers Borrelly. In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. The mission yielded results suggesting that the majority of a comet's water ice is below the surface and that these reservoirs feed the jets of vaporized water that form the coma of Tempel 1. Renamed EPOXI, it made a flyby of Comet Hartley 2 on 4 November 2010.

• Ulysses. In 2007, the Ulysses probe unexpectedly passed through the tail of the comet C/2006 P1 (McNaught) which was discovered in 2006. Ulysses was launched in 1990 and the intended mission was for Ulysses to orbit around the sun for further study at all latitudes.

• Stardust. Data from the Stardust mission show that materials retrieved from the tail of Wild 2 were crystalline and could only have been "born in fire", at extremely high temperatures of over . Although comets formed in the outer Solar System, radial mixing of material during the early formation of the Solar System is thought to have redistributed material throughout the proto-planetary disk. As a result, comets also contain crystalline grains that formed in the early, hot inner Solar System. This is seen in comet spectra as well as in sample return missions. More recent still, the materials retrieved demonstrate that the "comet dust resembles asteroid materials". These new results have forced scientists to rethink the nature of comets and their distinction from asteroids.

• Rosetta. The Rosetta probe orbited Comet Churyumov–Gerasimenko. On 12 November 2014, its lander Philae successfully landed on the comet's surface, the first time a spacecraft has ever landed on such an object in history.

Classification

Great comets

Approximately once a decade, a comet becomes bright enough to be noticed by a casual observer, leading such comets to be designated as great comets. Predicting whether a comet will become a great comet is notoriously difficult, as many factors may cause a comet's brightness to depart drastically from predictions. Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from Earth when at its brightest, it has a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular but failed to do so. Comet West, which appeared three years later, had much lower expectations but became an extremely impressive comet.

The Great Comet of 1577 is a well-known example of a great comet. It passed near Earth as a non-periodic comet and was seen by many, including well-known astronomers Tycho Brahe and Taqi ad-Din. Observations of this comet led to several significant findings regarding cometary science, especially for Brahe.

The late 20th century saw a lengthy gap without the appearance of any great comets, followed by the arrival of two in quick succession—Comet Hyakutake in 1996, followed by Hale–Bopp, which reached maximum brightness in 1997 having been discovered two years earlier. The first great comet of the 21st century was C/2006 P1 (McNaught), which became visible to naked eye observers in January 2007. It was the brightest in over 40 years.

Sungrazing comets

A sungrazing comet is a comet that passes extremely close to the Sun at perihelion, generally within a few million kilometers. Although small sungrazers can be completely evaporated during such a close approach to the Sun, larger sungrazers can survive many perihelion passages. However, the strong tidal forces they experience often lead to their fragmentation.

About 90% of the sungrazers observed with SOHO are members of the Kreutz group, which all originate from one giant comet that broke up into many smaller comets during its first passage through the inner Solar System. The remainder contains some sporadic sungrazers, but four other related groups of comets have been identified among them: the Kracht, Kracht 2a, Marsden, and Meyer groups. The Marsden and Kracht groups both appear to be related to Comet 96P/Machholz, which is also the parent of two meteor streams, the Quadrantids and the Arietids.

Unusual comets

Of the thousands of known comets, some exhibit unusual properties. Comet Encke (2P/Encke) orbits from outside the asteroid belt to just inside the orbit of the planet Mercury whereas the Comet 29P/Schwassmann–Wachmann currently travels in a nearly circular orbit entirely between the orbits of Jupiter and Saturn. 2060 Chiron, whose unstable orbit is between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed. Similarly, Comet Shoemaker–Levy 2 was originally designated asteroid .

Largest

The largest known periodic comet is 95P/Chiron at 200 km in diameter that comes to perihelion every 50 years just inside of Saturn's orbit at 8 AU. The largest known Oort cloud comet is suspected of being Comet Bernardinelli-Bernstein at ≈150 km that will not come to perihelion until January 2031 just outside of Saturn's orbit at 11 AU. The Comet of 1729 is estimated to have been ≈100 km in diameter and came to perihelion inside of Jupiter's orbit at 4 AU.

Centaurs

Centaurs typically behave with characteristics of both asteroids and comets. Centaurs can be classified as comets such as 60558 Echeclus, and 166P/NEAT. 166P/NEAT was discovered while it exhibited a coma, and so is classified as a comet despite its orbit, and 60558 Echeclus was discovered without a coma but later became active, and was then classified as both a comet and an asteroid (174P/Echeclus). One plan for Cassini involved sending it to a centaur, but NASA decided to destroy it instead.

Observation

A comet may be discovered photographically using a wide-field telescope or visually with binoculars. However, even without access to optical equipment, it is still possible for the amateur astronomer to discover a sungrazing comet online by downloading images accumulated by some satellite observatories such as SOHO. SOHO's 2000th comet was discovered by Polish amateur astronomer Michał Kusiak on 26 December 2010 and both discoverers of Hale–Bopp used amateur equipment (although Hale was not an amateur).

Lost

A number of periodic comets discovered in earlier decades or previous centuries are now lost comets. Their orbits were never known well enough to predict future appearances or the comets have disintegrated. However, occasionally a "new" comet is discovered, and calculation of its orbit shows it to be an old "lost" comet. An example is Comet 11P/Tempel–Swift–LINEAR, discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It was not found again until accidentally rediscovered by LINEAR in 2001. There are at least 18 comets that fit this category.

In popular culture

The depiction of comets in popular culture is firmly rooted in the long Western tradition of seeing comets as harbingers of doom and as omens of world-altering change. Halley's Comet alone has caused a slew of sensationalist publications of all sorts at each of its reappearances. It was especially noted that the birth and death of some notable persons coincided with separate appearances of the comet, such as with writers Mark Twain (who correctly speculated that he'd "go out with the comet" in 1910) and Eudora Welty, to whose life Mary Chapin Carpenter dedicated the song "Halley Came to Jackson".

In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley's Comet in 1910, Earth passed through the comet's tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions, whereas the appearance of Comet Hale–Bopp in 1997 triggered the mass suicide of the Heaven's Gate cult.

In science fiction, the impact of comets has been depicted as a threat overcome by technology and heroism (as in the 1998 films Deep Impact and Armageddon), or as a trigger of global apocalypse (Lucifer's Hammer, 1979) or zombies (Night of the Comet, 1984). In Jules Verne's Off on a Comet a group of people are stranded on a comet orbiting the Sun, while a large manned space expedition visits Halley's Comet in Sir Arthur C. Clarke's novel 2061: Odyssey Three.

Gallery

File:Comet_C2020F3_NEOWISE_over_California_desert_landscape.png|Comet C/2020 F3 NEOWISE

File:Comet P1 McNaught02 - 23-01-07-edited.jpg|Comet C/2006 P1 (McNaught) taken from Victoria, Australia 2007

File:Great Comet of 1882.jpg|The Great Comet of 1882 is a member of the Kreutz group

File:Great Comet 1861.jpg|Great Comet 1861

File:X-rays from Hyakutake.jpg|Comet Hyakutake (X-ray, ROSAT satellite)

File:Asteroid P2013 P5 v2.jpg|"Active asteroid" 311P/PANSTARRS with several tails

File:NASA-14090-Comet-C2013A1-SidingSpring-Hubble-20140311.jpg|Comet Siding Spring (Hubble; 11 March 2014)

File:Comets WISE.jpg|Mosaic of 20 comets discovered by the WISE space telescope

File:PIA22419-Neowise-1stFourYearsDataFromDec2013-20180420.gif|NEOWISE – first four years of data starting in December 2013

File:Lovejoy-hi1a srem dec12 14.gif|C/2011 W3 (Lovejoy) heads towards the Sun

File:ITS Impact.gif|View from the impactor in its last moments before hitting Comet Tempel 1 during the Deep Impact mission

;Videos

File:NASA Developing Comet Harpoon for Sample Return.ogv|NASA is developing a comet harpoon for returning samples to Earth

File:Encke tail rip off.ogg|Comet Encke loses its tail