asteroid

• 星状の、(天文)小惑星

関

astral、stellate

　 　 　

WordNet

1. shaped like a star (同)star-shaped
2. any of numerous small celestial bodies composed of rock and metal that move around the sun (mainly between the orbits of Mars and Jupiter)
3. the region of interplanetary space between Mars and Jupiter where most asteroids are found
4. an asteroid whose orbit crosses the Earths orbit
5. of or relating to or resembling an asteroid

PrepTutorEJDIC

1. 星状の / (また『planetoid』)小惑星 / ヒトデ
2. 星の,星からの

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2015/07/13 21:24:22」(JST)

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太陽系の天体の分類

恒星（太陽）
太陽の 周りを 回る 天体	惑星		地球型惑星
			木星型惑星
			天王星型惑星
	準惑星
			小惑星帯にあるもの （ケレスのみ）
			冥王星型天体
	太陽系 小天体		冥王星型天体以外の 太陽系外縁天体
			小惑星
			彗星
			惑星間塵
	太陽以外の 天体の周りを 回る天体		衛星（未定義）
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小惑星帯 小惑星は主として火星軌道と木星軌道の中間に分布する。このほか、木星軌道上の太陽から見て木星に対して前後60度の位置にトロヤ群と呼ばれる小惑星の集まりが存在する。図の単位は光分（左）と天文単位（右）。

小惑星の形と大きさ 近接探査が行われたガスプラ、エロス、イダ（中段左から中央、左上は拡大したもの）、唯一地球から肉眼で目視できるベスタ（中央右寄り）、小惑星帯では最大の天体であり、最初に発見された小惑星でもあったケレス（右）、そして火星（下）。小さな物ほど不規則な形状になっている。

メインベルト小惑星の分布。縦軸は軌道傾斜角。

軌道長半径 6 AU までの小惑星の分布。縦軸は軌道傾斜角。赤い点はメインベルト小惑星。

小惑星（しょうわくせい、英: asteroid）は、太陽系小天体のうち、星像に拡散成分がないものの総称。拡散成分（コマやそこから流出した尾）があるものは彗星と呼ばれる。

概要

ウィリアム・ハーシェルによって、（当時の）望遠鏡で見ると恒星のように見えることから、ギリシャ語の αστηρ（aster：恒星）と ειδος（eidos：姿、形）からアステロイド「asteroid：恒星のようなもの」と命名された。太陽系内の惑星より小さな天体であることから「minor planet：小さな惑星」、「planetoid：惑星のようなもの」などとも呼ばれた。現在では minor planet のうち、岩石を主成分とするものを asteroid と称する。

その多くは火星と木星の間の軌道を公転しているが、地球付近を通過する可能性のあるものも存在する。21世紀初頭まで最大の小惑星であった (1) ケレス（Ceres：数字は軌道が確定した小惑星に付けられる小惑星番号 (minor planet number)。以下同様）でも地球の月よりはるかに小さい。

また、惑星や衛星のような球形をしているのはケレスなどごく一部の大型の小惑星のみで、大半は丸みを帯びた不定形である。

位置

ほとんどの小惑星は、木星軌道と火星軌道の間に存在し、太陽からの距離が約2 - 4天文単位の範囲に集まっている。この領域を小惑星帯 (asteroid belt) と呼ぶ。現在では太陽系外縁部のエッジワース・カイパーベルトと区別するためにメインベルト (main belt) とも呼ばれる。小惑星は木星の摂動によって、いくつかの群をなして運動する。各群はその公転周期にしたがって分類される。群の中で特に注目されるのが、トロヤ群（周期約12年）と呼ばれる小惑星群であり、これは太陽と木星との間を一辺とする正三角形の一頂点、すなわち両天体の系でのラグランジュ点に位置することが知られている。なお、トロヤ群の名は、この群で最初に発見された小惑星 (588) アキレス (Achilles) にちなむ。

1990年代以降は (50000) クワオアー (Quaoar) や (90377) セドナ (Sedna) といった、エッジワース・カイパーベルトや、さらにその外側にあるtrans-Neptunian objects（太陽系外縁天体、TNO）が続々と発見されるようになった。これらはメインベルトの小惑星 (asteroid) とは起源が異なると考えられているが、同様に小惑星 (Minor planet) として登録されている。

歴史

1781年の天王星発見当時、ティティウス・ボーデの法則から、火星と木星の間に未知の惑星を探索する試みが行われた。1801年に (1) ケレスが発見されたが、翌1802年に (2) パラス、1804年に (3) ジュノー、1807年には (4) ベスタと、同じような位置に天体が相次いで発見されたこと、またいずれも惑星と呼ぶにはあまりに小さいことから、やがて惑星とは区別されるようになった。小惑星 (asteroid) という語は、1853年初めに考え出された。

2006年8月にプラハで開かれた国際天文学連合 (IAU) 総会で惑星の定義が採択された結果、それまで惑星とされていた冥王星および小惑星とされていたケレスと2003 UB₃₁₃（エリス）が dwarf planet（準惑星）に変更され、さらに小惑星のうち十数個が将来的に dwarf planet に変更される可能性があると考えられるようになった（2008年には、新たにマケマケとハウメアが dwarf planet に変更されている）。また小惑星はTNOや彗星とともにsmall solar system bodies (太陽系小天体、SSBO) というカテゴリーに包括されることになった。

これを受けて、日本学術会議の小委員会は2007年4月9日の対外報告（第一報告）において、dwarf planet、TNO、SSBO の訳語としてそれぞれ「準惑星」「太陽系外縁天体」「太陽系小天体」の使用を推奨することを提言した。なお、準惑星については当面の間、教育現場などでは積極的な使用を推奨しない方針（詳細は惑星#日本学術会議の対外報告を参照）。

数

2012年5月現在、軌道が確定して小惑星番号が付けられた天体は329,243個にのぼる（準惑星5個を含む。小惑星の一覧参照）。この他に仮符号のみが登録されている小惑星で、複数の衝を観測されたものが138,053個、1回の衝を観測されたものが117,390個あり、これらを合計すると584,686個に達する。番号登録されたもののうち、既に命名されたのは17,224個である[1]。

直径1km程度、ないしそれ以下の小惑星については未発見のものが数十万個あると推測されている。

軌道が確定した小惑星数の増え方については小惑星番号を参照。

なお、2009年10月までに地球近傍小惑星は仮符号のみのものを含めて6,398個、ケンタウルス族を含む太陽系外縁天体は同じく1,379個（準惑星4個を含む）が発見されている（『天文年鑑』2010年版）。

起源

メインベルトの軌道長半径がティティウス・ボーデの法則にほぼ合致するため、昔この位置にあった惑星が何らかの原因で破壊されて小惑星帯が作られたとする惑星破壊説が唱えられたこともあったが、メインベルトの小惑星の質量を合計しても惑星の質量には到底達しないことなどから、現在は支持されていない。またすべての小惑星が同一の起源を持つわけではなく、かつて彗星であったものなども含まれると考えられる。一方で、火星の衛星フォボスとダイモスなど、かつては小惑星だったものが他の天体に把捉されてその衛星となったと考えられている天体も存在する。

メインベルトにある小惑星発生には2つの要素が働いたと考えられる。1つは太陽系形成時にこの付近にダスト成分が少なかった事がある。通常原始太陽系円盤は内側から外側に向けてガスや塵が少なくなるが、メインベルト付近から外は水などの揮発成分が凍るため内側よりも固体成分が多くなり、結果的にメインベルト領域が固体存在量が最も少なくなる。もう1つは木星が先に形成された影響がある。巨大ガス惑星の木星が及ぼす重力によってメインベルト付近の微惑星の軌道が乱され、相対的な速度差が大きくなり、合体よりも破壊される傾向が強まったという。

命名規則

小惑星の名前については、現在では天体の中で唯一、発見者に命名提案権が与えられている。

まず、新天体と思われる天体を2夜以上にわたって位置観測し、その観測結果が小惑星センター (Minor Planet Center, MPC) に報告されると、発見順に仮符号が与えられる。 仮符号は以下の書式に従う英数字からなる [2]。

• 4桁の数字：発見年を表す。
• 空白
• アルファベット (A-Y)：発見時期（月の前後半）を表す。
• アルファベット (A-Z) + 数字： 発見時期内での発見順を表す。

詳細は仮符号#小惑星を参照。

仮符号を付けられた天体は既知の天体との軌道の同定作業が行なわれる。最終的に軌道が確定して新天体だと確認されると、小惑星番号が与えられた上で命名される。

発見者（既に死去している場合は軌道確定のための計算を行なった者）によって提案された新小惑星の名前は IAU の小天体命名委員会によって審査される。名前はラテン語化するのが好ましいというのが世界的な暗黙の了解事項であるが、現在ではそうでないものも多い。その他にも、「発音可能な英文字で16文字以内であること」、「公序良俗に反するもの、ペットの名前、既にある小惑星と紛らわしい名前は付けられない」、「政治・軍事に関連する事件や人物の名前は没後100年以上経過し評価が定まってからでないとつけられない」、「命名権の売買は禁止」などの基準がある[3]。

なお、トロヤ群はトロイア戦争に参加した戦士の中から、ケンタウルス族（後述）にはケンタウロス族の名前、太陽系外縁天体には各民族などの創世神話から命名を行なうという規則がある。

また、人名については、かつては『姓・名』を分けて命名できた（(3744) ジャック・ロンドン (Jack London) など）が、21世紀初頭には姓と名を結合した命名が為されている（(79896) ビルヘイリー (Billhaley) など）。また、別々の小惑星に命名提案された人名を結合するケースなども見られる（ノート:小惑星参照）。

基本的には、一度命名した小惑星名は変更できないことになっているが、何らかの問題が生じた際には例外的に変更された例がいくつかある（小惑星番号#例外を参照）。また、申請の際に名前の綴りが変更されてしまうことがある。

小惑星名を日本語で表記する方法は、メディア等によってまちまちである。片仮名もしくはアルファベットで表記する場合もあり、日本や中華人民共和国など、漢字文化圏に因んで命名された小惑星に関しては、漢字表記する場合もある。

命名の歴史

当初は他の惑星と同じように、小惑星に対してもローマ神話の神の名が与えられていた。やがて小惑星が多数見つかるようになると、他の神話の神や文学作品の登場人物、あるいは実在した人物や地名なども用いられるようになった。なお、初期に見つかった小惑星に女神の名が付けられたことから、男性の名前でも女性化して命名されていた。例としては (511) ダビダ（デイヴィッド・トッド (David Tod) →Davida）などがある。そして、1896年に最初の地球軌道に接近する小惑星、1906年に最初のトロヤ群小惑星が発見されると、それらのように特異な軌道を持つ小惑星には男性名（神または英雄など）が付けられることになった（上記の2個はそれぞれ (433) エロス、(588) アキレスと命名された）。その後、小惑星の数が更に増加するにつれて名前の数が足りなくなる恐れが出てきたため、比較的自由な命名が赦されるようになった。

第二次世界大戦後、アメリカ合衆国内に小惑星センターが設立され、小惑星および彗星の観測記録や番号登録、命名などを小惑星回報 (MPCs) として公表するようになり（戦前はベルリンに同様の業務を行う機関があったが、詳細は不明）、後に電子化 (MPECs) されている。しかし、すでに発見された小惑星との軌道の同定に手間取ることが多く、加えて20世紀末に小惑星の発見数が急増すると、提案された名前を審査するのが追いつかなくなり、固有名を付けるのをやめようという意見まで出るに至った。2003年、国際天文学連合総会第20委員会において、発見者1人当たり1ヶ月に1個以上の命名提案を控えるよう求めることが決定された。ただしそれは絶対ではなく、適切な理由があれば複数同時提案も認められる。例えば2012年には、東日本大震災で大きな被害を受けた地域に由来する小惑星の命名が、同一発見者の小惑星12個に対して同時にされたことがある。その一方、小惑星番号が付いてから10年以内に名前を提案しないと、命名権を放棄したと見なされるという「10年ルール」も存在する。こうしたことから、発見数に比して命名された小惑星の割合はあまり多くない。

MPCsによって名前とその由来が公表されるようになったのは、小惑星番号にして概ね1500番台以降である。1998年末以降に命名された小惑星（3000番台から9000番台の一部と、10000番台以降のすべて）については、ジェット推進研究所の小天体データベースにほぼ例外なくMPECsの命名文が収録されている。Lutz D. Schmadel の“Dictionary of Minor Planet Names”（2006年に第5版、2008年にその補遺が発行された）には、それまでに命名されたすべての小惑星が掲載されているが、MPCs以前に命名されたものについては由来が不明な場合もある。

分類

軌道による分類

軌道要素が似通っている小惑星のグループを、英語では代表的な小惑星名などの後に「family」ないし「asteroids」を付けるか、小惑星名自体に接尾辞を付けた名称で呼ぶ。日本語では、木星より内側を公転するグループについては「～ family」を「族」、それ以外を「群」と訳し、木星以遠については（海王星のトロヤ群を除き）一括して「族」と訳することが多い。

これらのグループは同一の母天体（原始惑星）が分裂して母天体に近い軌道を回り続けているものや、木星などの引力の影響で一定範囲の軌道に集まったものと考えられており、基本的には前者を「family」と呼ぶ。

「family」を最初に発見したのは日本の平山清次であり、21世紀初頭までにメインベルトで数十の「family」が発見されている。外縁天体については、2007年に2003 EL₆₁（後のハウメア）を含む「family」が存在する可能性が報告された。

メインベルト小惑星

木星との共鳴により、小惑星が分布しないカークウッドの空隙などを境界に、いくつかの族 (family) に分類されている。表は代表的な族のみ。

族	軌道長半径 (AU)	離心率	軌道傾斜角 (°)	代表的な 小惑星
フローラ族	2.15 - 2.35	0.03 - 0.23	1.5 - 8.0	(8) フローラ
ベスタ族	2.26 - 2.48	0.03 - 0.16	5.0 - 8.3	(4) ベスタ
マッサリア族	2.37 - 2.45	0.12 - 0.21	0.4 - 2.4	(20) マッサリア
ニサ族	2.41 - 2.5	0.12 - 0.21	1.5 - 4.3	(44) ニサ
マリア族	2.5 - 2.706	0.057 - 0.16	12 - 17	(170) マリア
エウノミア族	2.53 - 2.72	0.08 - 0.22	11.1 - 15.8	(15) エウノミア
パラス族	2.71 - 2.79	0.25 - 0.31	32 - 34	(2) パラス
ゲフィオン族	2.74 - 2.82	0.08 - 0.18	7.4 - 10.5	(1272) ゲフィオン
コロニス族	2.83 - 2.91	0 - 0.11	0 - 3.5	(158) コロニス
エオス族	2.99 - 3.03	0.01 - 0.13	8 - 12	(221) エオス
ヒギエア族	3.06 - 3.24	0.09 - 0.19	3.5 - 6.8	(10) ヒギエア
テミス族	3.08 - 3.24	0.09 - 0.22	0 - 3	(24) テミス
キュベレー族				(65) キュベレー

メインベルト以外の小惑星は特異小惑星と呼ばれる。

共鳴小惑星

公転周期が惑星と整数比の軌道の中には、安定で、多くの小惑星が分布するものがある。トロヤ群は、惑星と太陽を頂点とする正三角形の第3の頂点、つまりラグランジュ点のL₄、L₅点に位置する。

群など	軌道 長半径 (AU)	公転 周期 （年）	惑星	公転 周期 （年）	共鳴比	備考
地球の準衛星	1.00	1.00	地球	1.00	1:1	常に地球の近くに位置する。
火星トロヤ群	1.52	1.88	火星	1.88	1:1	太陽 - 火星のL4、L5点。
ハンガリア群	1.85	2.52	火星	1.88	4:3
アリンダ族	2.50	3.95	木星	11.86	1:3	地球近傍小惑星でもある。
ヒルダ群	3.97	7.91	木星	11.86	2:3	木星と衝の頃、近日点通過 （木星に近づかない）。
チューレ群	4.28	8.90	木星	11.86	3:4	2つのみ発見。
木星トロヤ群	5.20	11.86	木星	11.86	1:1	太陽 - 木星のL4、L5点。
海王星トロヤ群	30.11	164	海王星	164	1:1	太陽 - 海王星のL4、L5点。

小惑星
地球近傍小惑星
地球の準衛星
小惑星帯
木星のトロヤ群
ダモクレス族（逆行小惑星）
ケンタウルス族
太陽系外縁天体
関連項目	小惑星の衛星
	彗星・小惑星遷移天体
	太陽系小天体（彗星）
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地球近傍小惑星 (NEA)

地球軌道の近くを通るもの。いくつかのグループに分けられるが、特に上から3つを指すことが多い。

• アテン群…軌道長半径が1 AU以下で遠日点が0.983 AU以上のもの。　
• アポロ群…軌道長半径が1 AU以上で近日点が1.017 AU以下のもの。
• アモール群…軌道長半径が1 AU以上で近日点が1.017 AU以上1.3 AU以下のもの。アポロ群とまとめて、アポロ・アモール天体ということもある。
• アリンダ族…軌道長半径が約2.5 AUで離心率が0.4 - 0.65のもの。木星の公転周期の3分の1の公転周期を持ち、近日点は1AUに近い。
• ダモクレス族 (Damocloid)…長楕円軌道や、黄道平面から大きく傾いた軌道を取る。オールトの雲由来だと考えられている。
• 潜在的に危険な小惑星(PHA)…地球近傍小惑星の中でも特に衝突する可能性と衝突した場合の危険性が高い小惑星のこと。

○○横断小惑星

近日点と遠日点が、それぞれ対象となる惑星の公転軌道より内側と外側にある小惑星。地球近傍小惑星の多くは地球横断小惑星ということもできる。

• 水星横断小惑星
• 金星横断小惑星
• 地球横断小惑星
• 火星横断小惑星
• 木星横断小惑星
• 土星横断小惑星
• 天王星横断小惑星
• 海王星横断小惑星

ケンタウルス族 (Centaur)

軌道長半径が30 AU以下。近日点は木星軌道から天王星軌道の間に、遠日点は土星軌道から海王星軌道の間にあるものが多い。木星などの摂動を受けやすく、軌道は不安定。彗星起源と考えられており、太陽系外縁天体に分類されることもある。

逆行小惑星

軌道傾斜角が90度を超えるもの。ダモクレス族と重複するものも多い。

太陽系外縁天体
エッジワース ・カイパー ベルト （海王星との 軌道共鳴）	(3:4)
	冥王星族 (2:3)
	(3:5)
	キュビワノ族 ( - )
	(1:2)
散乱円盤天体
オールトの雲
類似天体	ケンタウルス族
	海王星のトロヤ群
	彗星（遷移天体）
関連項目	準惑星（冥王星型天体）
	太陽系小天体
■Portal ■Project ■Template

太陽系外縁天体も、いくつかのグループに分かれている。

エッジワース・カイパーベルト天体 (EKBO)

• 共鳴TNO
  • 3:4共鳴天体…軌道長半径が36 - 36.4 AUで、海王星の公転周期（166.5年）の4/3倍の周期を持つもの。
  • 冥王星族（Plutino、2:3共鳴天体）…軌道長半径が39 - 40.5AUで、海王星の公転周期の3/2倍の周期を持つもの。
  • 3:5共鳴天体…軌道長半径が42 - 42.5 AUで、海王星の公転周期の5/3倍の周期を持つもの。
  • トゥーティノ族（Twotino、1:2共鳴天体）…軌道長半径が48.0 - 48.5 AUで、海王星の公転周期の2倍の周期を持つもの。
  • その他の共鳴天体…海王星と4:7、3:7、2:5、3:8、1:3などの共鳴関係にあるかもしれない外縁天体が見つかっている。
• キュビワノ族（Cubewano、古典的TNO）…軌道長半径が41 AU以上で、離心率が0.15以下のもの。

散乱円盤天体 (SDO)

離心率が大きく、遠日点ではエッジワース・カイパーベルトの外縁を超えるもの。

さらに遠くの軌道を回る天体

E-SDO、内オールトの雲天体など。分類は進んでいない。

他にも多くの族・群がある (Asteroid groups and families)。

スペクトルによる分類

詳細は「小惑星のスペクトル分類」を参照

小惑星は色、アルベド（反射能）、スペクトルによって大きく3種類に分類される。

• C型小惑星 - 炭素質。発見されている小惑星の75%がここに含まれる。
• S型小惑星 - ケイ素質。発見されている小惑星の17%がここに含まれる。
• M型小惑星 - 金属質。ニッケルと鉄が主成分。

上記3つのサブグループに相当する型や、それら以外のマイナーな型も存在する。

その他の分類

• 彗星・小惑星遷移天体
• 小惑星の衛星
  • 二重小惑星（連小惑星）
  • 接触二重小惑星（接触連小惑星）
  • ラブルパイル（破砕集積体）

探査の歴史

太陽系生成論の研究や、将来的な資源利用への布石として、小惑星探査が進められている。

望遠鏡でも点状にしか見えないため、1990年代に入るまで、小惑星の研究は軌道の確定や光度の測定に留まり、その姿については想像の域を出なかった。しかし、恒星食による形状の推定、ハッブル宇宙望遠鏡などの高性能の望遠鏡による観察やレーダー測定により、大きさや形状など、その姿が徐々に明らかになってきた。

そして、1989年に打ち上げられた木星探査機ガリレオにより、1991年に (951) ガスプラ、1993年に (243) イダの映像が撮影され、人類は初めて小惑星の鮮明な映像を目にした。なお、ガリレオはイダに初めて衛星を発見し、ダクティルと名づけられた。その後も、主に地上での観測により170個以上（2010年現在）の小惑星に衛星の存在が確認されている（小惑星の衛星参照）。

1996年に打ち上げられたNEARシューメーカーは、1997年に (253) マティルド、2000年に (433) エロスの映像を撮影し、探査機はエロスの周回軌道に乗った後に着陸を果たした。

2003年に打ち上げられた日本の探査機はやぶさは、2005年に (25143) イトカワへ到達、至近距離からの詳細な観測を行った。はやぶさはイトカワへの着陸と離陸を行ない、サンプル採取を試みた後、2010年6月13日に地球へ帰還し、サンプル容器を納めたカプセルが回収されて容器内の微粒子の回収と分析が続いている。同年11月16日には、回収された微粒子のほとんど全てがイトカワ由来であることが発表された。これは世界初の小惑星からのサンプルリターンである。

2004年に打ち上げられたロゼッタは、2008年に (2867) シュテインス、2010年に (21) ルテティアへの接近観測を行った。

2007年に打ち上げられたドーンは、2011年に(4) ベスタの周回軌道に乗って観測を行い、2012年にベスタの軌道を離脱した。2015年に (1) ケレスに到達する予定である。

その他にも、彗星探査機などにより比較的遠距離からの、もしくは不鮮明な小惑星の映像がいくつか撮影されている。

2011年現在、はやぶさ2、マルコ・ポーロ、オシリスなどの小惑星サンプルリターン計画が検討中である。さらにドン・キホーテという計画では、小惑星にインパクターを衝突させる構想である。

アメリカではコンステレーション計画の中止により小天体探査に関心が集まりつつあり、2010年4月にオバマ大統領の発表した新宇宙政策によれば、月以降の有人探査の対象として地球近傍小惑星が有力な候補と見られている。

これまでに行われた近接探査

• ガリレオ… (951) ガスプラ、(243) イダ
• カッシーニ… (2685) マサースキー
• NEARシューメーカー… (253) マティルド、(433) エロス
• ディープ・スペース1号… (9969) ブライユ
• スターダスト… (5535) アンネフランク
• はやぶさ… (25143) イトカワ
• ニュー・ホライズンズ… (132524) APL
• ロゼッタ… (2867) シュテインス、(21) ルテティア
• ドーン… (4) ベスタ
• 嫦娥2号… (4179) トータティス

今後行われる近接探査

• ニュー・ホライズンズ… (134340) 冥王星（準惑星）、および少なくとも1個の太陽系外縁天体
• ドーン… (1) ケレス (準惑星)

計画が存在する近接探査

• オシリス… ベンヌ
• はやぶさ2… (162173) 1999 JU₃
• マルコ・ポーロ… (107P/ 4015) ウィルソン・ハリントン
• ドン・キホーテ… 未定

実現しなかった近接探査

中止または他の目標に変更されたもの

• クレメンタイン… (1620) ジオグラフォス
• ディープ・スペース1号… (107P/ 4015) ウィルソン・ハリントン、1999 KK₁
• ロゼッタ… (4979) 大田原、(140) シワ
• はやぶさ… (4660) ネレウス、(10302) 1989 ML

地球への衝突の可能性

詳細は「隕石衝突」を参照

小惑星は地球への衝突の可能性を有している。ユカタン半島にあるチュクシュルーブ・クレーターの調査から、約6550万年前に秒速10~20kmの速度で衝突した直径10kmの小惑星は、大型の恐竜を全滅させたと考えられている。クレーターは直径150km、深さ30km。周辺はマグニチュード11規模の地震と大規模の火災が発生し、海に落ちたために生じた津波は高さ300mと推定される。さらに、衝突で巻き上げられた塵が成層圏に及んで漂い、数ヶ月から数年間太陽光線を遮り、植物など光合成生物の死滅に端を発し生物全体の70%が滅んだと推測される。

直径10km規模の小惑星衝突は1億年に1回程の頻度で起こると考えられる。直径1kmの小惑星衝突でも地球規模の気候に変動を与えると考えられ、その頻度は100万年に1回程と推定される。これより小規模な衝突は影響こそ限定的になるが、その反面頻度は上昇する。直径1.2kmのバリンジャー・クレーターを作った隕石は直径50m規模であったが、頻度は1000年に1回程あると考えられる。

地球の公転軌道より1.3天文単位以内を通過する公転周期200年未満の小惑星はNEA(Near Earth Asteroid, 地球近傍小惑星)といい、2012年11月1日現在で9252個が確認されている。その中でも、地球に0.05天文単位（約750km）以下に近づく公転軌道を通り、直系が150m以上と考えられる小惑星はPHA(Potentially Hazardous Asteroid, 潜在的に危険な小惑星)と呼ばれ、1343個が該当する。しかもNEAは惑星重力の影響を受けやすいため、公転軌道は急に変化して予測どおりにならない可能性が高い。

小惑星の衝突を回避する技術は現在の科学技術では達成しておらず、現存するロケットを衝突させて軌道を変える方法でも、直径100m以下の小惑星でしか効果が無いと考えられている。ESAの「ドン・キホーテ計画」など有効な回避法が様々に模索されているが、今だ研究段階にあり効果はわかっていない。

NEAと、やはり衝突が懸念される彗星(Near Earth Comet, NEC)と合わたNEO(Near Earth Object, 地球近傍天体)を監視する計画は、NASAとアメリカ空軍、マサチューセッツ工科大学の共同によるLINEAR(Lincoln Near Earth Asteriod Research)、アリゾナ大学のSpace WatchとCatarina Sky Survey、NASAジェット推進研究所のNEAT(Near-Earth Asteroid Tracking)、ローウェル天文台のLONEOS(Lowell Observatory Near-Earth-Object Search)、ハワイ大学のPan-STARRAS(Panoramic Survey Telescope And Rapid Response Syastem)などがあり、日本でも美星スペースガードセンターが観測を行っている。このように多くの観測体制が敷かれる理由は、そもそもNEOが非常に観測しにくい事が背景にある。しかも現在、昼間に観測することは事実上不可能である。

脚注

参考文献

• 編集長：水谷仁「ニュートン2013年1月号、雑誌07047-01」、ニュートンプレス、2013年。

関連項目

• 小惑星の一覧
• 小惑星の植民
• 太陽系
• 太陽系小天体
• 太陽系外縁天体
• 準惑星
• エッジワース・カイパーベルト
• オールトの雲
• 平山清次
• 平山信

外部リンク

• 「Asteroids」 - Encyclopedia of Cosmos（エンサイクロペディア・オブ・コスモス）にある「小惑星」についての記事（英語）。
• ザ・ナインプラネッツ日本語版（小惑星）
• 小惑星 惑星になりそこなった星たち
• 小惑星の名前研究所
• 小惑星の画像集（英語）

wiki en

[Wiki en表示]

For other uses, see Asteroid (disambiguation).

253 Mathilde, a C-type asteroid measuring about 50 kilometres (30 mi) across, covered in craters half that size. Photograph taken in 1997 by the NEAR Shoemaker probe.

Asteroids are minor planets, especially those of the inner Solar System. The larger ones have also been called planetoids. These terms have historically been applied to any astronomical object orbiting the Sun that did not show the disc of a planet and was not observed to have the characteristics of an active comet, but as minor planets in the outer Solar System were discovered, they were often distinguished from traditional asteroids. Their volatile-based surfaces were found to resemble comets. They were named centaurs, Neptune trojans, and trans-Neptunian objects, types of minor planets that have properties distinct from those in the asteroid belt. In this article the term "asteroid" refers to the minor planets of the inner Solar System.

There are millions of asteroids, many thought to be the shattered remnants of planetesimals, bodies within the young Sun's solar nebula that never grew large enough to become planets. The large majority of known asteroids orbit in the asteroid belt between the orbits of Mars and Jupiter, or are co-orbital with Jupiter (the Jupiter Trojans). However, other orbital families exist with significant populations, including the near-Earth asteroids. Individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups: C-type, S-type, and M-type. These were named after and are generally identified with carbon-rich, stony, and metallic compositions, respectively.

Only one asteroid, 4 Vesta, which has a relatively reflective surface, is normally visible to the naked eye, and this only in very dark skies when it is favorably positioned. Rarely, small asteroids passing close to Earth may be visible to the naked eye for a short time. As of September 2013, the Minor Planet Center had data on more than one million objects in the inner and outer Solar System, of which 625,000 had enough information to be given numbered designations.

On 22 January 2014, ESA scientists reported the detection, for the first definitive time, of water vapor on 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."

Naming

Main article: Minor planet § Naming

2013 EC, shown here in radar images, has a provisional designation

A newly discovered asteroid is given a provisional designation (such as 2002 AT4) consisting of the year of discovery and an alphanumeric code indicating the half-month of discovery and the sequence within that half-month. Once an asteroid's orbit has been confirmed, it is given a number, and later may also be given a name (e.g. 433 Eros). The formal naming convention uses parentheses around the number (e.g. (433) Eros), but dropping the parentheses is quite common. Informally, it is common to drop the number altogether, or to drop it after the first mention when a name is repeated in running text.

Symbols

Main article: Astronomical symbols

The first asteroids to be discovered were assigned iconic symbols like the ones traditionally used to designate the planets. By 1855 there were two dozen asteroid symbols, which often occurred in multiple variants.

Asteroid	Symbol		Year
1 Ceres	⚳	Ceres' scythe, reversed to double as the letter C	1801
2 Pallas	⚴	Athena's (Pallas') spear	1801
3 Juno	⚵	A star mounted on a scepter, for Juno, the Queen of Heaven	1804
4 Vesta	⚶	The altar and sacred fire of Vesta	1807
5 Astraea		A scale, or an inverted anchor, symbols of justice	1845
6 Hebe		Hebe's cup	1847
7 Iris		A rainbow (iris) and a star	1847
8 Flora		A flower (flora) (specifically the Rose of England)	1847
9 Metis		The eye of wisdom and a star	1848
10 Hygiea		Hygiea's serpent and a star, or the Rod of Asclepius	1849
11 Parthenope		A harp, or a fish and a star; symbols of the sirens	1850
12 Victoria		The laurels of victory and a star	1850
13 Egeria		A shield, symbol of Egeria's protection, and a star	1850
14 Irene		A dove carrying an olive branch (symbol of irene 'peace') with a star on its head, or an olive branch, a flag of truce, and a star	1851
15 Eunomia		A heart, symbol of good order (eunomia), and a star	1851
16 Psyche		A butterfly's wing, symbol of the soul (psyche), and a star	1852
17 Thetis		A dolphin, symbol of Thetis, and a star	1852
18 Melpomene		The dagger of Melpomene, and a star	1852
19 Fortuna		The wheel of fortune and a star	1852
26 Proserpina		Proserpina's pomegranate	1853
28 Bellona		Bellona's whip and lance	1854
29 Amphitrite		The shell of Amphitrite and a star	1854
35 Leukothea		A lighthouse beacon, symbol of Leucothea	1855
37 Fides		The cross of faith (fides)	1855

In 1851, after the fifteenth asteroid (Eunomia) had been discovered, Johann Franz Encke made a major change in the upcoming 1854 edition of the Berliner Astronomisches Jahrbuch (BAJ, Berlin Astronomical Yearbook). He introduced a disk (circle), a traditional symbol for a star, as the generic symbol for an asteroid. The circle was then numbered in order of discovery to indicate a specific asteroid (although he assigned ① to the fifth, Astraea, while continuing to designate the first four only with their existing iconic symbols). The numbered-circle convention was quickly adopted by astronomers, and the next asteroid to be discovered (16 Psyche, in 1852) was the first to be designated in that way at the time of its discovery. However, Psyche was given an iconic symbol as well, as were a few other asteroids discovered over the next few years (see chart above). 20 Massalia was the first asteroid that was not assigned an iconic symbol, and no iconic symbols were created after the 1855 discovery of 37 Fides. That year Astraea's number was increased to ⑤, but the first four asteroids, Ceres to Vesta, were not listed by their numbers until the 1867 edition. The circle was soon abbreviated to a pair of parentheses, which were easier to typeset and sometimes omitted altogether over the next few decades, leading to the modern convention.

Discovery

243 Ida and its moon Dactyl. Dactyl is the first satellite of an asteroid to be discovered.

The first asteroid to be discovered, Ceres, was found in 1801 by Giuseppe Piazzi, and was originally considered to be a new planet. This was followed by the discovery of other similar bodies, which, with the equipment of the time, appeared to be points of light, like stars, showing little or no planetary disc, though readily distinguishable from stars due to their apparent motions. This prompted the astronomer Sir William Herschel to propose the term "asteroid", coined in Greek as ἀστεροειδής asteroeidēs 'star-like, star-shaped', from Ancient Greek ἀστήρ astēr 'star, planet'. In the early second half of the nineteenth century, the terms "asteroid" and "planet" (not always qualified as "minor") were still used interchangeably; for example, the Annual of Scientific Discovery for 1871, page 316, reads "Professor J. Watson has been awarded by the Paris Academy of Sciences, the astronomical prize, Lalande foundation, for the discovery of eight new asteroids in one year. The planet Lydia (No. 110), discovered by M. Borelly at the Marseilles Observatory [...] M. Borelly had previously discovered two planets bearing the numbers 91 and 99 in the system of asteroids revolving between Mars and Jupiter".

Historical methods

Asteroid discovery methods have dramatically improved over the past two centuries.

In the last years of the 18th century, Baron Franz Xaver von Zach organized a group of 24 astronomers to search the sky for the missing planet predicted at about 2.8 AU from the Sun by the Titius-Bode law, partly because of the discovery, by Sir William Herschel in 1781, of the planet Uranus at the distance predicted by the law. This task required that hand-drawn sky charts be prepared for all stars in the zodiacal band down to an agreed-upon limit of faintness. On subsequent nights, the sky would be charted again and any moving object would, hopefully, be spotted. The expected motion of the missing planet was about 30 seconds of arc per hour, readily discernible by observers.

First asteroid image (Ceres and Vesta) from Mars – viewed by Curiosity (20 April 2014).

The first object, Ceres, was not discovered by a member of the group, but rather by accident in 1801 by Giuseppe Piazzi, director of the observatory of Palermo in Sicily. He discovered a new star-like object in Taurus and followed the displacement of this object during several nights. His colleague, Carl Friedrich Gauss, used these observations to find the exact distance from this unknown object to Earth. Gauss's calculations placed the object between the planets Mars and Jupiter. Piazzi named it after Ceres, the Roman goddess of agriculture.

Three other asteroids (2 Pallas, 3 Juno, and 4 Vesta) were discovered over the next few years, with Vesta found in 1807. After eight more years of fruitless searches, most astronomers assumed that there were no more and abandoned any further searches.

However, Karl Ludwig Hencke persisted, and began searching for more asteroids in 1830. Fifteen years later, he found 5 Astraea, the first new asteroid in 38 years. He also found 6 Hebe less than two years later. After this, other astronomers joined in the search and at least one new asteroid was discovered every year after that (except the wartime year 1945). Notable asteroid hunters of this early era were J. R. Hind, Annibale de Gasparis, Robert Luther, H. M. S. Goldschmidt, Jean Chacornac, James Ferguson, Norman Robert Pogson, E. W. Tempel, J. C. Watson, C. H. F. Peters, A. Borrelly, J. Palisa, the Henry brothers and Auguste Charlois.

In 1891, Max Wolf pioneered the use of astrophotography to detect asteroids, which appeared as short streaks on long-exposure photographic plates. This dramatically increased the rate of detection compared with earlier visual methods: Wolf alone discovered 248 asteroids, beginning with 323 Brucia, whereas only slightly more than 300 had been discovered up to that point. It was known that there were many more, but most astronomers did not bother with them, calling them "vermin of the skies", a phrase variously attributed to Eduard Suess and Edmund Weiss. Even a century later, only a few thousand asteroids were identified, numbered and named.

Manual methods of the 1900s and modern reporting

Until 1998, asteroids were discovered by a four-step process. First, a region of the sky was photographed by a wide-field telescope, or astrograph. Pairs of photographs were taken, typically one hour apart. Multiple pairs could be taken over a series of days. Second, the two films or plates of the same region were viewed under a stereoscope. Any body in orbit around the Sun would move slightly between the pair of films. Under the stereoscope, the image of the body would seem to float slightly above the background of stars. Third, once a moving body was identified, its location would be measured precisely using a digitizing microscope. The location would be measured relative to known star locations.

These first three steps do not constitute asteroid discovery: the observer has only found an apparition, which gets a provisional designation, made up of the year of discovery, a letter representing the half-month of discovery, and finally a letter and a number indicating the discovery's sequential number (example: 1998 FJ₇₄).

The last step of discovery is to send the locations and time of observations to the Minor Planet Center, where computer programs determine whether an apparition ties together earlier apparitions into a single orbit. If so, the object receives a catalogue number and the observer of the first apparition with a calculated orbit is declared the discoverer, and granted the honor of naming the object subject to the approval of the International Astronomical Union.

Computerized methods

2004 FH is the center dot being followed by the sequence; the object that flashes by during the clip is an artificial satellite.

There is increasing interest in identifying asteroids whose orbits cross Earth's, and that could, given enough time, collide with Earth (see Earth-crosser asteroids). The three most important groups of near-Earth asteroids are the Apollos, Amors, and Atens. Various asteroid deflection strategies have been proposed, as early as the 1960s.

The near-Earth asteroid 433 Eros had been discovered as long ago as 1898, and the 1930s brought a flurry of similar objects. In order of discovery, these were: 1221 Amor, 1862 Apollo, 2101 Adonis, and finally 69230 Hermes, which approached within 0.005 AU of Earth in 1937. Astronomers began to realize the possibilities of Earth impact.

Two events in later decades increased the alarm: the increasing acceptance of Walter Alvarez' hypothesis that an impact event resulted in the Cretaceous–Paleogene extinction, and the 1994 observation of Comet Shoemaker-Levy 9 crashing into Jupiter. The U.S. military also declassified the information that its military satellites, built to detect nuclear explosions, had detected hundreds of upper-atmosphere impacts by objects ranging from one to 10 metres across.

All these considerations helped spur the launch of highly efficient surveys that consist of Charge-Coupled Device (CCD) cameras and computers directly connected to telescopes. As of spring 2011, it was estimated that 89% to 96% of near-Earth asteroids one kilometer or larger in diameter had been discovered. A list of teams using such systems includes:

• The Lincoln Near-Earth Asteroid Research (LINEAR) team
• The Near-Earth Asteroid Tracking (NEAT) team
• Spacewatch
• The Lowell Observatory Near-Earth-Object Search (LONEOS) team
• The Catalina Sky Survey (CSS)
• The Campo Imperatore Near-Earth Object Survey (CINEOS) team
• The Japanese Spaceguard Association
• The Asiago-DLR Asteroid Survey (ADAS)

The LINEAR system alone has discovered 138,393 asteroids, as of 20 September 2013. Among all the surveys, 4711 near-Earth asteroids have been discovered including over 600 more than 1 km (0.6 mi) in diameter.

Terminology

A composite image, to scale, of the asteroids which have been imaged at high resolution. As of 2011 they are, from largest to smallest: 4 Vesta, 21 Lutetia, 253 Mathilde, 243 Ida and its moon Dactyl, 433 Eros, 951 Gaspra, 2867 Šteins, 25143 Itokawa.

The largest asteroid in the previous image, Vesta (left), with Ceres (center) and the Moon (right) shown to scale.

Traditionally, small bodies orbiting the Sun were classified as asteroids, comets or meteoroids, with anything smaller than ten metres across being called a meteoroid. The term "asteroid" is ill-defined. It never had a formal definition, with the broader term minor planet being preferred by the International Astronomical Union. In 2006, the term "small Solar System body" was introduced to cover both most minor planets and comets. Other languages prefer "planetoid" (Greek for "planet-like"), and this term is occasionally used in English for larger minor planets such as the dwarf planets. The word "planetesimal" has a similar meaning, but refers specifically to the small building blocks of the planets that existed when the Solar System was forming. The term "planetule" was coined by the geologist William Daniel Conybeare to describe minor planets, but is not in common use. The three largest objects in the asteroid belt, Ceres, 2 Pallas, and 4 Vesta, grew to the stage of protoplanets. Ceres is a dwarf planet, the only one in the inner Solar System.

When found, asteroids were seen as a class of objects distinct from comets, and there was no unified term for the two until "small Solar System body" was coined in 2006. The main difference between an asteroid and a comet is that a comet shows a coma due to sublimation of near surface ices by solar radiation. A few objects have ended up being dual-listed because they were first classified as minor planets but later showed evidence of cometary activity. Conversely, some (perhaps all) comets are eventually depleted of their surface volatile ices and become asteroids. A further distinction is that comets typically have more eccentric orbits than most asteroids; most "asteroids" with notably eccentric orbits are probably dormant or extinct comets.

For almost two centuries, from the discovery of Ceres in 1801 until the discovery of the first centaur, 2060 Chiron, in 1977, all known asteroids spent most of their time at or within the orbit of Jupiter, though a few such as 944 Hidalgo ventured far beyond Jupiter for part of their orbit. When astronomers started finding more small bodies that permanently resided further out than Jupiter, now called centaurs, they numbered them among the traditional asteroids, though there was debate over whether they should be considered as asteroids or as a new type of object. Then, when the first trans-Neptunian object (other than Pluto), 1992 QB1, was discovered in 1992, and especially when large numbers of similar objects started turning up, new terms were invented to sidestep the issue: Kuiper-belt object, trans-Neptunian object, scattered-disc object, and so on. These inhabit the cold outer reaches of the Solar System where ices remain solid and comet-like bodies are not expected to exhibit much cometary activity; if centaurs or trans-Neptunian objects were to venture close to the Sun, their volatile ices would sublimate, and traditional approaches would classify them as comets and not asteroids.

The innermost of these are the Kuiper-belt objects, called "objects" partly to avoid the need to classify them as asteroids or comets. They are believed to be predominantly comet-like in composition, though some may be more akin to asteroids. Furthermore, most do not have the highly eccentric orbits associated with comets, and the ones so far discovered are larger than traditional comet nuclei. (The much more distant Oort cloud is hypothesized to be the main reservoir of dormant comets.) Other recent observations, such as the analysis of the cometary dust collected by the Stardust probe, are increasingly blurring the distinction between comets and asteroids, suggesting "a continuum between asteroids and comets" rather than a sharp dividing line.

The minor planets beyond Jupiter's orbit are sometimes also called "asteroids", especially in popular presentations. However, it is becoming increasingly common for the term "asteroid" to be restricted to minor planets of the inner Solar System. Therefore, this article will restrict itself for the most part to the classical asteroids: objects of the asteroid belt, Jupiter trojans, and near-Earth objects.

When the IAU introduced the class small Solar System bodies in 2006 to include most objects previously classified as minor planets and comets, they created the class of dwarf planets for the largest minor planets—those that have enough mass to have become ellipsoidal under their own gravity. According to the IAU, "the term 'minor planet' may still be used, but generally the term 'Small Solar System Body' will be preferred." Currently only the largest object in the asteroid belt, Ceres, at about 950 km (590 mi) across, has been placed in the dwarf planet category.

Formation

It is believed that planetesimals in the asteroid belt evolved much like the rest of the solar nebula until Jupiter neared its current mass, at which point excitation from orbital resonances with Jupiter ejected over 99% of planetesimals in the belt. Simulations and a discontinuity in spin rate and spectral properties suggest that asteroids larger than approximately 120 km (75 mi) in diameter accreted during that early era, whereas smaller bodies are fragments from collisions between asteroids during or after the Jovian disruption. Ceres and Vesta grew large enough to melt and differentiate, with heavy metallic elements sinking to the core, leaving rocky minerals in the crust.

In the Nice model, many Kuiper-belt objects are captured in the outer asteroid belt, at distances greater than 2.6 AU. Most were later ejected by Jupiter, but those that remained may be the D-type asteroids, and possibly include Ceres.

Distribution within the Solar System

See also: list of minor-planet groups, List of notable asteroids and List of minor planets

The asteroid belt (white) and the Trojan asteroids (green)

Various dynamical groups of asteroids have been discovered orbiting in the inner Solar System. Their orbits are perturbed by the gravity of other bodies in the Solar System and by the Yarkovsky effect. Significant populations include:

Asteroid belt

Main article: Asteroid belt

The majority of known asteroids orbit within the asteroid belt between the orbits of Mars and Jupiter, generally in relatively low-eccentricity (i.e. not very elongated) orbits. This belt is now estimated to contain between 1.1 and 1.9 million asteroids larger than 1 km (0.6 mi) in diameter, and millions of smaller ones. These asteroids may be remnants of the protoplanetary disk, and in this region the accretion of planetesimals into planets during the formative period of the Solar System was prevented by large gravitational perturbations by Jupiter.

Trojans

Main article: Trojan (astronomy)

Trojans are populations that share an orbit with a larger planet or moon, but do not collide with it because they orbit in one of the two Lagrangian points of stability, L4 and L5, which lie 60° ahead of and behind the larger body.

The most significant population of trojans are the Jupiter trojans. Although fewer Jupiter trojans have been discovered as of 2010, it is thought that they are as numerous as the asteroids in the asteroid belt.

A couple of trojans have also been found orbiting with Mars.

Near-Earth asteroids

Main article: Near-Earth asteroids

Near-Earth asteroids, or NEAs, are asteroids that have orbits that pass close to that of Earth. Asteroids that actually cross Earth's orbital path are known as Earth-crossers. As of November 2014, 11,600 near-Earth asteroids are known and the number over one kilometre in diameter is estimated to be 900–1,000.

Frequency of small asteroids roughly 1 to 20 meters in diameter impacting Earth's atmosphere.

Characteristics

Size distribution

Sizes of the first ten asteroids to be discovered, compared to the Moon

Ceres as imaged by Dawn on 4 February 2015

Asteroids vary greatly in size, from almost 7006100000000000000♠1000 km for the largest down to rocks just tens of metres across. The three largest are very much like miniature planets: they are roughly spherical, have at least partly differentiated interiors, and are thought to be surviving protoplanets. The vast majority, however, are much smaller and are irregularly shaped; they are thought to be either surviving planetesimals or fragments of larger bodies.

The dwarf planet Ceres is by far the largest asteroid, with a diameter of 975 km (610 mi). The next largest are 2 Pallas and 4 Vesta, both with diameters of just over 500 km (300 mi). Vesta is the only main-belt asteroid that can, on occasion, be visible to the naked eye. On some rare occasions, a near-Earth asteroid may briefly become visible without technical aid; see 99942 Apophis.

The mass of all the objects of the asteroid belt, lying between the orbits of Mars and Jupiter, is estimated to be about 2.8–7021320000000000000♠3.2×10²¹ kg, or about 4% of the mass of the Moon. Of this, Ceres comprises 7020950000000000000♠0.95×10²¹ kg, a third of the total. Adding in the next three most massive objects, Vesta (9%), Pallas (7%), and Hygiea (3%), brings this figure up to 51%; whereas the three after that, 511 Davida (1.2%), 704 Interamnia (1.0%), and 52 Europa (0.9%), only add another 3% to the total mass. The number of asteroids then increases rapidly as their individual masses decrease.

The number of asteroids decreases markedly with size. Although this generally follows a power law, there are 'bumps' at 7003500000000000000♠5 km and 7005100000000000000♠100 km, where more asteroids than expected from a logarithmic distribution are found.

The asteroids of the Solar System, categorized by size and number

Approximate number of asteroids (N) larger than a certain diameter (D)

D	100 m	300 m	500 m	1 km	3 km	5 km	10 km	30 km	50 km	100 km	200 km	300 km	500 km	900 km
N	~7007250000000000000♠25000000	7006400000000000000♠4000000	7006200000000000000♠2000000	7005750000000000000♠750000	7005200000000000000♠200000	7004900000000000000♠90000	7004100000000000000♠10000	7003110000000000000♠1100	600	200	30	5	3	1

Largest asteroids

See also: Largest asteroids

The relative masses of the twelve largest asteroids

known, compared to the remaining mass of the asteroid belt.

  1 Ceres

  4 Vesta

  2 Pallas

  10 Hygiea

  31 Euphrosyne

  704 Interamnia

 

  511 Davida

  532 Herculina

  15 Eunomia

  3 Juno

  16 Psyche

  52 Europa

  all others

Although their location in the asteroid belt excludes them from planet status, the three largest objects, Ceres, Vesta, and Pallas, are intact protoplanets that share many characteristics common to planets, and are atypical compared to the majority of "potato"-shaped asteroids.

Ceres is the only asteroid with a fully ellipsoidal shape and hence the only one that is a dwarf planet. It has a much higher absolute magnitude than the other asteroids, of around 3.32, and may possess a surface layer of ice. Like the planets, Ceres is differentiated: it has a crust, a mantle and a core. No meteorites from Ceres have been found on Earth.

Vesta, too, has a differentiated interior, though it formed inside the Solar System's frost line, and so is devoid of water; its composition is mainly of basaltic rock such as olivine. Aside from the large crater at its southern pole, Rheasilvia, Vesta also has an ellipsoidal shape. Vesta is the parent body of the Vestian family and other V-type asteroids, and is the source of the HED meteorites, which constitute 5% of all meteorites on Earth.

Pallas is unusual in that, like Uranus, it rotates on its side, with its axis of rotation tilted at high angles to its orbital plane. Its composition is similar to that of Ceres: high in carbon and silicon, and perhaps partially differentiated. Pallas is the parent body of the Palladian family of asteroids,

The fourth-most-massive asteroid, Hygiea, is the largest carbonaceous asteroid and, unlike the other largest asteroids, lies relatively close to the plane of the ecliptic. It is the largest member and presumed parent body of the Hygiean family of asteroids. Between them, the four largest asteroids constitute half the mass of the asteroid belt.

Attributes of largest asteroids
Name	Orbital radius (AU)	Orbital period (years)	Inclination to ecliptic	Orbital eccentricity	Diameter (km)	Diameter (% of Moon)	Mass (×1018 kg)	Mass (% of Ceres)	Density g/cm3	Rotation period (hr)	Axial tilt	Surface temperature
Vesta	2.36	3.63	7.1°	0.089	573×557×446 (mean 525)	15%	260	28%	3.44 ± 0.12	5.34	29°	85–270 K
Ceres	2.77	4.60	10.6°	0.079	975×975×909 (mean 952)	28%	940	100%	2.12 ± 0.04	9.07	≈ 3°	167 K
Pallas	2.77	4.62	34.8°	0.231	580×555×500 (mean 545)	16%	210	22%	2.71 ± 0.11	7.81	≈ 80°	164 K
Hygiea	3.14	5.56	3.8°	0.117	530×407×370 (mean 430)	12%	87	9%	2.76 ± 1.2	27.6	≈ 60°	164 K

Rotation

Measurements of the rotation rates of large asteroids in the asteroid belt show that there is an upper limit. No asteroid with a diameter larger than 100 meters has a rotation period smaller than 2.2 hours. For asteroids rotating faster than approximately this rate, the inertia at the surface is greater than the gravitational force, so any loose surface material would be flung out. However, a solid object should be able to rotate much more rapidly. This suggests that most asteroids with a diameter over 100 meters are rubble piles formed through accumulation of debris after collisions between asteroids.

Composition

Cratered terrain on 4 Vesta

The physical composition of asteroids is varied and in most cases poorly understood. Ceres appears to be composed of a rocky core covered by an icy mantle, where Vesta is thought to have a nickel-iron core, olivine mantle, and basaltic crust. 10 Hygiea, however, which appears to have a uniformly primitive composition of carbonaceous chondrite, is thought to be the largest undifferentiated asteroid. Most of the smaller asteroids are thought to be piles of rubble held together loosely by gravity, though the largest are probably solid. Some asteroids have moons or are co-orbiting binaries: Rubble piles, moons, binaries, and scattered asteroid families are believed to be the results of collisions that disrupted a parent asteroid.

Asteroids contain traces of amino acids and other organic compounds, and some speculate that asteroid impacts may have seeded the early Earth with the chemicals necessary to initiate life, or may have even brought life itself to Earth (also see panspermia). In August 2011, a report, based on NASA studies with 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 in outer space.

Asteroid collision – building planets (artist concept).

Composition is calculated from three primary sources: albedo, surface spectrum, and density. The last can only be determined accurately by observing the orbits of moons the asteroid might have. So far, every asteroid with moons has turned out to be a rubble pile, a loose conglomeration of rock and metal that may be half empty space by volume. The investigated asteroids are as large as 280 km in diameter, and include 121 Hermione (268×186×183 km), and 87 Sylvia (384×262×232 km). Only half a dozen asteroids are larger than 87 Sylvia, though none of them have moons; however, some smaller asteroids are thought to be more massive, suggesting they may not have been disrupted, and indeed 511 Davida, the same size as Sylvia to within measurement error, is estimated to be two and a half times as massive, though this is highly uncertain. The fact that such large asteroids as Sylvia can be rubble piles, presumably due to disruptive impacts, has important consequences for the formation of the Solar System: Computer simulations of collisions involving solid bodies show them destroying each other as often as merging, but colliding rubble piles are more likely to merge. This means that the cores of the planets could have formed relatively quickly.

On 7 October 2009, the presence of water ice was confirmed on the surface of 24 Themis using NASA’s Infrared Telescope Facility. The surface of the asteroid appears completely covered in ice. As this ice layer is sublimated, it may be getting replenished by a reservoir of ice under the surface. Organic compounds were also detected on the surface. Scientists hypothesize that some of the first water brought to Earth was delivered by asteroid impacts after the collision that produced the Moon. The presence of ice on 24 Themis supports this theory.

In October 2013, water was detected on an extrasolar body for the first time, on an asteroid orbiting the white dwarf GD 61.

Surface features

Most asteroids outside the "big four" (Ceres, Pallas, Vesta, and Hygiea) are likely to be broadly similar in appearance, if irregular in shape. 50-km (31-mi) 253 Mathilde is a rubble pile saturated with craters with diameters the size of the asteroid's radius, and Earth-based observations of 300-km (186-mi) 511 Davida, one of the largest asteroids after the big four, reveal a similarly angular profile, suggesting it is also saturated with radius-size craters. Medium-sized asteroids such as Mathilde and 243 Ida that have been observed up close also reveal a deep regolith covering the surface. Of the big four, Pallas and Hygiea are practically unknown. Vesta has compression fractures encircling a radius-size crater at its south pole but is otherwise a spheroid. Ceres seems quite different in the glimpses Hubble has provided, with surface features that are unlikely to be due to simple craters and impact basins, but details will be expanded with the Dawn spacecraft, which entered Ceres orbit on 6 March 2015.

Color

Asteroids become darker and redder with age due to space weathering. However evidence suggests most of the color change occurs rapidly, in the first hundred thousands years, limiting the usefulness of spectral measurement for determining the age of asteroids.

Classification

Asteroids are commonly classified according to two criteria: the characteristics of their orbits, and features of their reflectance spectrum.

Orbital classification

Main articles: Asteroid group and Asteroid family

Many asteroids have been placed in groups and families based on their orbital characteristics. Apart from the broadest divisions, it is customary to name a group of asteroids after the first member of that group to be discovered. Groups are relatively loose dynamical associations, whereas families are tighter and result from the catastrophic break-up of a large parent asteroid sometime in the past. Families have only been recognized within the asteroid belt. They were first recognized by Kiyotsugu Hirayama in 1918 and are often called Hirayama families in his honor.

About 30–35% of the bodies in the asteroid belt belong to dynamical families each thought to have a common origin in a past collision between asteroids. A family has also been associated with the plutoid dwarf planet Haumea.

Quasi-satellites and horseshoe objects

Some asteroids have unusual horseshoe orbits that are co-orbital with Earth or some other planet. Examples are 3753 Cruithne and 2002 AA29. The first instance of this type of orbital arrangement was discovered between Saturn's moons Epimetheus and Janus.

Sometimes these horseshoe objects temporarily become quasi-satellites for a few decades or a few hundred years, before returning to their earlier status. Both Earth and Venus are known to have quasi-satellites.

Such objects, if associated with Earth or Venus or even hypothetically Mercury, are a special class of Aten asteroids. However, such objects could be associated with outer planets as well.

Spectral classification

This picture of 433 Eros shows the view looking from one end of the asteroid across the gouge on its underside and toward the opposite end. Features as small as 35 m (115 ft) across can be seen.

Main article: Asteroid spectral types

In 1975, an asteroid taxonomic system based on color, albedo, and spectral shape was developed by Clark R. Chapman, David Morrison, and Ben Zellner. These properties are thought to correspond to the composition of the asteroid's surface material. The original classification system had three categories: C-types for dark carbonaceous objects (75% of known asteroids), S-types for stony (silicaceous) objects (17% of known asteroids) and U for those that did not fit into either C or S. This classification has since been expanded to include many other asteroid types. The number of types continues to grow as more asteroids are studied.

The two most widely used taxonomies now used are the Tholen classification and SMASS classification. The former was proposed in 1984 by David J. Tholen, and was based on data collected from an eight-color asteroid survey performed in the 1980s. This resulted in 14 asteroid categories. In 2002, the Small Main-Belt Asteroid Spectroscopic Survey resulted in a modified version of the Tholen taxonomy with 24 different types. Both systems have three broad categories of C, S, and X asteroids, where X consists of mostly metallic asteroids, such as the M-type. There are also several smaller classes.

The proportion of known asteroids falling into the various spectral types does not necessarily reflect the proportion of all asteroids that are of that type; some types are easier to detect than others, biasing the totals.

Problems

Originally, spectral designations were based on inferences of an asteroid's composition. However, the correspondence between spectral class and composition is not always very good, and a variety of classifications are in use. This has led to significant confusion. Although asteroids of different spectral classifications are likely to be composed of different materials, there are no assurances that asteroids within the same taxonomic class are composed of similar materials.

Exploration

See also: Sample return mission, Asteroid mining and Colonization of the asteroids

951 Gaspra is the first asteroid to be imaged in close-up (enhanced color).

Vesta imaged by the Dawn spacecraft

Several views of 433 Eros in natural colour

Until the age of space travel, objects in the asteroid belt were merely pinpricks of light in even the largest telescopes and their shapes and terrain remained a mystery. The best modern ground-based telescopes and the Earth-orbiting Hubble Space Telescope can resolve a small amount of detail on the surfaces of the largest asteroids, but even these mostly remain little more than fuzzy blobs. Limited information about the shapes and compositions of asteroids can be inferred from their light curves (their variation in brightness as they rotate) and their spectral properties, and asteroid sizes can be estimated by timing the lengths of star occulations (when an asteroid passes directly in front of a star). Radar imaging can yield good information about asteroid shapes and orbital and rotational parameters, especially for near-Earth asteroids. In terms of delta-v and propellant requirements, NEOs are more easily accessible than the Moon.

The first close-up photographs of asteroid-like objects were taken in 1971 when the Mariner 9 probe imaged Phobos and Deimos, the two small moons of Mars, which are probably captured asteroids. These images revealed the irregular, potato-like shapes of most asteroids, as did later images from the Voyager probes of the small moons of the gas giants.

The first true asteroid to be photographed in close-up was 951 Gaspra in 1991, followed in 1993 by 243 Ida and its moon Dactyl, all of which were imaged by the Galileo probe en route to Jupiter.

The first dedicated asteroid probe was NEAR Shoemaker, which photographed 253 Mathilde in 1997, before entering into orbit around 433 Eros, finally landing on its surface in 2001.

Other asteroids briefly visited by spacecraft en route to other destinations include 9969 Braille (by Deep Space 1 in 1999), and 5535 Annefrank (by Stardust in 2002).

In September 2005, the Japanese Hayabusa probe started studying 25143 Itokawa in detail and was plagued with difficulties, but returned samples of its surface to Earth on 13 June 2010.

The European Rosetta probe (launched in 2004) flew by 2867 Šteins in 2008 and 21 Lutetia, the third-largest asteroid visited to date, in 2010.

In September 2007, NASA launched the Dawn Mission, which orbited 4 Vesta from July 2011 to September 2012, and is planned to orbit the dwarf planet 1 Ceres in 2015. 4 Vesta is the second-largest asteroid visited to date.

On 13 December 2012, China's lunar orbiter Chang'e 2 flew within 2 miles (3.2 km) of the asteroid 4179 Toutatis on an extended mission.

Planned and future missions

The Japan Aerospace Exploration Agency (JAXA) plans to launch around 2015 the improved Hayabusa 2 space probe and to return asteroid samples by 2020. Current target for the mission is the C-type asteroid (162173) 1999 JU3.

In May 2011, NASA announced the OSIRIS-REx sample return mission to asteroid 1999 RQ36, and is expected to launch in 2016.

On 15 February 2013, an asteroid measuring approximately 18 metres (59 feet) with a mass of about 9,100 tonnes (10,000 short tons) exploded over Chelyabinsk, Russia causing 1,500 injuries and damaging 7,000 buildings. Small samples of the rocky Chelyabinsk meteorite were quickly recovered and analyzed with a larger fragment found several months later.

In early 2013, NASA announced the planning stages of a mission to capture a near-Earth asteroid and move it into lunar orbit where it could possibly be visited by astronauts and later impacted into the Moon. On 19 June 2014, NASA reported that asteroid 2011 MD was a prime candidate for capture by a robotic mission, perhaps in the early 2020s.

It has been suggested that asteroids might be used as a source of materials that may be rare or exhausted on Earth (asteroid mining), or materials for constructing space habitats (see Colonization of the asteroids). Materials that are heavy and expensive to launch from Earth may someday be mined from asteroids and used for space manufacturing and construction.

Fiction

Main article: Asteroids in fiction

Asteroids and the asteroid belt are a staple of science fiction stories. Asteroids play several potential roles in science fiction: as places human beings might colonize, resources for extracting minerals, hazards encountered by spaceships traveling between two other points, and as a threat to life on Earth by potential impact.

See also

• Asteroid Day
• Asteroid impact avoidance
• Atira asteroids, (Interior-Earth objects, asteroids with orbits fully within that of Earth).
• BOOTES (Burst Observer and Optical
  Transient Exploring System)
• Category:Asteroids
• Category:Asteroid groups and families
• Category:Binary asteroids
• Centaur (minor planet)
• Constellation program
• Dwarf planet
• Impact event
• List of asteroid close approaches to Earth
• List of minor planets named after people
• List of minor planets named after places
• List of minor planets
• List of notable asteroids
• Lost asteroid
• Marco Polo (spacecraft)
• Meanings of minor planet names
• Mesoplanet
• Minor planet
• Near-Earth object
• Near Earth Object Surveillance Satellite NEOsStat Canada's New Satellite
• Pioneer 10 space probe
• Pronunciation of asteroid names
• Rosetta probe

Notes

References

External links

• Alphabetical list of minor planet names (ASCII) (Minor Planet Center)
• Near Earth Asteroid Tracking (NEAT)
• Near Earth Objects Dynamic Site
• Spaceguard Centre
• IAU Committee on Small Body Nomenclature
• When Did the Asteroids Become Minor Planets?
• Asteroid articles in Planetary Science Research Discoveries
• TECA Table of next close approaches to the Earth
• NEO MAP (Armagh Observatory)

UpToDate Contents

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English Journal

• Spectral-Domain and Swept-Source OCT Imaging of Asteroid Hyalosis: A Case Report.

• Alasil T, Adhi M, Liu JJ, Fujimoto JG, Duker JS, Baumal CR.
• Ophthalmic surgery, lasers & imaging retina.Ophthalmic Surg Lasers Imaging Retina.2014 Sep 23:1-3. doi: 10.3928/23258160-20140909-01. [Epub ahead of print]
• A 72-year-old man with diabetes was referred to the retina clinic for diabetic retinopathy. Detailed funduscopic examination of the left eye was limited by prominent asteroid hyalosis. Spectral-domain (SD) and swept-source (SS) optical coherence tomography (OCT) were utilized to examine the vitreous
• PMID 25230400

• Comment on 'Photocoagulation guided by wide-field fundus autofluorescence in eyes with asteroid hyalosis': single and double pass of light in the ocular media.

• Davies N.
• Eye (London, England).Eye (Lond).2014 Sep 12. doi: 10.1038/eye.2014.215. [Epub ahead of print]
• PMID 25213999

• Retraction. X-ray fluorescence spectrometry of asteroid Itokawa by Hayabusa.

• Okada T1, Shirai K2, Yamamoto Y2, Arai T2, Ogawa K2, Hosono K2, Kato M2.
• Science (New York, N.Y.).Science.2014 Sep 12;345(6202):1254. doi: 10.1126/science.345.6202.1254-a.
• PMID 25214596

• Granuloma Annulare Mimicking Sarcoidosis: Report of Patient With Localized Granuloma Annulare Whose Skin Lesions Show 3 Clinical Morphologies and 2 Histology Patterns.

• Cohen PR1, Carlos CA.
• The American Journal of dermatopathology.Am J Dermatopathol.2014 Aug 19. [Epub ahead of print]
• : Granuloma annulare, a benign dermatosis of undetermined etiology, typically presents in a localized or generalized form. It has 3 distinctive histologic patterns: an infiltrative (interstitial) pattern, a palisading granuloma pattern, and an epithelioid nodule (sarcoidal granuloma) pattern. A man
• PMID 25140662

Japanese Journal

• CLINICAL INVESTIGATION : Silicone intraocular lens surface calcification in a patient with asteroid hyalosis

• Matsumura Kazuhiro,Takano Masahiko,Shimizu Kimiya [他]
• Japanese journal of ophthalmology : the official English-language journal of the Japanese Ophthalmological Society 56(4), 319-323, 2012-07
• NAID 40019342254

• 小惑星サンプルリターンミッション「はやぶさ2」の進捗状況 (宇宙・航行エレクトロニクス・宇宙応用シンポジウム)

• 吉川 真,南野 浩之,中澤 暁 [他]
• 電子情報通信学会技術研究報告 : 信学技報 112(107), 19-22, 2012-06-28
• NAID 40019342496

• 「はやぶさ」航法誘導を目的とした小惑星イトカワの3Dモデル作成 (特集 人工衛星・探査機からの映像およびリモートセンシング) -- (人工衛星・探査機による映像ならびに画像の取得)

• 丸家 誠
• 映像情報メディア学会誌 66(6), 452-454,435, 2012-06
• NAID 40019341274

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• asteroidの意味 - 英和辞書 - 英語辞書 - goo辞書

asteroidとは。意味や和訳。[名]1 《天文》小遊星，小惑星：主に火星と木星との軌道間にある.2 《動物》ヒトデ(starfish). [形]星状の.às・ter・ói・dal[形] - goo英和辞書は14万項目以上を収録し、発音、音声、慣用句、例文が分かる英和 ...

• ASTEROID - 日本スペースガード協会

ASTEROID編集室 ASTEROID編集室 〒151-0073東京都渋谷区笹塚1丁目60番地7号 2F E-mail:info@spaceguard.or.jp( 高橋） 室長: 高橋典嗣 編集員: 内田麻紀、下田哲郎、吉川真 顧問: 松島弘一 ホームページ担当: 山本威一郎 ...

Related Pictures

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★リンクテーブル★

リンク元	「astral」「星状」「stellate」「小惑星」
拡張検索	「Nocardia asteroides」

「astral」

　　[★]

• 星状の、星状体の

関

aster、asteroid、asteroid body、astral body、stellate

　

「星状」

　　[★]

英

stellate、asteroid、astral

関

星状体、小惑星

「stellate」

　　[★]

• adj.

• 星状の

関

asteroid、astral

「小惑星」

　　[★]

英

asteroid

関

星状

「Nocardia asteroides」

　　[★]

英

Nocardia asteroides

関

ノカルジア属、細菌

• グラム陽性桿菌
• 好気性細菌　(⇔[[]]

感染症

• ノカルジア症