Comet

Japanese: 彗星 - すいせい

It is also called a comet. In English, the word comet means hair. A comet is one of the celestial bodies that orbit the sun along with planets and asteroids. Most of them are dim and can only be seen with a telescope, but occasionally bright ones with long tails suddenly appear. They can be seen with the naked eye for one to two weeks and become the most prominent celestial body in the night sky. Since ancient times, they have often been considered a sign of disaster, and this tendency still exists to some extent today. However, the main body of a comet is merely a lump of ice less than 10 kilometers in diameter, and does not affect the earth or humans.

About 10 comets have been observed every year. On average, 4 of these are new comets, and the rest are short-period comets, which are the same comet that appears more than twice. Since the 1990s, the significance of small solar system bodies such as asteroids and comets has been reassessed, and observation methods have improved, resulting in a dramatic increase in the number of small bodies discovered each year. Unlike asteroids, comets are not disks or star-like points, but always appear as a vague extension. This part is called a coma (meaning hair). Many comets have no tails, but this is not because they do not have tails, but rather because they are too thin to be seen.

[Kaoru Saito]

Orbit

A comet with a period of more than 200 years is called a long-period comet. The longest periods can be hundreds of thousands of years. The orbits of observed comets are generally squashed ellipses, with the point where the comet is closest to the Sun (perihelion) being within 4 astronomical units (an astronomical unit is the average distance between the Earth and the Sun), and the point where the comet leaves the Sun (aphelion) being several astronomical units farther away. In the case of a comet with a 200-year period, the aphelion distance is 67 astronomical units, which is about twice the distance of the orbit of Pluto.

Comets with periods of several thousand years or more follow orbits that are almost parabolic when they are close to the Sun. Some comets follow hyperbolic orbits that are unlikely to ever return to the Sun. However, even though they are hyperbolic, the difference between a hyperbolic and a parabolic one is slight. Among these comets, there must be rogue comets that have entered the Solar System from interstellar space, but this has not yet been confirmed by observation.

Many short-period comets (with periods of less than 200 years) have aphelions inside or outside the orbit of Jupiter. These are called Jovian comets. It is thought that long-period comets changed into short-period comets by the gravitational force when they passed close to Jupiter. There are a few comets that reach the orbits of Saturn, Uranus, and Neptune, and are called the Saturnian and Uranian comets. Most periodic comets are faint, and the only great comet is Halley's Comet, which is in the Neptune family.

Cometary orbits are characterized not only by their shape but also by the angle that the orbital plane makes with the ecliptic plane (orbital inclination). Planets and asteroids have an inclination angle of less than 20 degrees, but this angle varies for long-period comets, and they do not tend to converge on the ecliptic plane. Half of them also orbit in the opposite direction to planets. Only Jovian family comets have a small inclination angle with the ecliptic plane of less than 30 degrees, and move in the same direction as planets. Of the newly discovered short-period comets, there are many with a perihelion distance of more than one celestial unit, and which do not come within the Earth's orbit. This is a characteristic that is not a coincidence.

[Kaoru Saito]

naming

Due to the orbital characteristics of long-period new comets, it is impossible to predict when and where they will appear in the sky. Amateur astronomers around the world search every night to discover comets. Discovery reports are sent to the Smithsonian Observatory in the United States via a local observatory (the National Astronomical Observatory in Japan). If a comet is determined to be a comet, it is given a provisional number with the year of appearance and the letters a, b, c, etc., in that order, and if it is a new comet, it is called by the name of the discoverer (up to three people if there are multiple discoverers). Recently, even faint comets have been discovered, and more short-period comets can be observed throughout their orbits, so the way provisional numbers are assigned changed in 1995. Following the year of discovery, the provisional numbers are assigned according to the time of discovery of each comet: A1, A2, etc. for the first half of January, B1, B2, etc. for the second half, and Y1, Y2, etc. for the second half of December (I is not used). Periodic comets are only assigned provisional numbers when they are first discovered. After the orbit is calculated, the comet is given a definitive number based on the year of its perihelion and the order of its passage: I, II, III, ... For example, the provisional number of Comet Ikeya-Seki is 1965f, and its definitive number is 1965VIII.

There are also a few comets that are named after researchers rather than their discoverers; Comet Harry and Comet Encke are famous examples.

[Kaoru Saito]

Main unit

The tail of a comet's coma is made up of thin gas and small solid particles that shine in the sunlight. At the center of the coma is the main body of the comet, which is invisible from Earth, and emits gas and dust particles.

In the mid-19th century, it was discovered that the spatial orbits of meteor showers matched certain comets, and it was determined that comets were the parent bodies of meteors. As a result, the idea that the main body of a comet is a dense mass of meteoroid material became popular. However, comets repeatedly release solid particles and large amounts of gas every time they approach the sun, and they do not easily break down no matter how many times they return. To explain this, in 1950, American Fred Lawrence Whipple (1906-2004) proposed the theory that the main body of a comet is a "snowball" made of gaseous ice and solid particles.

There are slight disturbances in the motion of comets that cannot be explained by gravity alone. This was discovered in Comet Encke, and has been recognized in many short-period comets. The snowball theory states that this disturbance is a jet action caused by gas being released from a specific part of the comet's body. If we think about it this way, we can explain the amount of disturbance in the motion. In March 1986, the probes Vega 1 and 2 and Giotto approached Comet Harry and succeeded in taking television images of the comet's body, proving the correctness of the Whipple theory.

The main body of a comet is also called the nucleus. When a comet is far from the Sun and the light of its coma is faint, the size of the nucleus can be estimated by measuring the brightness of the comet. For large comets, the radius is several kilometers, and for Jovian comets, the radius is between one kilometer and several hundred meters.

The most abundant element in the universe is hydrogen (H), followed by oxygen (O), carbon (C), and nitrogen (N). According to the snowball hypothesis, the gaseous matter of comets is a mixture of these gaseous elements, namely water, carbon dioxide, methane, and ammonia, which condensed and mixed together at the low temperatures of space to form ice, with other complex carbon compounds contained as trace components. The solid particles are solid compounds of non-volatile elements such as silicon, iron, and magnesium.

[Kaoru Saito]

Top and tail

When a comet approaches within 3 astronomical units of the Sun, the evaporation of ice becomes intense, and solid particles are also released by the gas pressure. The evaporated gas is also called the parent substance, and it decomposes under the action of the sun's ultraviolet rays to produce daughter substances. For example, water _H2O splits into OH, H, and O, and carbon dioxide _CO2 becomes CO ⁺ , O, and C. The parent substance is transparent to visible light, but some of the daughter substances absorb and re-emit sunlight in the visible range. This makes the daughter substances glow, and their spread becomes visible to the eye. This forms the shape of the coma and tail.

The coma suddenly becomes brighter at 1.5 astronomical units from the Sun, and reaches its maximum at 1 astronomical unit, reaching a radius of tens of thousands to hundreds of thousands of kilometers. Within 1 astronomical unit, the power of solar ultraviolet rays to break down daughter molecules becomes stronger, so the coma actually shrinks. The tail begins to form at 1.5 astronomical units, and in large comets, it can exceed 100 million kilometers.

The luminous molecules in the coma that are most visible to the eye are carbon _C2 and the cyanide radical CN. In the ultraviolet region there are atoms such as hydrogen and oxygen. All of the luminous molecules in the tail are positively charged ions. _C2 and CN have been observed since the 19th century. _C2 has a spectral group in the green and blue region, which determines the extent of the coma seen by the eye. This is why comets appear green to the eye. CN emits strong light in the purple region, which determines the shape of the coma seen in photographs. In the 1970s, acetonitrile _CH3CN , hydrogen cyanide HCN, and water were detected by radio telescopes. These are thought to be among the parent molecules.

Comets that came 0.8 AU from the Sun showed a spectrum of sodium atoms. After Comet Ikeya-Seki approached the Sun, metal atoms such as iron, nickel, chromium, and copper appeared, indicating that solid particles had evaporated.

A comet's tail is usually divided into two parts. One is thin and straight, pointing away from the sun, with streaks, small bends, and irregularities in brightness. It is a plasma consisting of molecular ions and negative electrons that is carried away by the solar wind and interplanetary magnetic field.

The other tail is short but thick, with a curved tip that makes it seem as if it is being left slightly behind in the comet's motion. There is no unevenness. It is very bright, and comets with a prominent tail are called great comets. Looking at the spectrum, we can see that it is made up of solid particles caused by reflected sunlight. Because the particles are extremely small, they are pushed away by the light pressure of the sunlight. The degree of curvature of the tail can indicate the light pressure, and the size of the particles and sometimes their type can be estimated.

On rare occasions, a short tail can be seen in the direction of the Sun, called the "anti-tail." This is a group of meteoroid material trailing behind a comet, which can be seen due to the relationship between the comet's orbit and the Earth's position, proving that meteoroid material is actually produced from comets.

Since the mid-1960s, infrared light has been observed emanating from the central region of the coma. Solid particles are heated by sunlight and emit infrared light, providing information on the size, shape, and type of particles. Analysis of the infrared light and the shape of the tail indicates that the size of the solid particles is less than 10 micrometers, with most of the particles in the tail being around 1 micrometer in size. It is thought that the material is made up of silicate (similar to meteorites) and iron particles, with the possibility of containing small amounts of carbonaceous particles.

[Kaoru Saito]

Origin and End

Comets release material each time they return to Earth, approaching the Sun hundreds of times before disappearing. However, even today, 4.6 billion years after the birth of the solar system, new comets appear. Ernst Julius Öpik (1893-1985) from Estonia and Oort from the Netherlands considered the size of the orbits of long-period comets and came to the conclusion that there is a swarm of comets at a distance of 40,000 to 150,000 astronomical units from the Sun, orbiting the Sun. According to their theory, there are about 100 billion of them, and sometimes, due to the action of a star approaching the solar system, some of them change their orbit to fall toward the Sun, and eventually emerge as new comets. The Sun and the solar system were born from the condensation of interstellar gas and interstellar dust, and it is believed that planetesimals that could not grow into planets at that time were dispersed to the outer edge of the solar system by the action of the planets, forming comet clouds. If this is the case, comets hold the mystery of the birth of the solar system. Support for this idea is the presence of interstellar molecules in the parent material of comets, and the similarity of the chemical composition of cometary grains to interstellar dust.

It seems that short-period comets become celestial bodies similar to a type of meteorite when they release gaseous matter. The solid matter of comets is the parent material of meteors, and small particles are thought to spread into the space of the solar system and become the source of zodiacal light.

[Kaoru Saito]

"Starry Sky Traveler" by Hasegawa Ichiro (1975, Seibundo Shinkosha)" ▽ "The Story of Comets" by Tomita Koichiro (1977, Iwanami Shinsho)" ▽ "Comets - Exploring Their True Nature" by Saito Keiji (1983, Kodansha, Bluebacks)" ▽ "Comets - The Cutting Edge of Comet Science" by Nakamura Takeshi and Yamamoto Tetsuo (1984, Koseisha Koseisha)" ▽ "Capturing Halley's Comet" edited by the Astronomical Society of Japan (1986, University of Tokyo Press)" ▽ "Comets - Their Nature and Origin" edited by Sakurai Kunitomo and Shimizu Mikio (1989, Asakura Shoten)" ▽ "The Close Approach of a Comet - Where Do Comets Come From?" by Fujii Akira (2002, Kaiseisha)"

Orbits of major comets
Several comets whose orbits are known, such as Encke, Harry (Halley), and Ikeya-Seki, are shown here. The aphelion of comets with periods of up to 11 years, such as Encke, is near the orbit of Jupiter, while the aphelion of comets with periods of 60 to 85 years, such as Halley, is near the orbit of Neptune. This provides clues for studying the origin of each comet .

Orbits of major comets

The inclination of the comet's orbital plane
The orbital planes of Earth, Jupiter, Saturn, Uranus, and Neptune are almost circular on the ecliptic plane, as shown in the square. Some of the orbital planes of short-period comets with aphelions near the orbit of Jupiter are inclined to the ecliptic plane, but generally they are distributed on the ecliptic plane. The orbital planes of long-period comets, as shown in the oblong ellipse, tend to be distributed in a disorderly manner. (1) Comet Ikeya-Seki (2) Comet IRAS-Araki-Alcock (3) Comet Harry (4) Comet Schwassmann-Wachmann 1 (5) Comet Honda (6) Comet Giacobini-Zinnel (7) Comet Gunn (8) Comet Encke © Shigemi Numazawa ">

The inclination of the comet's orbital plane

Structure of a comet
©Shigemi Numazawa ">

Structure of a comet

The direction of the comet's tail
©Shigemi Numazawa ">

The direction of the comet's tail

Halley's Comet
A long-known periodic comet. Photographed on Easter Island on March 8, 1986 (Chilean time) ©NASA/NSSDC/W.Liller ">

Halley's Comet

Comet Ikeya-Seki
A long-period comet with a period of about 900 years. Photographed on November 4, 1965 (Showa 40) in Toyama City, Toyama Prefecture © Toyama City Museum of Science ">

Comet Ikeya-Seki

Comet Encke
A short-period comet with a period of 3.3 years. Photographed on October 11, 2013 (Heisei 25) using the Murikabushi Telescope at Ishigakijima Astronomical Observatory © National Astronomical Observatory of Japan ">

Comet Encke

Comet Ison
A non-periodic comet discovered in September 2012. Photographed by the Hubble Space Telescope on October 9, 2013 ©NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

Comet Ison

Pan-STARRS Comet
A non-periodic comet discovered in June 2011. Photographed on May 26, 2013 (Heisei 25) using the Murikabushi Telescope at Ishigakijima Astronomical Observatory © National Astronomical Observatory of Japan ">

Pan-STARRS Comet

Comet Lovejoy
A collective name for five comets discovered by amateur astronomer Lovejoy. The photo shows C/2014 Q2 (Lovejoy). Photographed on January 30, 2015 by the Wide-field Infrared Survey Explorer (NEOWISE). ©NASA/JPL-Caltech

Comet Lovejoy

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

ほうき星ともいう。英語ではコメットcometで髪を意味する。彗星は惑星・小惑星などとともに太陽を巡る天体の一つであり、その多くは暗く、望遠鏡でだけ見えるが、まれに明るいものが長い尾をもって突然に現れる。そのときは1～2週間は肉眼でも見え、夜空でもっとも目だつ天体になる。昔から災いの兆しとされることが多く、今日もその傾向が一部にある。しかし彗星の本体は直径10キロメートル以下の氷の塊にすぎず、地球や人間に影響を及ぼすことなどはない。

　彗星は毎年10個ほど観測されてきた。そのうち平均4個は新彗星であり、残りは短周期彗星とよばれる同じ彗星が二度以上現れる場合である。1990年代以降、小惑星、彗星など太陽系小天体の意味が見直されるようになり、観測手段も進んで、毎年の小天体発見数は格段に増している。彗星は小惑星と違い、円板や恒星状の点でなく、かならずぼんやりした広がりが見える。その部分をコマ（髪の意）という。尾のない彗星も多いが、それは、ないのではなく、薄くて見えないというのが正しい。

［斉藤馨児］

軌道

周期200年以上の彗星を長周期彗星という。長いものは周期が数十万年に及ぶ。観測される彗星の軌道は、一般につぶれた楕円(だえん)で、太陽にもっとも近づく点（近日点）は4天文単位（天文単位とは地球―太陽間の平均距離）以内にあり、離れる点（遠日点）は数天文単位より遠くにある。周期200年の場合、遠日点距離は67天文単位で冥王(めいおう)星軌道の約2倍の距離にあたる。

　周期が数千年以上の彗星は太陽の近くではほとんど放物線と変わらない軌道を描く。一部の彗星は二度とは太陽を訪れないはずの双曲線軌道をたどる。ただし双曲線とはいっても放物線との差はわずかである。このなかに星間空間から太陽系へ飛び込んできた放浪彗星があるはずであるが、まだ観測で確かめられたことはない。

　短周期彗星（周期200年以内）のなかには木星の軌道の内外に遠日点をもつものが多い。これを木星族彗星という。長周期彗星が木星の近くを通過したとき引力で短周期に変わったと考えられている。土星・天王星・海王星の軌道まで行く彗星がすこしあり、土星族・天王星族などという。周期彗星は暗いものが多く、大彗星は海王星族のハリー彗星（ハレー彗星）だけである。

　彗星軌道は形ばかりか軌道面の黄道面となす角度（軌道面傾斜角）に特徴がある。惑星と小惑星は傾斜角が20度以下であるが、長周期彗星ではこの角度はまちまちで、軌道が黄道面に集中する傾向はない。また惑星などとは逆回りに軌道を巡るものが半数を占める。木星族彗星だけは黄道面との傾斜角が30度以下と小さく、運動の向きも惑星と同じである。新たに発見される短周期彗星は、近日点の距離が1天体単位を超えていて、地球軌道の内側までは入ってこないものが目だつ。これは偶然ではない一つの特徴になっている。

［斉藤馨児］

命名

長周期の新彗星は軌道の特徴のため、いつ、空のどこに出てくるか予想できない。彗星の発見のために世界中でアマチュア天文家が毎夜捜索をしている。発見報告は地域の天文台（日本では国立天文台）を経てアメリカのスミソニアン天文台へ送られる。彗星と決まればその順に出現年と文字a、b、c、……をつけて仮の番号とし、新彗星なら発見者の名前（複数のときは3人まで）でよぶ。最近では暗い彗星までみつかるほか、短周期彗星は軌道の全周で観測できるものが増えたため、1995年から仮番号のつけ方が変わった。発見年に続き各彗星の発見時期によって、1月前半ならA1、A2……、後半はB1、B2……、12月後半であればY1、Y2……とする（Iは使わない）。周期彗星は最初の発見のときだけ仮番号をつける。軌道が計算されたあと、近日点通過の年と、通過の順にⅠ、Ⅱ、Ⅲ、……の番号をつけて確定番号とする。たとえば池谷(いけや)‐関彗星の仮番号は1965f、確定番号は1965Ⅷである。

　また、わずかだが発見者でなく研究者の名でよぶ彗星があり、ハリー彗星、エンケ彗星が有名である。

［斉藤馨児］

本体

彗星のコマの尾は、薄いガスや小さな固体の粒が太陽の光を受けて光っているものである。コマの中心部に、地球からは見えない彗星の本体があって、ガスと塵粒(ちりつぶ)を出している。

　19世紀の中ごろ、流星群の空間軌道のなかに特定の彗星と一致するものが、初めていくつもみつかり、彗星は流星の母天体とわかった。そのため、彗星の本体は流星物質の濃密な集団とする考えが有力となった。しかし彗星は、太陽に近づくたびに繰り返し固体粒と多量のガスとを放出し、何度回帰してもたやすくは分解しない。これらを説明するため、1950年アメリカのホイップルFred Lawrence Whipple（1906―2004）は、彗星の本体はガス物質の氷と固体の粒とからなる「雪玉」だとする説をたてた。

　彗星の運動には万有引力だけでは理解できないわずかな乱れがある。これはエンケ彗星でみつかり、多数の短周期彗星で認められてきた。雪玉説ではこの乱れを、本体の特定な部分からガス放出がおこるためのジェット作用だとする。そう考えると運動の乱れが量の点まで説明できる。1986年3月、ハリー彗星に近づいた探査機「ベガ」1号・2号と「ジオット（ジョット）」は、彗星本体のテレビ撮影に成功して、ホイップル説の正しさを実証した。

　彗星の本体を核ともいう。彗星が太陽から遠く、コマの光が薄いとき、彗星の光度を測ると核の大きさが推定できる。大彗星で半径数キロメートル、木星族彗星では1キロメートルから数百メートルである。

　宇宙にもっとも多量に存在する元素は水素（H）で、次に多いのは酸素（O）、炭素（C）、窒素（N）である。雪玉説によると彗星のガス物質はこれら気体元素の化合物すなわち水や二酸化炭素、メタン、アンモニアなどが宇宙の低温で凝結し、混じり合って氷となったもので、ほかに複雑な炭素化合物を微量成分として含んでいる。固体の粒はケイ素、鉄、マグネシウムなど非揮発性元素の固体化合物である。

［斉藤馨児］

コマと尾

彗星が太陽まで3天文単位に近づくと氷の蒸発が激しくなり、固体粒もガス圧にのって出てくる。蒸発ガスは母物質(ぼぶっしつ)ともいい、太陽紫外線の作用で分解して娘物質(むすめぶっしつ)を生じる。たとえば水H₂Oは、OH、H、Oに分かれ、二酸化炭素CO₂はCO⁺、O、Cになる。母物質は可視光に対して透明だが、娘物質のなかには可視域の太陽光を吸収し再放出するものがある。そのため娘物質は光り、その広がりが目に見えるようになる。それがコマと尾との形をつくる。

　コマは太陽から1.5天文単位で急に明るさを増し、1天文単位で最大になって半径数万～十数万キロメートルに達する。1天文単位以内では太陽紫外線が娘分子を分解する力が強くなるため、コマはかえって縮む。尾は1.5天文単位からでき始め、大彗星では1億キロメートルを超えることもある。

　コマの発光分子は、目に感じるものとしては炭素C₂、シアン基CNが強い。紫外域では水素、酸素などの原子がある。尾の発光分子はすべてプラス・イオンになっている。C₂とCNは19世紀から観測された。C₂は緑・青域にスペクトル群をもち、目に映るコマの広がりを決める。そのため彗星は目で緑色に見える。CNは紫域に強い光を出していて写真に写るコマの形を決める。1970年代に電波望遠鏡により、アセトニトリルCH₃CN、シアン化水素HCN、水が検出された。これらは母分子の一つと考えられる。

　太陽まで0.8天文単位にきた彗星はナトリウム原子のスペクトルを示す。池谷‐関彗星は太陽に近づいたあと、鉄、ニッケル、クロム、銅などの金属原子が現れた。これは固体粒が蒸発したことを示す。

　彗星の尾は普通2筋(すじ)に分かれる。一つは細くて、太陽と反対方向にまっすぐ伸び、内部に筋や細かな屈曲、明るさのむらを認める。分子イオンとマイナス電子とからなるプラズマが太陽風と惑星間磁場の流れにのって飛び去っているものである。

　もう一つの尾は短いが太く、彗星の運動の後ろへすこし取り残されるように先が曲がる。むらはない。明るさが強く、この尾の著しい彗星は大彗星となる。スペクトルをみると太陽光の反射で、固体粒からなるとわかる。粒がきわめて小さいため、太陽光の光圧を受けて押し流されているのである。尾の曲がりの程度から光圧がわかり、粒の大きさと、ときには種類が推定できる。

　まれに太陽の方向に短い尾を見ることがあり、「反対尾」という。彗星の後ろからついていく流星物質の集団が、彗星軌道と地球との位置の関係で見えるもので、彗星から流星物質の生まれる現実の証明になる。

　1960年中ごろから、コマ中心域から出てくる赤外線が観測されている。固体粒が太陽光で温められ赤外線を出しているもので、粒の大きさと形・種類などを知る資料が得られる。赤外光と尾の形との分析によると、固体粒の大きさは10マイクロメートル以下、尾では1マイクロメートルほどの微粒が多い。ケイ酸塩（隕石(いんせき)と似る）と鉄質の粒からなり少量の炭素質の粒を含むことも考えられる。

［斉藤馨児］

起源と終末

彗星は回帰のたびに物質を放出し数百回太陽に近づいたあとは消えてしまう。しかし太陽系の誕生以来46億年を経た今日でも新彗星が現れる。エストニアのエーピクErnst Julius Öpik（1893―1985）やオランダのオールトらは長周期彗星の軌道の大きさを検討して、太陽まで4万～15万天文単位の遠距離に彗星の大群があり、太陽を巡っているという考えに達した。その考えによれば、その数は1000億個ほどあり、それらはときどき太陽系に近づく恒星の作用で、一部分が太陽へ落ち込む軌道に変わり、やがて新彗星として出てくる。太陽と太陽系は星間ガスと星間塵(じん)とが凝集して生まれたが、そのとき惑星まで成長できなかった微惑星が、惑星の作用で太陽系の外縁へ拡散して彗星雲をつくったのではないかという。そしてもしそうであるなら彗星は太陽系の誕生の謎(なぞ)を秘めていることになる。彗星の母物質のなかに星間分子と似たものがあり、彗星の固体粒の化学組成が星間塵と似ていることは、この考えの支えとなる。

　短周期彗星はガス物質を放出してしまうと隕石の一種に似た天体になるらしい。彗星の固体物質は流星の母物質であるほか、小さな粒は太陽系の空間に広がって黄道光の原因物質になるとみられている。

［斉藤馨児］

『長谷川一郎著『星空のトラベラー』（1975・誠文堂新光社）』▽『冨田弘一郎著『彗星の話』（1977・岩波新書）』▽『斉藤馨児著『彗星――その実像を探る』（1983・講談社・ブルーバックス）』▽『中村士・山本哲生著『彗星――彗星科学の最前線』（1984・恒星社厚生閣）』▽『日本天文学会編『ハレー彗星をとらえた』（1986・東京大学出版会）』▽『桜井邦朋・清水幹夫編『彗星――その本性と起源』（1989・朝倉書店）』▽『藤井旭著『彗星大接近――彗星はどこからやってくる？』（2002・偕成社）』

おもな彗星の軌道
エンケ、ハリー（ハレー）、池谷-関など、軌道が判明しているいくつかの彗星を示した。エンケなど周期11年ぐらいまでの彗星の軌道遠日点は木星軌道付近にあり、ハリーなど周期60～85年の彗星の遠日点は海王星軌道付近にある。このことは、それぞれの彗星の起源を研究する手掛りとなる©Shogakukan">

おもな彗星の軌道

彗星の軌道面の傾き
四角に図示された黄道面上には地球、木星、土星、天王星、海王星のほぼ円形の軌道がある。木星の軌道付近に遠日点をもつ短い周期の彗星の軌道面のいくつかは黄道面に対して傾きが大きいが、総じて黄道面に分布する。長楕円に図示された長い周期の彗星の軌道面は、無秩序に分布する傾向にある(1)池谷-関彗星(2)IRAS-荒貴-オルコック彗星(3)ハリー彗星(4)シュワッスマン-ワッフマン第1彗星(5)本田彗星(6)ジャコビニ-ツィンネル彗星(7)ガン彗星(8)エンケ彗星©沼澤茂美">

彗星の軌道面の傾き

彗星の構造
©沼澤茂美">

彗星の構造

彗星の尾の伸びる方向

ハリー彗星（ハレー彗星）

池谷‐関彗星

エンケ彗星

アイソン彗星

パンスターズ彗星

ラブジョイ彗星

出典　小学館　日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について　情報 | 凡例

<<: Water gas - Suiseigasu (English spelling)