Satellite - Eisei (English spelling)

Japanese: 衛星 - えいせい（英語表記）satellite

A celestial body that moves around a planet or other object under the force of gravity. To date, no moons have been discovered on Mercury or Venus in the solar system, but all other planets have moons. In general, compared to the parent planet, a moon's diameter is anywhere from a few tens to a few thousandths, and its mass is also less than a few thousandths, with the exception of Earth's moon, which is about a quarter of Earth's diameter and 1/81st of Earth's mass. Since the 20th century, many artificial satellites have been built that orbit the Earth, but this article will focus on natural satellites. For more information on artificial satellites, please refer to the relevant article.

[Naoaki Owaki]

Satellite Observation History

Galileo was the first to discover that other planets, excluding the Moon, had moons, and in 1610 he observed four moons around Jupiter (Io, Europa, Ganymede, and Callisto). These are now called the Galilean moons. Later, in the late 17th century, the Dutchman Huygens discovered a moon around Saturn (Titan), and the Paris Observatory's Cassini also discovered four moons around Saturn (Iapetus, Rhea, Dione, and Tethys). In the 19th century, the American astronaut Hall discovered two moons around Mars (Deimos and Phobos), but it is well known that the two moons of Mars were already described in detail in Gulliver's Travels, published about 150 years earlier. Since then, moons have been discovered one after another around each planet through observations from the ground, leaving only Mercury and Venus as undiscovered planets. Furthermore, since the 1970s, the existence of new moons on the outer planets has been confirmed by planetary probes. It is likely that these planets still have small moons, and there is a possibility that more will be discovered in the future.

Moons have long been important subjects of research in celestial mechanics, with their movements being the subject of study. Since the 1980s, space exploration and other research has shed light on the size, shape, physical and chemical properties, and various phenomena of moons, providing clues not only about the origin and evolution of the moons themselves, but also about various issues surrounding their parent planets and, ultimately, the solar system.

[Naoaki Owaki]

Characteristics of satellite movement

(1) In general, the eccentricity of a satellite (orbital eccentricity; a constant that determines the shape of a conic section; if it is 0, it is a circle; if it is greater than 0 and less than 1, it is an ellipse; if it is equal to 1, it is a parabola; and if it is greater than 1, it is a hyperbola) is small (with the exception of Nereid, the second moon of Neptune).

(2) The inclination angle of the satellite's orbital plane (the plane of the circle that the satellite traces as it orbits the planet) to the parent planet's equatorial plane (the plane when the celestial body is cut in a cross section at the equator) is less than 90 degrees. In other words, most of the satellites orbit in the same direction as the parent planet's rotation (prograde). However, there are exceptions for Jupiter, Saturn, and Neptune.

(3) The orbital period (the time it takes for one celestial body to orbit another celestial body; in this case, the time it takes for a moon to orbit a planet) is generally longer than the rotation period of the parent planet (both relative to the star). (Exceptions are the first moon of Mars and the 15th and 16th moons of Jupiter.)

[Naoaki Owaki]

Characteristics of each satellite

(1) As stated in the previous section (3), the orbital period of Phobos, the first moon of Mars, is shorter than the rotation period of Mars.

(2) Jupiter's four so-called Galilean moons have small inclinations and eccentricities, but are larger than the other moons of Jupiter. The third moon, Ganymede, is the largest in the solar system. The ratio of the orbital periods of the first moon, Io, the second moon, Europa, and the third moon, Ganymede, is a rational ratio of 1:2:4, and when the mean motions of the three moons are _n1 , _n2 , and _n3, respectively, there is a relationship of _n1 - _3n2 + _2n3 = 0, which appears as a special situation in the movement of the three moons. The outer eighth moon, Pasiphae, the ninth moon, Sinope, the eleventh moon, Carme, and the twelfth moon, Ananke, have orbital inclinations greater than 90 degrees (opposite the direction of Jupiter's rotation). The existence of atmospheres, ice, water, and rockspheres has been revealed in the Galilean moons.

(3) In Saturn, the moons at a medium distance from the parent planet are large, and the sixth moon Titan is the second largest in the solar system. The ninth moon Phoebe is retrograde and has a large eccentricity. It is an interesting fact that the outer moons of both Jupiter and Saturn have retrograde motion. In addition, the moons of Saturn have an approximate relationship similar to the Titius-Bode law, and if the semi-major axis of the orbit of the second moon Enceladus is 4, the semi-major axis of each moon can be expressed as 4 + ²ⁿ (n is the moon number, and n = -∞ for the first moon Mimas, excluding the eleventh moon Epimetheus and the rings). The orbital period of Mimas and the third moon Tethys is about 1:2, that of Enceladus and the fourth moon Dione is also 1:2, and that of Titan and the seventh moon Hyperion is 3:4, providing important problems in celestial mechanics. Titan has been found to have an atmosphere, ice, water, and a rocksphere. The eighth moon, Iapetus, exhibits the greatest variation (change in brightness) of any of the moons, with an amplitude of 1 magnitude, probably due to the rotation of its uneven surface. Saturn appears to have many other small moons.

(4) The orbital planes of the moons of Uranus are all nearly parallel to the equatorial plane of Uranus, but since the equatorial plane of Uranus is tilted by 97 degrees and 9 minutes to the orbital plane of Uranus (the plane of the circle that Uranus traces as it revolves around the Sun), the orbital planes of the moons are nearly perpendicular to the orbital plane of Uranus. The moons on the outside are larger in size.

(5) Neptune's innermost moon, Triton, is retrograde and large, but its outermost moon, Nereid, is prograde and only one-eighth the size of Triton. In return, its eccentricity is abnormally large at 0.75 (the largest in the solar system, excluding comets and some asteroids), and its orbit is extremely elongated. By comparison, the eccentricity of the Moon is 0.0549. As such, there are many interesting facts about the motion of the moons and the moons themselves, which provide important clues to solving various questions about not only moons, but also planets and the solar system.

The fact that Saturn has rings was already discovered by Huygens in the 17th century, but since the 1970s it has been discovered that Jupiter, Uranus, and Neptune also have rings. The existence and characteristics of rings of these planets outside the asteroid belt, as well as the characteristics of their moons, provide important questions.

[Naoaki Owaki]

"The 35 Planets and Moons of the Solar System" by Ron Miller and William K. Hartman, supervised by Oobi Shinya (1982, Obunsha)" ▽ "Our Solar System" by Isaac Asimov, translated by Ohara Takahiro (1989, Fukutake Shoten)" ▽ "Modern Planetology" by Komori Nagao (1992, Tokai University Press)" ▽ "The Solar System - Stars and Galaxies" by Wada Akio (1995, Kindai Bungeisha) ▽ "The Origin and Evolution of the Solar System - Exploring the Mystery of the Birth of the Earth" by Alexei Alexandrovich Malakshev, translated by Oshide Takashi, Komori Nagao, and Aoki Bin (1997, Tokai University Press)" ▽ "Iwanami Lectures on Earth and Planetary Sciences 12: Comparative Planetology" by Matsui Takanori, Nagahara Yuko, Fujiwara Akira, and others (1997, Iwanami Shoten)" ▽ "Ancient Records of the Stars" by Kuniharu Saito (Iwanami Shinsho)" ▽ "A Grand Tour of the Solar System - The Voyager 1 and 2 Voyage" by Nobuo Nakatomi (Shincho Bunko) ▽ "Flying Through the Solar System" by Nagao Komori (Shinnihon Shuppansha, Shinnihon Shinsho)"

Phobos
The first moon of Mars. It is an irregular ellipsoid with a major axis of about 27 km, and its surface is marked by craters and grooves. Photographed by the Mars Reconnaissance Orbiter (MRO) © NASA / JPL-Caltech / University of Arizona ">

Phobos

Deimos
The second moon of Mars. It is an irregular ellipsoid with a major axis of about 16 km. Photographed by the Mars Reconnaissance Orbiter (MRO) © NASA / JPL-Caltech / University of Arizona ">

Deimos

Io
Jupiter's first moon. Diameter 3,640 km. The ring-shaped areas scattered on the surface are areas of active volcanic activity. Photographed by the Jupiter probe "Galileo" © NASA / JPL / University of Arizona ">

Europa
Jupiter's second moon. Diameter 3,130 km. The surface is covered with ice and has irregular stripes. Photographed by the Jupiter probe Galileo © NASA/JPL/DLR ">

Europa

Ganymede
Jupiter's third moon. Diameter 5,280 km. It is the largest moon in the solar system. Photographed by the Jupiter probe "Galileo" © NASA/JPL ">

Ganymede

Callisto
Jupiter's fourth moon. Diameter 4,820 km. There are many craters on the surface. Photographed by the Jupiter probe "Galileo" © NASA / JPL / DLR ">

Callisto

Mimas
Saturn's first moon. Diameter 396 km. Mimas's largest crater, Herschel, can be seen on the right. Photographed by the Cassini spacecraft © NASA/JPL-Caltech/Space Science Institute ">

Mimas

Enceladus
Saturn's second moon. Its diameter is about 500 km. Its surface is covered with ice, and it is thought that there is a layer of liquid underneath. Photographed by the planetary probe "Voyager 2" © NASA/JPL ">

Enceladus

Tethys
Saturn's third moon. Diameter: 1070 km. The upper left shows Odysseus, the largest crater on Tethys. Photographed by the Cassini spacecraft © NASA/JPL/Space Science Institute ">

Tethys

Dione
Saturn's fourth moon. Diameter: 1,120 km. The line at the back is Saturn's ring. Photographed by the Cassini spacecraft © NASA/JPL-Caltech/Space Science Institute ">

Dione

rare
Saturn's fifth moon. Diameter 1,528 km. Photographed by the Cassini spacecraft © NASA/JPL/Space Science Institute ">

rare

Titanium
The sixth moon of Saturn. It is the largest of Saturn's moons, with a diameter of 5,150 km. It is covered in haze due to its thick atmosphere, which is mainly composed of nitrogen. Photographed by the Cassini spacecraft © NASA / JPL-Caltech / Space Science Institute ">

Titanium

Hyperion
Saturn's seventh moon. It is an irregular ellipsoid with a major axis of about 360 km, and its surface is covered with numerous deep craters. False-color image taken by the Cassini spacecraft © NASA/JPL/Space Science Institute ">

Hyperion

Iapetus
Saturn's eighth moon. Also called Iapetus or Iapetus. Diameter: 1,470 km. Photographed by the Cassini spacecraft © NASA/JPL/Space Science Institute ">

Iapetus

Triton
The first moon of Neptune. It has a diameter of 2,706 km. It is a retrograde moon that revolves in the opposite direction to the rotation of its parent planet. The photo shows the south pole of Triton. Photographed by the planetary probe "Voyager 2" © NASA/JPL .

Triton

month
Near side (the side seen from Earth). The large dark area in the upper left of the center is Mare Imbrium, followed by Mare Serenidae, Mare Tranquillitatis, and Mare Abundantia to the lower right. The white dot on the left of the center is the crater Copernicus, and below you can see the rayed crater Tycho. ©National Astronomical Observatory of Japan ">

month

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

惑星などの周囲をその引力のもとに運動する天体をいう。現在のところ、太陽系では水星と金星には発見されていないが、その他の惑星はすべて衛星をもっている。一般に母惑星に比べ、直径は数十分の1から数千分の1、質量も数千分の1以下であるが、地球の衛星（月）は例外であって、直径は地球の約4分の1、質量は81分の1である。なお20世紀以降、地球を周回する人工衛星が多数つくられたが、ここでは天然の衛星について述べる。「人工衛星」については、その項目を参照されたい。

［大脇直明］

衛星の観測史

月を除き、他の惑星にも衛星があることを初めて発見したのはガリレイで、1610年に木星の4個（イオ、ユーロパ、ガニメデ、カリスト）を観測した。今日ガリレオ衛星とよばれるものである。その後、17世紀後半にオランダのホイヘンスが土星の衛星（チタン）を発見し、さらにパリ天文台のカッシーニが同じく土星に4個の衛星（イアペトス、レア、ディオネ、テチス）を発見した。19世紀にはアメリカのホールが火星に2個の衛星（デイモス、フォボス）を発見したが、この150年ほど前に刊行された『ガリバー旅行記』に、すでに火星の2衛星について詳細に述べられていたというのは有名な話である。以後、地上からの観測により相次いで各惑星に衛星が発見され、未発見の惑星は水星と金星のみとなった。さらに1970年代以降は、惑星探査機によって外惑星に新しい衛星の存在が確認された。おそらくこれらの惑星には微小な衛星がまだ存在し、これからも発見される可能性があろう。

　衛星は、その運動に関して古来天体力学の重要な研究対象であったが、1980年代以降の宇宙探査などにより、衛星の大きさ、形状、物理的・化学的性質や諸現象が明らかとなり、衛星自体の成因や進化のみならず、母惑星の、ひいては太陽系のさまざまな諸問題を明らかにする手掛りを与えつつある。

［大脇直明］

衛星の運動の特徴

(1)一般に衛星の離心率（軌道離心率。円錐曲線の形を決める定数で、0だと円、0より大きく1より小さいと楕円(だえん)、1と等しいと放物線、1より大きいと双曲線となる）は小さい（海王星第2衛星ネレイドなどは例外）。

(2)母惑星の赤道面（天体を赤道で輪切りにしたときの平面）に対する衛星軌道面（衛星が惑星を周回するときに描く円の平面）の傾斜角は90度より小さい。すなわち、母惑星の自転方向と同じ方向に公転する（順行）ものが多い。ただし、木星・土星・海王星にはそれぞれ例外がある。

(3)公転周期（天体が他の天体を周回するのに要する時間。この場合は惑星の周りを衛星が周回するのに要する時間）は一般に母惑星の自転周期（いずれも対恒星）より長い（例外は火星の第1衛星および木星の第15、第16衛星）。

［大脇直明］

各衛星の特徴

(1)火星の第1衛星フォボスは、前項(3)のように公転周期は火星自転周期より短い。

(2)木星では、4個のいわゆるガリレオ衛星は、対赤道面傾斜角も離心率も小さく、そのかわり、木星の他の衛星に比べ大形である。第3衛星ガニメデは太陽系中最大である。また、第1衛星イオ、第2衛星ユーロパ、第3衛星ガニメデの公転周期の比は1：2：4という有理数比で、それぞれの平均運動をn₁、n₂、n₃とするとき、n₁－3n₂＋2n₃＝0という関係があり、3衛星の運行に特別な状況となって現れる。外側の第8衛星パシファエ、第9衛星シノペ、第11衛星カルメ、第12衛星アナンケなどは軌道傾斜が90度より大きい（木星自転の方向に対し逆行する）。ガリレオ衛星では大気、氷、水、岩石圏の存在が明らかになってきた。

(3)土星では、母惑星から中ごろの距離にあるものが大形で、第6衛星チタンは太陽系第2の大きさである。第9衛星フェーベは逆行で離心率も大きい。木星、土星とも外側のものに逆行があるのは興味深い事実である。なお、土星の衛星系にはティティウス‐ボーデの法則に似た近似関係があり、第2衛星エンケラドゥスの軌道長半径を4とすると、各衛星の長半径は4＋2ⁿで表される（nは衛星番号、第1衛星ミマスはn＝－∞とし、第11衛星エピメテウスと環は除く）。ミマスと第3衛星テチスとの公転周期は約1：2、エンケラドゥスと第4衛星ディオネも1：2、チタンと第7衛星ヒペリオンは3：4で、天体力学上、重要な問題を提供する。チタンには大気、氷、水、岩石圏がみいだされている。また第8衛星イアペトスは振幅が1等に及ぶ変光（明るさが変化すること）をし、諸衛星の変光中最大である。たぶん不均一な表面の自転によるものであろう。土星には、このほか多くの微小衛星が存在するようである。

(4)天王星の衛星軌道面は、すべて天王星赤道面に対しほぼ平行であるが、天王星の赤道面は、天王星公転軌道面（天王星が太陽の周りを公転するときに描く円の平面）に対して97度9分傾いているので、衛星軌道面は天王星公転軌道面に対してほぼ直角である。衛星の大きさは外側のものが大きい。

(5)海王星では、内側の第1衛星トリトンが逆行で、大きさも大きいが、外側の第2衛星ネレイドは順行で、大きさはトリトンの8分の1にすぎない。そのかわり離心率は0.75と異常に大きく（太陽系内で彗星(すいせい)と一部の小惑星を除いて最大）、著しく細長い軌道をもつ。ちなみに月の離心率は0.0549である。このように衛星の運動および衛星自体にはいろいろ興味ある事実があり、これらは、衛星はもちろん、惑星や太陽系についての諸問題を解明する重要な手掛りとなる。

　なお、土星に環のあることは、すでに17世紀、ホイヘンスにより発見されているが、1970年代以降、木星、天王星、海王星にも環のあることがわかった。これらの、小惑星帯より外側の惑星に関し、環の存在やその特性は衛星の特性とともに重要な課題を提供するものである。

［大脇直明］

『ロン・ミラー、ウィリアム・K・ハートマン著、小尾信彌監修『太陽系35の惑星と衛星』（1982・旺文社）』▽『アイザック・アシモフ著、小原隆博訳『わたしたちの太陽系』（1989・福武書店）』▽『小森長生著『現代の惑星学』（1992・東海大学出版会）』▽『和田昭夫著『太陽系――恒星とギャラクシー』（1995・近代文芸社）』▽『アレクセイ・アレクサンドロヴィチ・マラークシェフ著、押手敬・小森長生・青木斌訳『太陽系の起源と進化――地球誕生の謎をさぐる』（1997・東海大学出版会）』▽『松井孝典・永原裕子・藤原顕他著『岩波講座　地球惑星科学12　比較惑星学』（1997・岩波書店）』▽『斉藤国治著『星の古記録』（岩波新書）』▽『中冨信夫著『太陽系グランドツアー――ボイジャー1・2号の旅』（新潮文庫）』▽『小森長生著『太陽系を翔ける』（新日本出版社・新日本新書）』

[参照項目] | イオ | 引力 | 海王星 | カッシーニ | 火星 | ガニメデ | カリスト | ガリレイ | 軌道 | 人工衛星 | 太陽系 | チタン | ティティウス‐ボーデの法則 | デイモス | 天王星 | 土星 | フォボス | ホイヘンス | ホール | 木星 | ユーロパ | 惑星 | 惑星探査機

フォボス
火星の第1衛星。長径約27kmの不規則な楕円体で、表面にはクレーターや溝状の地形がみられる。火星偵察軌道周回機（MRO）により撮影©NASA／JPL-Caltech／University of Arizona">

フォボス

デイモス

イオ

ユーロパ

ガニメデ

カリスト

ミマス

エンケラドゥス

テチス

ディオネ

レア