Milky Way Stars and gases at a wide range of distances from the Galactic center orbit at approximately 220 kilometers per second. The constant rotation speed contradicts the laws of Keplerian dynamics and suggests that much of the mass of the Milky Way does not emit or absorb electromagnetic radiation. This mass has been given the name “dark matter”.[22] The rotational period is about 240 million years at the position of the Sun.[9] The Galaxy as a whole is moving at a velocity of approximately 600 km per second with respect to extragalactic frames of reference. The oldest known star in the Galaxy is at least 13.6 billion years old and thus must have formed shortly after the Big Bang.[6] Surrounded by several smaller satellite galaxies, the Milky Way is part of the Local Group of galaxies, which forms a subcomponent of the Virgo Supercluster. Appearance[edit] The Milky Way has a relatively low surface brightness. Size and mass[edit] Schematic illustration showing the galaxy in profile
Phobos Phobos (systematic designation: Mars I) is the larger and closer of the two natural satellites of Mars. Both moons were discovered in 1877. Phobos has dimensions of 27 × 22 × 18 km,[1] and is too small to be rounded under its own gravity. Faint dust rings produced by Phobos and Deimos have long been predicted but attempts to observe these rings have, to date, failed.[23] Recent images from Mars Global Surveyor indicate that Phobos is covered with a layer of fine-grained regolith at least 100 meters thick; it is hypothesized to have been created by impacts from other bodies, but it is not known how the material stuck to an object with almost no gravity.[24] The unique Kaidun meteorite is thought to be a piece of Phobos, but this has been difficult to verify since little is known about the detailed composition of the moon.[25][26] Labeled Map of Phobos - Moon of Mars (USGS).[30] Tidal deceleration is gradually decreasing the orbital radius of Phobos.
Moon The Moon is in synchronous rotation with Earth, always showing the same face with its near side marked by dark volcanic maria that fill between the bright ancient crustal highlands and the prominent impact craters. It is the second-brightest regularly visible celestial object in Earth's sky (after the Sun), as measured by illuminance on the surface of Earth. Although it can appear a very bright white, its surface is actually dark, with a reflectance just slightly higher than that of worn asphalt. The Moon is thought to have formed nearly 4.5 billion years ago, not long after Earth. As of November 2014[update], the Moon is the only celestial body other than Earth on which humans have set foot. Future manned missions to the Moon have been planned, including government as well as privately funded efforts. Name and etymology The principal modern English adjective pertaining to the Moon is lunar, derived from the Latin Luna. Formation The evolution of the Moon and a tour of the Moon.
Mercury Mercury is gravitationally locked and rotates in a way that is unique in the Solar System. As seen relative to the fixed stars, it rotates exactly three times for every two revolutions[b] it makes around its orbit.[13] As seen from the Sun, in a frame of reference that rotates with the orbital motion, it appears to rotate only once every two Mercurian years. An observer on Mercury would therefore see only one day every two years. Because Mercury's orbit lies within Earth's orbit (as does Venus's), it can appear in Earth's sky in the morning or the evening, but not in the middle of the night. Internal structure Internal structure of Mercury: 1. Mercury's density can be used to infer details of its inner structure. Mercury's core has a higher iron content than that of any other major planet in the Solar System, and several theories have been proposed to explain this. Alternatively, Mercury may have formed from the solar nebula before the Sun's energy output had stabilized. Surface geology
Saturn Saturn's interior is probably composed of a core of iron, nickel and rock (silicon and oxygen compounds), surrounded by a deep layer of metallic hydrogen, an intermediate layer of liquid hydrogen and liquid helium and an outer gaseous layer.[15] The planet exhibits a pale yellow hue due to ammonia crystals in its upper atmosphere. Electrical current within the metallic hydrogen layer is thought to give rise to Saturn's planetary magnetic field, which is weaker than Earth's magnetic field but has a magnetic moment 580 times that of the Earth due to Saturn's larger body radius. Saturn's magnetic field strength is around one-twentieth the strength of Jupiter's.[16] The outer atmosphere is generally bland and lacking in contrast, although long-lived features can appear. Wind speeds on Saturn can reach 1,800 km/h (1,100 mph), faster than on Jupiter, but not as fast as those on Neptune.[17] Physical characteristics Composite image roughly comparing the sizes of Saturn and Earth Atmosphere
Asteriod Belt The asteroid belt (shown in white) is located between the orbits of Mars and Jupiter. History of observation[edit] In an anonymous footnote to his 1766 translation of Charles Bonnet's Contemplation de la Nature,[8] the astronomer Johann Daniel Titius of Wittenberg[9][10] noted an apparent pattern in the layout of the planets. If one began a numerical sequence at 0, then included 3, 6, 12, 24, 48, etc., doubling each time, and added four to each number and divided by 10, this produced a remarkably close approximation to the radii of the orbits of the known planets as measured in astronomical units. This pattern, now known as the Titius–Bode law, predicted the semi-major axes of the six planets of the time (Mercury, Venus, Earth, Mars, Jupiter and Saturn) provided one allowed for a "gap" between the orbits of Mars and Jupiter. In his footnote Titius declared, "But should the Lord Architect have left that space empty? Origin[edit] Formation[edit] Evolution[edit] Characteristics[edit]
Mars Animation of Mars' rotation from the vantage of an observer who moves south, then north, to hover over both poles, showing the planet's major topographic features. Mars is currently host to five functioning spacecraft: three in orbit – the Mars Odyssey, Mars Express, and Mars Reconnaissance Orbiter – and two on the surface – Mars Exploration Rover Opportunity and the Mars Science Laboratory Curiosity. Defunct spacecraft on the surface include MER-A Spirit and several other inert landers and rovers such as the Phoenix lander, which completed its mission in 2008. Observations by the Mars Reconnaissance Orbiter have revealed possible flowing water during the warmest months on Mars.[25] In 2013, NASA's Curiosity rover discovered that Mars' soil contains between 1.5% and 3% water by mass (about two pints of water per cubic foot or 33 liters per cubic meter, albeit attached to other compounds and thus not freely accessible).[26] Physical characteristics Size comparison of Earth and Mars. Soil
Alpha Centauri Location of Alpha Centauri in Centaurus (right-click on starmap to enlarge) From Earth to Alpha Centauri. Alpha Centauri (α Centauri, α Cen; also known as Rigil Kent /ˈraɪdʒəl ˈkɛnt/—see Names) is the brightest star in the southern constellation of Centaurus, and the third brightest star in the night sky.[10][11] The Alpha Centauri system is located 1.34 parsecs or 4.37 light years from the Sun, making it the closest star system to the Solar System.[12] Although it appears to the unaided eye as a single object, Alpha Centauri is actually a binary star system (designated Alpha Centauri AB or α Cen AB) whose combined visual magnitude of −0.27 makes it the third brightest star (other than the Sun) seen from Earth after the −1.46 magnitude Sirius and the −0.72 magnitude Canopus. Its component stars are named Alpha Centauri A (α Cen A), with 110% of the mass and 151.9% the luminosity of the Sun, and Alpha Centauri B (α Cen B), at 90.7% of the Sun's mass and 44.5% of its luminosity.
Jupiter Structure Jupiter is composed primarily of gaseous and liquid matter. It is the largest of four gas giants as well as the largest planet in the Solar System with a diameter of 142,984 km (88,846 mi) at its equator. The density of Jupiter, 1.326 g/cm3, is the second highest of the gas giants, but lower than for any of the four terrestrial planets. Composition Jupiter's upper atmosphere is composed of about 88–92% hydrogen and 8–12% helium by percent volume or fraction of gas molecules. Based on spectroscopy, Saturn is thought to be similar in composition to Jupiter, but the other gas giants Uranus and Neptune have relatively much less hydrogen and helium.[21] Because of the lack of atmospheric entry probes, high-quality abundance numbers of the heavier elements are lacking for the outer planets beyond Jupiter. Mass Jupiter's diameter is one order of magnitude smaller (×0.10045) than the Sun, and one order of magnitude larger (×10.9733) than the Earth. Internal structure Atmosphere Cloud layers
Earth Earth is the third planet from the Sun. It is the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets. It is sometimes referred to as the world or the Blue Planet.[23] Earth formed approximately 4.54 billion years ago, and life appeared on its surface within its first billion years.[24] Earth's biosphere then significantly altered the atmospheric and other basic physical conditions, which enabled the proliferation of organisms as well as the formation of the ozone layer, which together with Earth's magnetic field blocked harmful solar radiation, and permitted formerly ocean-confined life to move safely to land.[25] The physical properties of the Earth, as well as its geological history and orbit, have allowed life to persist. Name and etymology In general English usage, the name earth can be capitalized or spelled in lowercase interchangeably, either when used absolutely or prefixed with "the" (i.e. Heat
Pluto In 2015, the Pluto system is due to be visited by spacecraft for the first time. The New Horizons probe will perform a flyby during which it will attempt to take detailed measurements and images of the plutoid and its moons. Discovery Discovery photographs of Pluto In the 1840s, using Newtonian mechanics, Urbain Le Verrier predicted the position of the then-undiscovered planet Neptune after analysing perturbations in the orbit of Uranus. Subsequent observations of Neptune in the late 19th century caused astronomers to speculate that Uranus' orbit was being disturbed by another planet besides Neptune. In 1906, Percival Lowell, a wealthy Bostonian who had founded the Lowell Observatory in Flagstaff, Arizona in 1894, started an extensive project in search of a possible ninth planet, which he termed "Planet X".[25] By 1909, Lowell and William H. Name The discovery made headlines across the globe. ), but has a circle in place of the middle prong of the trident ( Demise of Planet X Other factors
Deimos Deimos was discovered by Asaph Hall, Sr. at the United States Naval Observatory in Washington, D.C on 12 August 1877, at about 07:48 UTC (given in contemporary sources as "11 August 14:40" Washington mean time, using an astronomical convention of beginning a day at noon, so 12 hours must be added to get the actual local mean time).[9][10][11][12] Hall also discovered Phobos on 18 August 1877, at about 09:14 GMT, after deliberately searching for Martian moons. It has an escape velocity of 5.6 m/s[3] and apparent magnitude of 12.45.[4] Only two geological features on Deimos have been given names. The craters Swift and Voltaire are named after writers who speculated on the existence of two Martian moons before Phobos and Deimos were discovered.[16] Deimos's orbit is nearly circular and is close to Mars's equatorial plane. Unlike Phobos, which orbits so fast that it actually rises in the west and sets in the east, Deimos rises in the east and sets in the west. ^ Jump up to: a b "Apsis".
Uranus Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. Uranus is similar in composition to Neptune, and both are of different chemical composition than the larger gas giants Jupiter and Saturn. For this reason, astronomers sometimes place them in a separate category called "ice giants". Uranus's atmosphere, although similar to Jupiter's and Saturn's in its primary composition of hydrogen and helium, contains more "ices" such as water, ammonia, and methane, along with traces of hydrocarbons.[12] It is the coldest planetary atmosphere in the Solar System, with a minimum temperature of 49 K (−224.2 °C), and has a complex, layered cloud structure, with water thought to make up the lowest clouds, and methane the uppermost layer of clouds.[12] In contrast, the interior of Uranus is mainly composed of ices and rock.[11] History Discovery The power I had on when I first saw the comet was 227. Naming Name Axial tilt
Venus Venus is a terrestrial planet and is sometimes called Earth's "sister planet" because of their similar size, gravity, and bulk composition (Venus is both the closest planet to Earth and the planet closest in size to Earth). However, it has also been shown to be very different from Earth in other respects. It has the densest atmosphere of the four terrestrial planets, consisting of more than 96% carbon dioxide. The atmospheric pressure at the planet's surface is 92 times that of Earth's. Physical characteristics Venus is one of the four terrestrial planets in the Solar System, meaning that, like Earth, it is a rocky body. Geography The Venusian surface was a subject of speculation until some of its secrets were revealed by planetary science in the 20th century. Most Venusian surface features are named after historical and mythological women.[26] Exceptions are Maxwell Montes, named after James Clerk Maxwell, and highland regions Alpha Regio, Beta Regio and Ovda Regio. Surface geology