Minor Space Bodies

Minor Space Bodies


Comets

A comet is a relatively small Solar System body. Comet nuclei are loose collections of ice, dust, and small rocky particles, ranging from a few hundred meters to tens of kilometers across. When a comet is close enough to the Sun, it displays a thin, fuzzy, temporary atmosphere called a coma, and sometimes also has a tail. These phenomenon are the result of the solar radiation and the solar wind interacting with the comet nucleus. Comets have been observed since ancient times and have historically been considered bad omens. Particularly bright or notable examples are called "Great Comets".


Halley's Comet

A color image of Halley's Comet

Halley's Comet or Comet Halley (officially designated 1P/Halley)[1] is the best-known of the short-period comets, and is visible from Earth every 75 to 76 years.[1][10] Halley is the only short-period comet that is clearly visible to the naked eye from Earth, and thus the only naked-eye comet that might appear twice in a human lifetime.[11] Other naked-eye comets may be brighter and more spectacular, but will appear only once in thousands of years. Halley's returns to the inner Solar System have been observed by astronomers since at least 240 BC, and recorded by Chinese, Babylonian, and medieval European chroniclers, but were not recognized as reappearances of the same object. The comet's periodicity was first determined in 1705 by English astronomer Edmond Halley, after whom it is now named. Halley's Comet last appeared in the inner Solar System in 1986 and will next appear in mid-2061.[12]

During its 1986 apparition, Halley's Comet became the first to be observed in detail by spacecraft, providing the first observational data on the structure of a comet nucleus and the mechanism of coma and tail formation.[13][14] These observations supported a number of longstanding hypotheses about comet construction, particularly Fred Whipple's "dirty snowball" model, which correctly surmised that Halley would be composed of a mixture of volatile ices—such as water, carbon dioxide and ammonia—and dust. However, the missions also provided data which substantially reformed and reconfigured these ideas; for instance it is now understood that Halley's surface is largely composed of dusty, non-volatile materials, and that only a small portion of it is icy.



Comet Hale Bopp

Image of Hale Bopp comet taken on 4th April 1997

Comet Hale-Bopp (formally designated C/1995 O1) was arguably the most widely observed comet of the twentieth century, and one of the brightest seen for many decades. It was visible to the naked eye for a record 18 months, twice as long as the previous record holder, the Great Comet of 1811.

Hale-Bopp was discovered on July 23, 1995 at a great distance from the Sun, raising expectations that the comet would brighten considerably by the time it passed close to Earth. Although predicting the brightness of comets with any degree of accuracy is very difficult, Hale-Bopp met or exceeded most predictions when it passed perihelion on April 1, 1997. The comet was dubbed the Great Comet of 1997.

The passage of Hale-Bopp was notable also for inciting a comet-related panic. Rumours that an alien spacecraft was following the comet gained remarkable currency, and inspired a mass suicide among followers of the Heaven's Gate cult.


Comet McNaught

Comet McNaught taken from Swift's Creek, Victoria, Australia

Comet McNaught, also known as the Great Comet of 2007 and given the designation C/2006 P1, is a non-periodic comet discovered on August 7, 2006 by British-Australian astronomer Robert H. McNaught.[2] It was the brightest comet for over 40 years, and was easily visible to the naked eye for observers in the Southern Hemisphere in January and February 2007.

With an estimated peak magnitude of -5.5, the comet was the second brightest since 1935.[3] Around perihelion on January 12, it was visible worldwide in broad daylight. Its tail measured an estimated 35 degrees in length at its peak.[4]

Given its estimated eccentricity over 1.0, suggesting a hyperbolic trajectory, it is believed McNaught will leave the Solar System and never be seen again.


Comet Holmes

Comet Holmes on November 4, 2007,
showing the blue ion tail on the right taken from Hungary

Comet Holmes (official designation: 17P/Holmes) is a periodic comet in our solar system, discovered by the British amateur astronomer Edwin Holmes on November 6, 1892. Although normally a very faint object, Holmes became notable during its October 2007 return when it temporarily brightened by a factor of about half a million, in what was the largest known outburst by a comet, and became visible to the naked eye. It also briefly became the largest object in the solar system, as its coma (the thin dissipating dust ball around the comet) expanded to a diameter greater than that of the Sun (although its mass remained minuscule

Comet Holmes was discovered by Edwin Holmes on November 6, 1892 while he was conducting regular observations of the Andromeda Galaxy (M31). Its discovery in 1892 was possible because of an increase in its magnitude similar to the 2007 outburst; it brightened to an approximate magnitude of 4 or 5 before fading from visibility over a period of several weeks.[3]


The comet's discovery was confirmed by Edward Walter Maunder (Royal Observatory, Greenwich, England), William Henry Maw (England), and Kidd (Bramley, England) and independent discoveries were made by Thomas David Anderson (Edinburgh, Scotland) on November 8 and by Mike Brown , (Wilkes) , (USA) and by John Ewen Davidson (Mackay, Queensland, Australia) on November 9.[4]

The first calculations of the elliptical orbits of 17P/Holmes were done independently by Heinrich Kreutz and George Mary Searle. Additional orbits eventually established the perihelion date as June 13 and the orbital period as 6.9 years. These calculations proved that the comet was not a return of Biela's Comet.

The 1899 and 1906 appearances were observed, but the comet was lost (see Lost comet) after 1906 until it was recovered on July 16, 1964 by Elizabeth Roemer (US Naval Observatory Flagstaff Station, Arizona, USA). Aided by the computer predictions of Brian G. Marsden, the comet has been observed on every subsequent return.

Planets

Planets


Mercury

Mercury is the innermost and smallest planet in the Solar System,[a] orbiting the Sun once every 87.969 days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about the axis for every two orbits. The perihelion of Mercury's orbit precesses around the Sun at an excess of 43 arcseconds per century; a phenomenon that was explained in the 20th century by Albert Einstein's General Theory of Relativity.[9] Mercury is bright when viewed from Earth, ranging from −1.9 to 5.7 in apparent magnitude, but is not easily seen as its greatest angular separation from the Sun is only 28.3°. Since Mercury is normally lost in the glare of the Sun, unless there is a solar eclipse, Mercury can only be viewed in morning or evening twilight.

Venus

Venus is the second-closest planet to the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows. Because Venus is an inferior planet from Earth, it never appears to venture far from the Sun: its elongation reaches a maximum of 47.8°. Venus reaches its maximum brightness shortly before sunrise or shortly after sunset, for which reason it is often called the Morning Star or the Evening Star.

Earth

Earth (or the Earth) is the third planet from the Sun, the fifth-largest and the densest 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, the Blue Planet,[note 7] or by its Latin name, Terra.[note 8]

Home to millions of species[16] including humans, Earth is the only place in the universe where life is known to exist. The planet formed 4.54 billion years ago,[17] and life appeared on its surface within a billion years. Since then, Earth's biosphere has significantly altered the atmosphere and other abiotic conditions on the planet, enabling the proliferation of aerobic organisms as well as the formation of the ozone layer which, together with Earth's magnetic field, blocks harmful solar radiation, permitting life on land.[18] The physical properties of the Earth, as well as its geological history and orbit, have allowed life to persist during this period. Without intervention, the planet could be expected to continue supporting life for between 0.5[19] to 2.3 billion[20] years, after which the rising luminosity and expansion of the Sun – as a result of the gradual but inexorable depletion of its hydrogen fuel – would eventually eliminate the planet's biosphere.[21]

Earth's outer surface is divided into several rigid segments, or tectonic plates, that gradually migrate across the surface over periods of many millions of years. About 71% of the surface is covered with salt water oceans, the remainder consisting of continents and islands which together have many lakes and other sources of water contributing to the hydrosphere. Liquid water, necessary for all known life, is not known to exist on any other planet's surface.[note 9][note 10] Earth's poles are mostly covered with solid ice (Antarctic ice sheet) or sea ice (Arctic ice cap). The planet's interior remains active, with a thick layer of relatively solid mantle, a liquid outer core that generates a magnetic field, and a solid iron inner core.

Earth interacts with other objects in space, especially the Sun and the Moon. At present, Earth orbits the Sun once for every roughly 366.26 times it rotates about its axis. This is a sidereal year, which is equal to 365.26 solar days.[note 11] The Earth's axis of rotation is tilted 23.4° away from the perpendicular to its orbital plane,[22] producing seasonal variations on the planet's surface with a period of one tropical year (365.24 solar days). Earth's only known natural satellite, the Moon, which began orbiting it about 4.53 billion years ago, provides ocean tides, stabilizes the axial tilt and gradually slows the planet's rotation. Between approximately 3.8 billion and 4.1 billion years ago, numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the greater surface environment.

Both the mineral resources of the planet, as well as the products of the biosphere, contribute resources that are used to support a global human population. These inhabitants are grouped into about 200 independent sovereign states, which interact through diplomacy, travel, trade, and military action. Human cultures have developed many views of the planet, including personification as a deity, a belief in a flat Earth or in Earth as the center of the universe, and a modern perspective of the world as an integrated environment that requires stewardship.



Mars

Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance.[11] Mars is a terrestrial planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon and the volcanoes, valleys, deserts, and polar ice caps of Earth. Mars’ rotational period and seasonal cycles are likewise similar to those of Earth. Mars is the site of Olympus Mons, the highest known mountain in the Solar System, and of Valles Marineris, the largest canyon. The smooth Borealis basin in the northern hemisphere covers 40% of the planet and may be a giant impact feature.[12][13] Unlike Earth, Mars is now geologically and tectonically inactive.

Jupiter

Jupiter is the fifth planet from the Sun and the largest planet within the Solar System.[13] It is a gas giant with a mass slightly less than one-thousandth of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn, Uranus and Neptune. Together, these four planets are sometimes referred to as the Jovian planets.

The planet was known by astronomers of ancient times and was associated with the mythology and religious beliefs of many cultures. The Romans named the planet after the Roman god Jupiter.[14] When viewed from Earth, Jupiter can reach an apparent magnitude of −2.94, making it on average the third-brightest object in the night sky after the Moon and Venus. (Mars can briefly match Jupiter's brightness at certain points in its orbit.)

Jupiter is primarily composed of hydrogen with a quarter of its mass being helium; it may also have a rocky core of heavier elements. Because of its rapid rotation, Jupiter's shape is that of an oblate spheroid (it possesses a slight but noticeable bulge around the equator). The outer atmosphere is visibly segregated into several bands at different latitudes, resulting in turbulence and storms along their interacting boundaries. A prominent result is the Great Red Spot, a giant storm that is known to have existed since at least the 17th century when it was first seen by telescope. Surrounding the planet is a faint planetary ring system and a powerful magnetosphere. There are also at least 63 moons, including the four large moons called the Galilean moons that were first discovered by Galileo Galilei in 1610. Ganymede, the largest of these moons, has a diameter greater than that of the planet Mercury.

Jupiter has been explored on several occasions by robotic spacecraft, most notably during the early Pioneer and Voyager flyby missions and later by the Galileo orbiter. The most recent probe to visit Jupiter was the Pluto-bound New Horizons spacecraft in late February 2007. The probe used the gravity from Jupiter to increase its speed. Future targets for exploration in the Jovian system include the possible ice-covered liquid ocean on the moon Europa.



Saturn

Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn, along with Jupiter, Uranus and Neptune, is classified as a gas giant. Together, these four planets are sometimes referred to as the Jovian, meaning "Jupiter-like", planets.

Saturn is named after the Roman god Saturn, equated to the Greek Cronus (the Titan father of Zeus) the Babylonian Ninurta and to the Hindu Shani. Saturn's symbol represents the god's sickle (Unicode: ♄).

The planet Saturn is composed of hydrogen, with small proportions of helium and trace elements.[12] The interior consists of a small core of rock and ice, surrounded by a thick layer of metallic hydrogen and a gaseous outer layer. The outer atmosphere is generally bland in appearance, although long-lived features can appear. Wind speeds on Saturn can reach 1,800 km/h, significantly faster than those on Jupiter. Saturn has a planetary magnetic field intermediate in strength between that of Earth and the more powerful field around Jupiter.

Saturn has a prominent system of rings, consisting mostly of ice particles with a smaller amount of rocky debris and dust. Sixty-one known moons orbit the planet, not counting hundreds of "moonlets" within the rings. Titan, Saturn's largest and the Solar System's second largest moon (after Jupiter's Ganymede), is larger than the planet Mercury and is the only moon in the Solar System to possess a significant atmosphere.[13]



Uranus

Uranus is the seventh planet from the Sun, and the third-largest and fourth most massive planet in the Solar System. It is named after the ancient Greek deity of the sky Uranus (Ancient Greek: Οὐρανός) the father of Cronus (Saturn) and grandfather of Zeus (Jupiter). Though it is visible to the naked eye like the five classical planets, it was never recognized as a planet by ancient observers because of its dimness and slow orbit.[16] Sir William Herschel announced its discovery on March 13, 1781, expanding the known boundaries of the Solar System for the first time in modern history. Uranus was also the first planet discovered with a telescope.

Uranus is similar in composition to Neptune, and both have different compositions from those of the larger gas giants Jupiter and Saturn. As such, astronomers sometimes place them in a separate category, the "ice giants". Uranus's atmosphere, while similar to Jupiter 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 °C). It has a complex, layered cloud structure, with water thought to make up the lowest clouds, and methane thought to make up the uppermost layer of clouds.[12] In contrast the interior of Uranus is mainly composed of ices and rock.[11]

Like the other giant planets, Uranus has a ring system, a magnetosphere, and numerous moons. The Uranian system has a unique configuration among the planets because its axis of rotation is tilted sideways, nearly into the plane of its revolution about the Sun. As such, its north and south poles lie where most other planets have their equators.[17] Seen from Earth, Uranus's rings can sometimes appear to circle the planet like an archery target and its moons revolve around it like the hands of a clock, though in 2007 and 2008 the rings appeared edge-on. In 1986, images from Voyager 2 showed Uranus as a virtually featureless planet in visible light without the cloud bands or storms associated with the other giants.[17] However, terrestrial observers have seen signs of seasonal change and increased weather activity in recent years as Uranus approached its equinox. The wind speeds on Uranus can reach 250 meters per second (900 km/h, 560 mph).[18]



Neptune

Neptune is the eighth and farthest planet from the Sun in our Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third-largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 Earth masses and not as dense.[12] On average, Neptune orbits the Sun at a distance of 30.1 AU, approximately 30 times the Earth-Sun distance. Its astronomical symbol is , a stylized version of the god Neptune's trident.

Discovered on September 23, 1846,[1] Neptune was the first planet found by mathematical prediction rather than by empirical observation. Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit was subject to gravitational perturbation by an unknown planet. Neptune was subsequently observed by Johann Galle within a degree of the position predicted by Urbain Le Verrier, and its largest moon, Triton, was discovered shortly thereafter, though none of the planet's remaining 12 moons were located telescopically until the 20th century. Neptune has been visited by only one spacecraft, Voyager 2, which flew by the planet on August 25, 1989.

Neptune is similar in composition to Uranus, and both have compositions which differ from those of the larger gas giants Jupiter and Saturn. Neptune's atmosphere, while similar to Jupiter's and Saturn's in that it is composed primarily of hydrogen and helium, along with traces of hydrocarbons and possibly nitrogen, contains a higher proportion of "ices" such as water, ammonia and methane. Astronomers sometimes categorize Uranus and Neptune as "ice giants" in order to emphasize these distinctions.[13] The interior of Neptune, like that of Uranus, is primarily composed of ices and rock.[14] Traces of methane in the outermost regions in part account for the planet's blue appearance.[15]

In contrast to the relatively featureless atmosphere of Uranus, Neptune's atmosphere is notable for its active and visible weather patterns. At the time of the 1989 Voyager 2 flyby, for example, the planet's southern hemisphere possessed a Great Dark Spot comparable to the Great Red Spot on Jupiter. These weather patterns are driven by the strongest sustained winds of any planet in the Solar System, with recorded wind speeds as high as 2,100 km/h.[16] Because of its great distance from the Sun, Neptune's outer atmosphere is one of the coldest places in the Solar System, with temperatures at its cloud tops approaching −218 °C (55 K). Temperatures at the planet's centre, however, are approximately 5,400 K (5,000 °C).[17][18] Neptune has a faint and fragmented ring system, which may have been detected during the 1960s but was only indisputably confirmed in 1989 by Voyager 2.[19]


Dwarf Planets

Pluto

Pluto, formal designation 134340 Pluto, is the second-largest known dwarf planet in the Solar System (after Eris) and the tenth-largest body observed directly orbiting the Sun. Originally classified as a planet, Pluto is now considered the largest member of a distinct population called the Kuiper belt.[note 9]

Like other members of the Kuiper belt, Pluto is composed primarily of rock and ice and is relatively small: approximately a fifth the mass of the Earth's Moon and a third its volume. It has an eccentric and highly inclined orbit that takes it from 30 to 49 AU (4.4–7.4 billion km) from the Sun. This causes Pluto to periodically come closer to the Sun than Neptune.

From its discovery in 1930 until 2006, Pluto was considered the Solar System's ninth planet. In the late 1970s, following the discovery of minor planet 2060 Chiron in the outer Solar System and the recognition of Pluto's very low mass, its status as a major planet began to be questioned.[6] Later, in the early 21st century, many objects similar to Pluto were discovered in the outer Solar System, notably the scattered disc object Eris, which is 27% more massive than Pluto.[7] On August 24, 2006, the International Astronomical Union (IAU) defined the term "planet" for the first time. This definition excluded Pluto as a planet, and added it as a member of the new category "dwarf planet" along with Eris and Ceres.[8] After the reclassification, Pluto was added to the list of minor planets and given the number 134340.[9][10] A number of scientists continue to hold that Pluto should be classified as a planet.[11]

Pluto and its largest moon, Charon, are sometimes treated together as a binary system because the barycentre of their orbits does not lie within either body.[12] The IAU has yet to formalise a definition for binary dwarf planets, and until it passes such a ruling, they classify Charon as a moon of Pluto.[13] Pluto has two known smaller moons, Nix and Hydra, discovered in 2005.[14]


Haumea

Haumea with its moons, Hi'aka (above) and Namaka

Haumea, formal designation 136108 Haumea, is a dwarf planet in the Kuiper belt. Its mass is one-third the mass of Pluto.[note 2] It was discovered in 2004 by a team headed by Mike Brown of Caltech at the Palomar Observatory in the United States and, in 2005, by a team headed by J. L. Ortiz at the Sierra Nevada Observatory in Spain, though the latter claim has been contested. On September 17, 2008, it was designated a dwarf planet by the International Astronomical Union (IAU) and named after Haumea, the Hawaiian goddess of childbirth.


Haumea's extreme elongation makes it unique among known trans-Neptunian objects (TNOs). Although its shape has not been directly observed, calculations from its light curve suggest it is an ellipsoid, with its greatest axis twice as long as its shortest. Nonetheless, its gravity is believed sufficient for it to have relaxed into hydrostatic equilibrium, thereby meeting the definition of a dwarf planet. This elongation, along with its unusually rapid rotation, high density, and high albedo (from a surface of crystalline water ice), are thought to be the results of a giant collision, which left Haumea the largest member of a collisional family that includes several large TNOs and its two known moons.


Makemake

Makemake, formally designated (136472) Makemake, is the third-largest known dwarf planet in the Solar System and one of the two largest Kuiper belt objects (KBO) in the classical KBO population.[b] Its diameter is roughly three-quarters that of Pluto.[9] Makemake has no known satellites, which makes it unique among the largest KBOs. Its extremely low average temperature, about 30 K (−243.2 °C), means its surface is covered with methane, ethane, and possibly nitrogen ices.[10] 

Initially known as 2005 FY9 and later given the minor planet number 136472, it was discovered on March 31, 2005, by a team led by Michael Brown, and announced on July 29, 2005. Its name derives from the Rapanui god Makemake. On June 11, 2008, the International Astronomical Union (IAU) included Makemake in its list of potential candidates to be given "plutoid" status, a term for dwarf planets beyond the orbit of Neptune that would place the object alongside Pluto, Haumea and Eris. Makemake was formally classified as a plutoid in July 2008.

Eris

Eris along with its moon, Dysnomia

Eris, formal designation 136199 Eris, is the largest known dwarf planet in the Solar System and the ninth-largest body known to orbit the Sun directly. It is approximately 2,500 kilometres in diameter and 27% more massive than Pluto.[9][13]

Eris was first identified in January 2005 by a Palomar Observatory-based team led by Mike Brown, and its identity verified later that year. It is a trans-Neptunian object (TNO) native to a region of space beyond the Kuiper belt known as the scattered disc. Eris has one moon, Dysnomia; recent observations have found no evidence of further satellites. The current distance from the Sun is 96.7 AU,[11] roughly three times that of Pluto. With the exception of some comets the pair are the most distant known natural objects in the Solar System.[2]

Because Eris is larger than Pluto, its discoverers[14] and NASA initially described it as the Solar System’s tenth planet. This, along with the prospect of other similarly sized objects being discovered in the future, motivated the International Astronomical Union (IAU) to define the term planet for the first time. Under a then-new IAU definition approved on August 24, 2006, Eris is a "dwarf planet" along with Pluto, Ceres, Haumea and Makemake.


Ceres

Ceres, formally designated 1 Ceres, is the smallest identified dwarf planet in the Solar System and the only one in the asteroid belt. It was discovered on 1 January 1801, by Giuseppe Piazzi,[17] and for half a century it was classified as the eighth planet. It is named after Ceres, the Roman goddess of growing plants, the harvest, and motherly love.


With a diameter of about 950 km (590 mi), Ceres is by far the largest and most massive body in the asteroid belt, and contains almost a third (32%) of the belt's total mass.[18][19] Recent observations have revealed that it is spherical, unlike the irregular shapes of smaller bodies with lower gravity.[11] The surface of Ceres is probably a mixture of water ice and various hydrated minerals such as carbonates and clays.[12] Ceres appears to be differentiated into a rocky core and ice mantle.[6] It may harbour an ocean of liquid water underneath its surface.[20][21]

From the Earth, Ceres' apparent magnitude ranges from 6.7 to 9.3, and hence at its brightest it is still too dim to be seen with the naked eye.[13] On 27 September 2007, NASA launched the Dawn space probe to explore Vesta (2011–2012) and Ceres (2015).