The mail or post is a system for physically transporting postcards and parcels. A postal service can be private or public, though many governments place restrictions on private systems. Since the mid-19th century, national postal systems have been established as government monopolies, with a fee on the article prepaid. Proof of payment is in the form of adhesive postage stamps, but postage meters are used for bulk mailing. Modern private postal systems are distinguished from national postal agencies by the names "courier" or "delivery service". Postal authorities have functions other than transporting letters. In some countries, a postal and telephone service oversees the postal system, in addition to telephone and telegraph systems; some countries' postal systems allow for savings handle applications for passports. The Universal Postal Union, established in 1874, includes 192 member countries and sets the rules for international mail exchanges; the word mail comes from the Medieval English word male, referring to pack.
It was spelled that way until the 17th century, is distinct from the word male. The French have a similar word, malle for a trunk or large box, mála is the Irish term for a bag. In the 17th century, the word mail began to appear as a reference for a bag that contained letters: "bag full of letter". Over the next hundred years the word mail began to be applied to the letters themselves, the sack as the mailbag. In the 19th century the British referred to mail as being letters that were being sent abroad, post as letters that were for localized delivery. S. the U. S. Postal Service delivers the mail; the term email first appeared in the 1970s. The term snail-mail is a retronym to distinguish it from the quicker email. Various dates have been given for its first use. Post is derived from Medieval French poste, which stems from the past participle of the Latin verb ponere; the practice of communication by written documents carried by an intermediary from one person or place to another certainly dates back nearly to the invention of writing.
However, development of formal postal systems occurred much later. The first documented use of an organized courier service for the diffusion of written documents is in Egypt, where Pharaohs used couriers for the diffusion of their decrees in the territory of the State; the earliest surviving piece of mail is Egyptian, dating to 255 BC. The first credible claim for the development of a real postal system comes from Ancient Persia, but the point of invention remains in question; the best documented claim attributes the invention to the Persian King Cyrus the Great, who mandated that every province in his kingdom would organize reception and delivery of post to each of its citizens. He negotiated with neighbouring countries to do the same and had roads built from the city of Post in Western Iran all the way up to the city of Hakha in the East. Other writers credit his successor Darius I of Persia. Other sources claim much earlier dates for an Assyrian postal system, with credit given to Hammurabi and Sargon II.
Mail may not have been the primary mission of this postal service, however. The role of the system as an intelligence gathering apparatus is well documented, the service was called angariae, a term that in time came to indicate a tax system; the Old Testament makes mention of this system: Ahasuerus, king of Medes, used couriers for communicating his decisions. The Persian system worked on stations, where the message carrier would ride to the next post, whereupon he would swap his horse with a fresh one, for maximum performance and delivery speed. Herodotus described the system in this way: "It is said that as many days as there are in the whole journey, so many are the men and horses that stand along the road, each horse and man at the interval of a day's journey; the verse prominently features on New York's James Farley Post Office, although it has been rephrased to Neither snow nor rain nor heat nor gloom of night stays these couriers from the swift completion of their appointed rounds. The economic growth and political stability under the Mauryan empire saw the development of impressive civil infrastructure in ancient India.
The Mauryans developed early Indian mail service as well as public wells, rest houses, other facilities for the common public. Common chariots called. Couriers were used militarily by kings and local rulers to deliver information through runners and other carriers; the postmaster, the head of the intelligence service, was responsible for ensuring the maintenance of the courier system. Couriers were used to deliver personal letters. In South India, the Wodeyar dynasty of the Kingdom of Mysore used mail service for espionage purposes thereby acquiring knowledge related to matters that took place at great distances. By the end of the 18th century, the postal system in India had reached impressive levels of efficiency. According to British national Thomas Broughton, the Maharaja of Jodhpur sent daily offerings of fresh flowers from his capital to Nathadvara, they arrived in time for the first religious Darshan at sunrise; this system underwent complete modernization when the British Raj established its full control over India.
The Post Offi
A natural satellite or moon is, in the most common usage, an astronomical body that orbits a planet or minor planet. In the Solar System there are six planetary satellite systems containing 185 known natural satellites. Four IAU-listed dwarf planets are known to have natural satellites: Pluto, Haumea and Eris; as of September 2018, there are 334 other minor planets known to have moons. The Earth–Moon system is unique in that the ratio of the mass of the Moon to the mass of Earth is much greater than that of any other natural-satellite–planet ratio in the Solar System. At 3,474 km across, the Moon is 0.27 times the diameter of Earth. The first known natural satellite was the Moon, but it was considered a "planet" until Copernicus' introduction of De revolutionibus orbium coelestium in 1543; until the discovery of the Galilean satellites in 1610, there was no opportunity for referring to such objects as a class. Galileo chose to refer to his discoveries as Planetæ, but discoverers chose other terms to distinguish them from the objects they orbited.
The first to use of the term satellite to describe orbiting bodies was the German astronomer Johannes Kepler in his pamphlet Narratio de Observatis a se quatuor Iouis satellitibus erronibus in 1610. He derived the term from the Latin word satelles, meaning "guard", "attendant", or "companion", because the satellites accompanied their primary planet in their journey through the heavens; the term satellite thus became the normal one for referring to an object orbiting a planet, as it avoided the ambiguity of "moon". In 1957, the launching of the artificial object Sputnik created a need for new terminology. Sputnik was created by Soviet Union, it was the first satellite ever; the terms man-made satellite and artificial moon were quickly abandoned in favor of the simpler satellite, as a consequence, the term has become linked with artificial objects flown in space – including, sometimes those not in orbit around a planet. Because of this shift in meaning, the term moon, which had continued to be used in a generic sense in works of popular science and in fiction, has regained respectability and is now used interchangeably with natural satellite in scientific articles.
When it is necessary to avoid both the ambiguity of confusion with Earth's natural satellite the Moon and the natural satellites of the other planets on the one hand, artificial satellites on the other, the term natural satellite is used. To further avoid ambiguity, the convention is to capitalize the word Moon when referring to Earth's natural satellite, but not when referring to other natural satellites. Many authors define "satellite" or "natural satellite" as orbiting some planet or minor planet, synonymous with "moon" – by such a definition all natural satellites are moons, but Earth and other planets are not satellites. A few recent authors define "moon" as "a satellite of a planet or minor planet", "planet" as "a satellite of a star" – such authors consider Earth as a "natural satellite of the sun". There is no established lower limit on what is considered a "moon"; every natural celestial body with an identified orbit around a planet of the Solar System, some as small as a kilometer across, has been considered a moon, though objects a tenth that size within Saturn's rings, which have not been directly observed, have been called moonlets.
Small asteroid moons, such as Dactyl, have been called moonlets. The upper limit is vague. Two orbiting bodies are sometimes described as a double planet rather than satellite. Asteroids such as 90 Antiope are considered double asteroids, but they have not forced a clear definition of what constitutes a moon; some authors consider the Pluto–Charon system to be a double planet. The most common dividing line on what is considered a moon rests upon whether the barycentre is below the surface of the larger body, though this is somewhat arbitrary, because it depends on distance as well as relative mass; the natural satellites orbiting close to the planet on prograde, uninclined circular orbits are thought to have been formed out of the same collapsing region of the protoplanetary disk that created its primary. In contrast, irregular satellites are thought to be captured asteroids further fragmented by collisions. Most of the major natural satellites of the Solar System have regular orbits, while most of the small natural satellites have irregular orbits.
The Moon and Charon are exceptions among large bodies in that they are thought to have originated by the collision of two large proto-planetary objects. The material that would have been placed in orbit around the central body is predicted to have reaccreted to form one or more orbiting natural satellites; as opposed to planetary-sized bodies, asteroid moons are thought to form by this process. Triton is another exception; the capture of an asteroid from a heliocentric orbit is not always permanent. According to simulations, temporary satellites should be a common phenomenon; the only observed example is 2006 RH120, a temporary satellite of Earth for nine months in 2006 and 2007. Most regular moons (natural satellites following close and prograde orbits with small orb
World War I
World War I known as the First World War or the Great War, was a global war originating in Europe that lasted from 28 July 1914 to 11 November 1918. Contemporaneously described as "the war to end all wars", it led to the mobilisation of more than 70 million military personnel, including 60 million Europeans, making it one of the largest wars in history, it is one of the deadliest conflicts in history, with an estimated nine million combatants and seven million civilian deaths as a direct result of the war, while resulting genocides and the 1918 influenza pandemic caused another 50 to 100 million deaths worldwide. On 28 June 1914, Gavrilo Princip, a Bosnian Serb Yugoslav nationalist, assassinated the Austro-Hungarian heir Archduke Franz Ferdinand in Sarajevo, leading to the July Crisis. In response, on 23 July Austria-Hungary issued an ultimatum to Serbia. Serbia's reply failed to satisfy the Austrians, the two moved to a war footing. A network of interlocking alliances enlarged the crisis from a bilateral issue in the Balkans to one involving most of Europe.
By July 1914, the great powers of Europe were divided into two coalitions: the Triple Entente—consisting of France and Britain—and the Triple Alliance of Germany, Austria-Hungary and Italy. Russia felt it necessary to back Serbia and, after Austria-Hungary shelled the Serbian capital of Belgrade on the 28th, partial mobilisation was approved. General Russian mobilisation was announced on the evening of 30 July; when Russia failed to comply, Germany declared war on 1 August in support of Austria-Hungary, with Austria-Hungary following suit on 6th. German strategy for a war on two fronts against France and Russia was to concentrate the bulk of its army in the West to defeat France within four weeks shift forces to the East before Russia could mobilise. On 2 August, Germany demanded free passage through Belgium, an essential element in achieving a quick victory over France; when this was refused, German forces invaded Belgium on 3 August and declared war on France the same day. On 12 August and France declared war on Austria-Hungary.
In November 1914, the Ottoman Empire entered the war on the side of the Alliance, opening fronts in the Caucasus and the Sinai Peninsula. The war was fought in and drew upon each power's colonial empire as well, spreading the conflict to Africa and across the globe; the Entente and its allies would become known as the Allied Powers, while the grouping of Austria-Hungary and their allies would become known as the Central Powers. The German advance into France was halted at the Battle of the Marne and by the end of 1914, the Western Front settled into a battle of attrition, marked by a long series of trench lines that changed little until 1917. In 1915, Italy opened a front in the Alps. Bulgaria joined the Central Powers in 1915 and Greece joined the Allies in 1917, expanding the war in the Balkans; the United States remained neutral, although by doing nothing to prevent the Allies from procuring American supplies whilst the Allied blockade prevented the Germans from doing the same the U. S. became an important supplier of war material to the Allies.
After the sinking of American merchant ships by German submarines, the revelation that the Germans were trying to incite Mexico to make war on the United States, the U. S. declared war on Germany on 6 April 1917. Trained American forces would not begin arriving at the front in large numbers until mid-1918, but the American Expeditionary Force would reach some two million troops. Though Serbia was defeated in 1915, Romania joined the Allied Powers in 1916 only to be defeated in 1917, none of the great powers were knocked out of the war until 1918; the 1917 February Revolution in Russia replaced the Tsarist autocracy with the Provisional Government, but continuing discontent at the cost of the war led to the October Revolution, the creation of the Soviet Socialist Republic, the signing of the Treaty of Brest-Litovsk by the new government in March 1918, ending Russia's involvement in the war. This allowed the transfer of large numbers of German troops from the East to the Western Front, resulting in the German March 1918 Offensive.
This offensive was successful, but the Allies rallied and drove the Germans back in their Hundred Days Offensive. Bulgaria was the first Central Power to sign an armistice—the Armistice of Salonica on 29 September 1918. On 30 October, the Ottoman Empire capitulated. On 4 November, the Austro-Hungarian empire agreed to the Armistice of Villa Giusti after being decisively defeated by Italy in the Battle of Vittorio Veneto. With its allies defeated, revolution at home, the military no longer willing to fight, Kaiser Wilhelm abdicated on 9 November and Germany signed an armistice on 11 November 1918. World War I was a significant turning point in the political, cultural and social climate of the world; the war and its immediate aftermath sparked numerous uprisings. The Big Four (Britain, the United States, It
Kiel is the capital and most populous city in the northern German state of Schleswig-Holstein, with a population of 249,023. Kiel lies 90 kilometres north of Hamburg. Due to its geographic location in the north of Germany, the southeast of the Jutland peninsula and the southwestern shore of the Baltic Sea, Kiel has become one of the major maritime centres of Germany. For instance, the city is known for a variety of international sailing events, including the annual Kiel Week, the biggest sailing event in the world; the Olympic sailing competitions of the 1936 and the 1972 Summer Olympics were held in the Bay of Kiel. Kiel has been one of the traditional homes of the German Navy's Baltic fleet, continues to be a major high-tech shipbuilding centre. Located in Kiel is the GEOMAR - Helmholtz Centre for Ocean Research Kiel at the University of Kiel. Kiel is an important sea transport hub, thanks to its location on the Kiel Fjord and the busiest artificial waterway in the world, Kiel Canal. A number of passenger ferries to Sweden, Norway and other countries operate from here.
Moreover, today Kiel Harbour is an important port of call for cruise ships touring the Baltic Sea. Kiel's recorded history began in the 13th century, but the city was a Danish village, in the 8th century; until 1864 it was administered by Denmark in personal union. In 1866 the city was annexed by Prussia and in 1871 it became part of Germany. Kiel was one of the founding cities of original European Green Regi51 Award in 2006. In 2005 Kiel's GDP per capita was €35,618, well above Germany's national average, 159% of the European Union's average; the city is home to the University of Kiel. Kiel Fjord and the village of Kiel was first settled by Vikings who wanted to colonise the land that they had raided, for many years they settled in German villages; this is evidenced by the architecture of the fjord. The city of Kiel was founded in 1233 as Holstenstadt tom Kyle by Count Adolf IV of Holstein, granted Lübeck city rights in 1242 by Adolf's eldest son, John I of Schauenburg. Being a part of Holstein, Kiel belonged to the Holy Roman Empire and was situated only a few kilometres south of the Danish border.
Kiel, the capital of the county of Holstein, was a member of the Hanseatic League from 1284 until it was expelled in 1518 for harbouring pirates. In 1431, the Kieler Umschlag was first held, which became the central market for goods and money in Schleswig-Holstein, until it began to lose significance from 1850 on, being held for the last time in 1900, until when it has been restarted; the University of Kiel was founded on 29 September 1665 by Christian Albert, Duke of Holstein-Gottorp. A number of important scholars, including Theodor Mommsen, Felix Jacoby, Hans Geiger and Max Planck, studied or taught there. From 1773 to 1864, the town belonged to the king of Denmark. However, because the king ruled Holstein as a fief of the Holy Roman Empire only through a personal union, the town was not incorporated as part of Denmark proper, thus Kiel belonged to Germany. Though the empire was abolished in 1806, the Danish king continued to rule Kiel only through his position as Duke of Holstein, which became a member of the German Confederation in 1815.
When Schleswig and Holstein rebelled against Denmark in 1848, Kiel became the capital of Schleswig-Holstein until the Danish victory in 1850. During the Second Schleswig War in 1864, Kiel and the rest of the duchies of Schleswig and Holstein were conquered by a German Confederation alliance of the Austrian Empire and the Kingdom of Prussia. After the war, Kiel was administered by both the Austrians and the Prussians, but the Austro-Prussian War in 1866 led to the formation of the Province of Schleswig-Holstein and the annexation of Kiel by Prussia in 1867. On 24 March 1865 King William I based Prussia's Baltic Sea fleet in Kiel instead of Danzig; the Imperial shipyard Kiel was established in 1867 in the town. When William I of Prussia became Emperor William I of the German Empire in 1871, he designated Kiel and Wilhelmshaven as Reichskriegshäfen; the prestigious Kiel Yacht Club was established in 1887 with Prince Henry of Prussia as its patron. Emperor Wilhelm II became its commodore in 1891.
Because of its new role as Germany's main naval base, Kiel quickly increased in size in the following years, from 18,770 in 1864 to about 200,000 in 1910. Much of the old town centre and other surroundings were levelled and redeveloped to provide for the growing city; the Kiel tramway network, opened in 1881, had been enlarged to 10 lines, with a total route length of 40 km, before the end of the First World War. Kiel was the site of the sailors' mutiny which sparked the German Revolution in late 1918. Just before the end of the First World War, the German fleet stationed at Kiel was ordered to be sent out on a last great battle with the Royal Navy; the sailors, who thought of this as a suicide mission which would have no effect on the outcome of the war, decided they had nothing to lose and refused to leave the safety of the port. The sailors' actions and the lack of response of the government to them, fuelled by an critical view of the Kaiser, sparked a revolution which caused the abolition of the monarchy and the creation of the Weimar Republic.
During the Second World War, Kiel remained one of the major naval bases and shipbuilding centres of the German Reich. There was a slave labour camp for the local industry; because of its status as a naval port and as production site for submarines, Kiel was bombed by the Allies d
A supernova is an event that occurs upon the death of certain types of stars. Supernovae are more energetic than novae. In Latin, nova means "new", referring astronomically to what appears to be a temporary new bright star. Adding the prefix "super-" distinguishes supernovae from ordinary novae, which are far less luminous; the word supernova was coined by Walter Baade and Fritz Zwicky in 1931. Only three Milky Way, naked-eye supernova events have been observed during the last thousand years, though many have been seen in other galaxies; the most recent directly observed supernova in the Milky Way was Kepler's Supernova in 1604, but two more recent supernova remnants have been found. Statistical observations of supernovae in other galaxies suggest they occur on average about three times every century in the Milky Way, that any galactic supernova would certainly be observable with modern astronomical telescopes. Supernovae may expel much, if not all, of the material away from a star at velocities up to 30,000 km/s or 10% of the speed of light.
This drives an expanding and fast-moving shock wave into the surrounding interstellar medium, in turn, sweeping up an expanding shell of gas and dust, observed as a supernova remnant. Supernovae create and eject the bulk of the chemical elements produced by nucleosynthesis. Supernovae play a significant role in enriching the interstellar medium with the heavier atomic mass chemical elements. Furthermore, the expanding shock waves from supernovae can trigger the formation of new stars. Supernova remnants are expected to accelerate a large fraction of galactic primary cosmic rays, but direct evidence for cosmic ray production was found only in a few of them so far, they are potentially strong galactic sources of gravitational waves. Theoretical studies indicate that most supernovae are triggered by one of two basic mechanisms: the sudden re-ignition of nuclear fusion in a degenerate star or the sudden gravitational collapse of a massive star's core. In the first instance, a degenerate white dwarf may accumulate sufficient material from a binary companion, either through accretion or via a merger, to raise its core temperature enough to trigger runaway nuclear fusion disrupting the star.
In the second case, the core of a massive star may undergo sudden gravitational collapse, releasing gravitational potential energy as a supernova. While some observed supernovae are more complex than these two simplified theories, the astrophysical collapse mechanics have been established and accepted by most astronomers for some time. Owing to the wide range of astrophysical consequences of these events, astronomers now deem supernova research, across the fields of stellar and galactic evolution, as an important area for investigation; the earliest recorded supernova HB9 was viewed by Indians 5,000-years ago and recorded in the oldest Star chart. The SN 185, was viewed by Chinese astronomers in 185 AD; the brightest recorded supernova was SN 1006, which occurred in 1006 AD and was described by observers across China, Iraq and Europe. The observed supernova SN 1054 produced the Crab Nebula. Supernovae SN 1572 and SN 1604, the latest to be observed with the naked eye in the Milky Way galaxy, had notable effects on the development of astronomy in Europe because they were used to argue against the Aristotelian idea that the universe beyond the Moon and planets was static and unchanging.
Johannes Kepler began observing SN 1604 at its peak on October 17, 1604, continued to make estimates of its brightness until it faded from naked eye view a year later. It was the second supernova to be observed in a generation. There is some evidence that the youngest galactic supernova, G1.9+0.3, occurred in the late 19th century more than Cassiopeia A from around 1680. Neither supernova was noted at the time. In the case of G1.9+0.3, high extinction along the plane of the galaxy could have dimmed the event sufficiently to go unnoticed. The situation for Cassiopeia A is less clear. Infrared light echos have been detected showing that it was a type IIb supernova and was not in a region of high extinction. Before the development of the telescope, only five supernovae were seen in the last millennium. Compared to a star's entire history, the visual appearance of a galactic supernova is brief spanning several months, so that the chances of observing one is once in a lifetime. Only a tiny fraction of the 100 billion stars in a typical galaxy have the capacity to become a supernova, restricted to either those having large mass or extraordinarily rare kinds of binary stars containing white dwarfs.
However and discovery of extragalactic supernovae are now far more common. The first such observation was of SN 1885A in the Andromeda galaxy. Today and professional astronomers are finding several hundred every year, some when near maximum brightness, others on old astronomical photographs or plates. American astronomers Rudolph Minkowski and Fritz Zwicky developed the modern supernova classification scheme beginning in 1941. During the 1960s, astronomers found that the maximum intensities of supernovae could be used as standard candles, hence indicators of astronomical distances; some of the most distant supernovae observed in 2003, appeared dimmer than expected. This supports the view. Techniques were developed for reconstructing supernovae events that have no written records of being observed; the date of the Cassiopeia A supernova event was determined from light echoes off nebulae, while the age of supernova remnant RX J0852.0-4622 was estimated from temperature
Harvard College Observatory
The Harvard College Observatory is an institution managing a complex of buildings and multiple instruments used for astronomical research by the Harvard University Department of Astronomy. It is located in Cambridge, Massachusetts, USA, was founded in 1839. With the Smithsonian Astrophysical Observatory, it forms part of the Harvard–Smithsonian Center for Astrophysics. HCO houses a collection of 500,000 astronomical plates taken between the mid-1880s and 1989; this 100-year coverage is a unique resource for studying temporal variations in the universe. The Digital Access to a Sky Century @ Harvard project is digitally scanning and archiving these photographic plates. In 1839, the Harvard Corporation voted to appoint William Cranch Bond, a prominent Boston clockmaker, as "Astronomical Observer to the University"; this marked the founding of the Harvard College Observatory. HCO's first telescope, the 15-inch Great Refractor, was installed in 1847; that telescope was the largest in the United States from installation until 1867.
Between 1847 and 1852 Bond and pioneer photographer John Adams Whipple used the Great Refractor telescope to produce images of the moon that are remarkable in their clarity of detail and aesthetic power. This was the largest telescope in North America at that time, their images of the moon took the prize for technical excellence in photography at the 1851 Great Exhibition at The Crystal Palace in London. On the night of July 16–17, 1850, Whipple and Bond made the first daguerreotype of a star. Harvard College Observatory is important to astronomy, as many women including Annie Jump Cannon, Henrietta Swan Leavitt, Cecilia Payne-Gaposchkin, Williamina Fleming performed pivotal stellar classification research. Cannon and Leavitt were hired as "computers" to perform calculations and examine stellar photographs, but made insightful connections in their research. From 1898 to 1926, a series of Bulletins were issued containing many of the major discoveries of the period; these were replaced by Announcement Cards which continued to be issued until 1952.
In 1908, the observatory published the Harvard Revised Photometry Catalogue, which gave rise to the HR star catalogue, now maintained by the Yale University Observatory as the Bright Star Catalogue. William Cranch Bond 1839–1859 George Phillips Bond 1859–1865 Joseph Winlock 1866–1875 Edward Charles Pickering 1877–1919 Solon Irving Bailey 1919–1921 Harlow Shapley 1921–1952 Donald H. Menzel 1952–1953. Field 1971–1972 Harvard Computers Sears Tower – Harvard Observatory The Minor Planet Center credits many asteroid discoveries to "Harvard Observatory." See List of largest optical refracting telescopes, for other'great refractors' Dava Sobel. The Glass Universe:. Viking. ISBN 978-0670016952. HCO home page Harvard-Smithsonian Center for Astrophysics Harvard College Observatory Bulletins Harvard College Announcement Cards
A comet is an icy, small Solar System body that, when passing close to the Sun and begins to release gases, a process called outgassing. This produces a visible atmosphere or coma, sometimes a tail; these phenomena are due to the effects of solar radiation and the solar wind acting upon the nucleus of the comet. Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of loose collections of ice and small rocky particles; the coma may be up to 15 times the Earth's diameter, while the tail may stretch one astronomical unit. If sufficiently bright, a comet may be seen from the Earth without the aid of a telescope and may subtend an arc of 30° across the sky. Comets have been recorded since ancient times by many cultures. Comets have eccentric elliptical orbits, they have a wide range of orbital periods, ranging from several years to several millions of years. Short-period comets originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune.
Long-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper belt to halfway to the nearest star. Long-period comets are set in motion towards the Sun from the Oort cloud by gravitational perturbations caused by passing stars and the galactic tide. Hyperbolic comets may pass once through the inner Solar System before being flung to interstellar space; the appearance of a comet is called an apparition. Comets are distinguished from asteroids by the presence of an extended, gravitationally unbound atmosphere surrounding their central nucleus; this atmosphere has parts termed the tail. However, extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids. Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System; the discovery of main-belt comets and active centaur minor planets has blurred the distinction between asteroids and comets.
In the early 21st century, the discovery of some minor bodies with long-period comet orbits, but characteristics of inner solar system asteroids, were called Manx comets. They are still classified as comets, such as C/2014 S3. 27 Manx comets were found from 2013 to 2017. As of July 2018 there are 6,339 known comets, a number, increasing as they are discovered. However, this represents only a tiny fraction of the total potential comet population, as the reservoir of comet-like bodies in the outer Solar System is estimated to be one trillion. One comet per year is visible to the naked eye, though many of those are faint and unspectacular. Bright examples are called "great comets". Comets have been visited by unmanned probes such as the European Space Agency's Rosetta, which became the first to land a robotic spacecraft on a comet, NASA's Deep Impact, which blasted a crater on Comet Tempel 1 to study its interior; the word comet comētēs. That, in turn, is a latinisation of the Greek κομήτης, the Oxford English Dictionary notes that the term κομήτης meant "long-haired star, comet" in Greek.
Κομήτης was derived from κομᾶν, itself derived from κόμη and was used to mean "the tail of a comet". The astronomical symbol for comets is ☄; the solid, core structure of a comet is known as the nucleus. Cometary nuclei are composed of an amalgamation of rock, water ice, frozen carbon dioxide, carbon monoxide and ammonia; as such, they are popularly described as "dirty snowballs" after Fred Whipple's model. However, some comets may have a higher dust content, leading them to be called "icy dirtballs". Research conducted in 2014 suggests that comets are like "deep fried ice cream", in that their surfaces are formed of dense crystalline ice mixed with organic compounds, while the interior ice is colder and less dense; the surface of the nucleus is dry, dusty or rocky, suggesting that the ices are hidden beneath a surface crust several metres thick. In addition to the gases mentioned, the nuclei contain a variety of organic compounds, which may include methanol, hydrogen cyanide, formaldehyde and ethane and more complex molecules such as long-chain hydrocarbons and amino acids.
In 2009, it was confirmed that the amino acid glycine had been found in the comet dust recovered by NASA's Stardust mission. In August 2011, a report, based on NASA studies of meteorites found on Earth, was published suggesting DNA and RNA components may have been formed on asteroids and comets; the outer surfaces of cometary nuclei have a low albedo, making them among the least reflective objects found in the Solar System. The Giotto space probe found that the nucleus of Halley's Comet reflects about four percent of the light that falls on it, Deep Space 1 discovered that Comet Borrelly's surface reflects less than 3.0%. The dark surface material of the nucleus may consist of complex organic compounds. Solar heating drives off lighter volatile compounds, leaving behind larger organic compounds that tend to be dark, like tar or crude oil; the low reflectivity of cometary surfaces causes them to absorb t