Jet Propulsion Laboratory
The Jet Propulsion Laboratory is a federally funded research and development center and NASA field center in La Cañada Flintridge, United States, though it is referred to as residing in Pasadena, because it has a Pasadena ZIP Code. Founded in the 1930s, the JPL is owned by NASA and managed by the nearby California Institute of Technology for NASA; the laboratory's primary function is the construction and operation of planetary robotic spacecraft, though it conducts Earth-orbit and astronomy missions. It is responsible for operating NASA's Deep Space Network. Among the laboratory's major active projects are the Mars Science Laboratory mission, the Mars Reconnaissance Orbiter, the Juno spacecraft orbiting Jupiter, the NuSTAR X-ray telescope, the SMAP satellite for earth surface soil moisture monitoring, the Spitzer Space Telescope, it is responsible for managing the JPL Small-Body Database, provides physical data and lists of publications for all known small Solar System bodies. The JPL's Space Flight Operations Facility and Twenty-Five-Foot Space Simulator are designated National Historic Landmarks.
JPL traces its beginnings to 1936 in the Guggenheim Aeronautical Laboratory at the California Institute of Technology when the first set of rocket experiments were carried out in the Arroyo Seco. Caltech graduate students Frank Malina, Qian Xuesen, Weld Arnold, Apollo M. O. Smith, along with Jack Parsons and Edward S. Forman, tested a small, alcohol-fueled motor to gather data for Malina's graduate thesis. Malina's thesis advisor was engineer/aerodynamicist Theodore von Kármán, who arranged for U. S. Army financial support for this "GALCIT Rocket Project" in 1939. In 1941, Parsons, Martin Summerfield, pilot Homer Bushey demonstrated the first jet-assisted takeoff rockets to the Army. In 1943, von Kármán, Malina and Forman established the Aerojet Corporation to manufacture JATO rockets; the project took on the name Jet Propulsion Laboratory in November 1943, formally becoming an Army facility operated under contract by the university. During JPL's Army years, the laboratory developed two deployed weapon systems, the MGM-5 Corporal and MGM-29 Sergeant intermediate-range ballistic missiles.
These missiles were the first US ballistic missiles developed at JPL. It developed a number of other weapons system prototypes, such as the Loki anti-aircraft missile system, the forerunner of the Aerobee sounding rocket. At various times, it carried out rocket testing at the White Sands Proving Ground, Edwards Air Force Base, Goldstone, California. In 1954, JPL teamed up with Wernher von Braun's engineers at the Army Ballistic Missile Agency's Redstone Arsenal in Huntsville, Alabama, to propose orbiting a satellite during the International Geophysical Year; the team lost that proposal to Project Vanguard, instead embarked on a classified project to demonstrate ablative re-entry technology using a Jupiter-C rocket. They carried out three successful sub-orbital flights in 1956 and 1957. Using a spare Juno I, the two organizations launched the United States' first satellite, Explorer 1, on January 31, 1958. JPL was transferred to NASA in December 1958, becoming the agency's primary planetary spacecraft center.
JPL engineers designed and operated Ranger and Surveyor missions to the Moon that prepared the way for Apollo. JPL led the way in interplanetary exploration with the Mariner missions to Venus and Mercury. In 1998, JPL opened the Near-Earth Object Program Office for NASA; as of 2013, it has found 95% of asteroids that are a kilometer or more in diameter that cross Earth's orbit. JPL was early to employ female mathematicians. In the 1940s and 1950s, using mechanical calculators, women in an all-female computations group performed trajectory calculations. In 1961, JPL hired Dana Ulery as the first female engineer to work alongside male engineers as part of the Ranger and Mariner mission tracking teams. JPL has been recognized four times by the Space Foundation: with the Douglas S. Morrow Public Outreach Award, given annually to an individual or organization that has made significant contributions to public awareness of space programs, in 1998; when it was founded, JPL's site was west of a rocky flood-plain – the Arroyo Seco riverbed – above the Devil's Gate dam in the northwestern panhandle of the city of Pasadena.
While the first few buildings were constructed in land bought from the city of Pasadena, subsequent buildings were constructed in neighboring unincorporated land that became part of La Cañada Flintridge. Nowadays, most of the 177 acres of the U. S. federal government-owned NASA property that makes up the JPL campus is located in La Cañada Flintridge. Despite this, JPL still uses a Pasadena address as its official mailing address; the city of La Cañada Flintridge was incorporated in 1976, well after JPL attained international recognition as a Pasadena institution. There has been occasional rivalry between the two cities over the issue of which one should be mentioned in the media as the home of the laboratory. There are 6,000 full-time Caltech employees, a few thousand additional contractors working on any given day. NASA has a resident office at the facility staffed by federal managers who oversee JPL's activities and work for NASA. There are some Caltech graduate students, college student interns and co-op students.
The JPL Education Office serves educators and students by providi
Georges Bizet, registered at birth as Alexandre César Léopold Bizet, was a French composer of the Romantic era. Best known for his operas in a career cut short by his early death, Bizet achieved few successes before his final work, which has become one of the most popular and performed works in the entire opera repertoire. During a brilliant student career at the Conservatoire de Paris, Bizet won many prizes, including the prestigious Prix de Rome in 1857, he was recognised as an outstanding pianist, though he chose not to capitalise on this skill and performed in public. Returning to Paris after three years in Italy, he found that the main Parisian opera theatres preferred the established classical repertoire to the works of newcomers, his keyboard and orchestral compositions were largely ignored. Restless for success, he began many theatrical projects during the 1860s, most of which were abandoned. Neither of his two operas that reached the stage in this time—Les pêcheurs de perles and La jolie fille de Perth—were successful.
After the Franco-Prussian War of 1870–1871, during which Bizet served in the National Guard, he had little success with his one-act opera Djamileh, though an orchestral suite derived from his incidental music to Alphonse Daudet's play L'Arlésienne was popular. The production of Bizet's final opera, was delayed because of fears that its themes of betrayal and murder would offend audiences. After its premiere on 3 March 1875, Bizet was convinced. Bizet's marriage to Geneviève Halévy produced one son. After his death, his work, apart from Carmen, was neglected. Manuscripts were given away or lost, published versions of his works were revised and adapted by other hands, he had no obvious disciples or successors. After years of neglect, his works began to be performed more in the 20th century. Commentators have acclaimed him as a composer of brilliance and originality whose premature death was a significant loss to French musical theatre. Georges Bizet was born in Paris on 25 October 1838, he was registered as Alexandre César Léopold, but baptised as "Georges" on 16 March 1840, was known by this name for the rest of his life.
His father, Adolphe Bizet, had been a hairdresser and wigmaker before becoming a singing teacher despite his lack of formal training. He composed a few works, including at least one published song. In 1837, Adolphe married Aimée Delsarte, against the wishes of her family who considered him a poor prospect. Aimée was an accomplished pianist, while her brother François Delsarte was a distinguished singer and teacher who performed at the courts of both Louis Philippe and Napoleon III. François Delsarte's wife Rosine, a musical prodigy, had been an assistant professor of solfège at the Conservatoire de Paris at the age of 13. At least one author has alleged that his mother was from a Jewish family but this is not substantiated in any of his official biographies. Georges, an only child, showed early aptitude for music and picked up the basics of musical notation from his mother, who gave him his first piano lessons. By listening at the door of the room where Adolphe conducted his classes, Georges learned to sing difficult songs from memory and developed an ability to identify and analyse complex chordal structures.
This precocity convinced his ambitious parents that he was ready to begin studying at the Conservatoire though he was still only nine years old. Georges was interviewed by Joseph Meifred, the horn virtuoso, a member of the Conservatoire's Committee of Studies. Meifred was so struck by the boy's demonstration of his skills that he waived the age rule and offered to take him as soon as a place became available. Bizet was admitted to the Conservatoire on 9 October two weeks before his 10th birthday, he made an early impression. Zimmerman gave Bizet private lessons in counterpoint and fugue, which continued until the old man's death in 1853. Through these classes, Bizet met Zimmerman's son-in-law, the composer Charles Gounod, who became a lasting influence on the young pupil's musical style—although their relationship was strained in years, he met another of Gounod's young students, the 13-year-old Camille Saint-Saëns, who remained a firm friend of Bizet's. Under the tuition of Antoine François Marmontel, the Conservatoire's professor of piano, Bizet's pianism developed rapidly.
Bizet would write to Marmontel: "In your class one learns something besides the piano. Bizet's first preserved compositions, two wordless songs for soprano, date from around 1850. In 1853, he joined Fromental Halévy's composition class and began to produce works of increasing sophistication and quality. Two of his songs, "Petite Marguerite" and "La Rose et l'abeille", were published in 1854. In 1855, he wrote an ambitious overture for a large orchestra, prepared four-hand piano versions of two of Gounod's works: the opera La nonne sanglante and the Symphony in D. Bizet's work on the Gounod symphony inspired him, shortly after his seventeenth birthd
A trans-Neptunian object written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a semi-major axis of 30.1 astronomical units. TNOs are further divided into the classical and resonant objects of the Kuiper belt, the scattered disc and detached objects with the sednoids being the most distant ones; as of October 2018, the catalog of minor planets contains 528 numbered and more than 2,000 unnumbered TNOs. The first trans-Neptunian object to be discovered was Pluto in 1930, it took until 1992 to discover a second trans-Neptunian object orbiting the Sun directly, 15760 Albion. The most massive TNO known is Eris, followed by Pluto, 2007 Makemake and Haumea. More than 80 satellites have been discovered in orbit of trans-Neptunian objects. TNOs vary in color and are either grey-blue or red, they are thought to be composed of mixtures of rock, amorphous carbon and volatile ices such as water and methane, coated with tholins and other organic compounds.
Twelve minor planets with a semi-major axis greater than 150 AU and perihelion greater than 30 AU are known, which are called extreme trans-Neptunian objects. The orbit of each of the planets is affected by the gravitational influences of the other planets. Discrepancies in the early 1900s between the observed and expected orbits of Uranus and Neptune suggested that there were one or more additional planets beyond Neptune; the search for these led to the discovery of Pluto in February 1930, too small to explain the discrepancies. Revised estimates of Neptune's mass from the Voyager 2 flyby in 1989 showed that the problem was spurious. Pluto was easiest to find because it has the highest apparent magnitude of all known trans-Neptunian objects, it has a lower inclination to the ecliptic than most other large TNOs. After Pluto's discovery, American astronomer Clyde Tombaugh continued searching for some years for similar objects, but found none. For a long time, no one searched for other TNOs as it was believed that Pluto, which up to August 2006 was classified a planet, was the only major object beyond Neptune.
Only after the 1992 discovery of a second TNO, 15760 Albion, did systematic searches for further such objects begin. A broad strip of the sky around the ecliptic was photographed and digitally evaluated for moving objects. Hundreds of TNOs were found, with diameters in the range of 50 to 2,500 kilometers. Eris, the most massive TNO, was discovered in 2005, revisiting a long-running dispute within the scientific community over the classification of large TNOs, whether objects like Pluto can be considered planets. Pluto and Eris were classified as dwarf planets by the International Astronomical Union. On Monday, December 17, 2018 the discovery of 2018 VG18, nicknamed “Farout”, was announced. Farout is the most distant solar system object so-far observed and is about 120 AU away from the sun taking more than 1,000 years to complete one orbit. According to their distance from the Sun and their orbital parameters, TNOs are classified in two large groups: the Kuiper belt objects and the scattered disc objects.
The diagram to the right illustrates the distribution of known trans-Neptunian objects in relation to the orbits of the planets and the centaurs for reference. Different classes are represented in different colours. Resonant objects are plotted in classical Kuiper belt objects in blue; the scattered disc extends to the right, far beyond the diagram, with known objects at mean distances beyond 500 AU and aphelia beyond 1000 AU. The Edgeworth-Kuiper belt contains objects with an average distance to the Sun of 30 to about 55 AU having close-to-circular orbits with a small inclination from the ecliptic. Edgeworth-Kuiper belt objects are further classified into the resonant trans-Neptunian object, that are locked in an orbital resonance with Neptune, the classical Kuiper belt objects called "cubewanos", that have no such resonance, moving on circular orbits, unperturbed by Neptune. There are a large number of resonant subgroups, the largest being the twotinos and the plutinos, named after their most prominent member, Pluto.
Members of the classical Edgeworth-Kuiper belt include 50000 Quaoar and Makemake. The scattered disc contains objects farther from the Sun, with eccentric and inclined orbits; these orbits are non-planetary-orbit-crossing. A typical example is the most massive known Eris. Based on the Tisserand parameter relative to Neptune, the objects in the scattered disc can be further divided into the "typical" scattered disc objects with a TN of less than 3, into the detached objects with a TN greater than 3. In addition, detached objects have a time-averaged eccentricity greater than 0.2 The Sednoids are a further extreme sub-grouping of the detached objects with perihelia so distant that it is confirmed that their orbits cannot be explained by perturbations from the giant planets, nor by interaction with the galactic tides. Given the apparent magnitude of all but the biggest trans-Neptunian objects, the physical studies are limited to the following: thermal emissions for the largest objects colour indices, i.e. comparisons of the apparent magnitudes using different filters analysis of spectra and infraredStudying colours and spectra provides insight into the objects' origin and a potential correlation with other classes of objects, namely centaurs and some satellites of giant planets, suspected to originate in the Kuiper belt.
The kilometre, or kilometer is a unit of length in the metric system, equal to one thousand metres. It is now the measurement unit used for expressing distances between geographical places on land in most of the world. K is used in some English-speaking countries as an alternative for the word kilometre in colloquial writing and speech. A slang term for the kilometre in the US and UK military is klick. There are two common pronunciations for the word; the former follows a pattern in English whereby metric units are pronounced with the stress on the first syllable and the pronunciation of the actual base unit does not change irrespective of the prefix. It is preferred by the British Broadcasting Corporation and the Australian Broadcasting Corporation. Many scientists and other users in countries where the metric system is not used, use the pronunciation with stress on the second syllable; the latter pronunciation follows the stress pattern used for the names of measuring instruments. The problem with this reasoning, however, is that the word meter in those usages refers to a measuring device, not a unit of length.
The contrast is more obvious in countries using the British rather than American spelling of the word metre. When Australia introduced the metric system in 1975, the first pronunciation was declared official by the government's Metric Conversion Board. However, the Australian prime minister at the time, Gough Whitlam, insisted that the second pronunciation was the correct one because of the Greek origins of the two parts of the word. By the 8 May 1790 decree, the Constituent assembly ordered the French Academy of Sciences to develop a new measurement system. In August 1793, the French National Convention decreed the metre as the sole length measurement system in the French Republic; the first name of the kilometre was "Millaire". Although the metre was formally defined in 1799, the myriametre was preferred to the "kilometre" for everyday use; the term "myriamètre" appeared a number of times in the text of Develey's book Physique d'Emile: ou, Principes de la science de la nature, while the term kilometre only appeared in an appendix.
French maps published in 1835 had scales showing myriametres and "lieues de Poste". The Dutch gave it the local name of the mijl, it was only in 1867 that the term "kilometer" became the only official unit of measure in the Netherlands to represent 1000 metres. Two German textbooks dated 1842 and 1848 give a snapshot of the use of the kilometre across Europe - the kilometre was in use in the Netherlands and in Italy and the myriametre was in use in France. In 1935, the International Committee for Weights and Measures abolished the prefix "myria-" and with it the "myriametre", leaving the kilometre as the recognised unit of length for measurements of that magnitude. In the United Kingdom, road signs show distances in miles and location marker posts that are used for reference purposes by road engineers and emergency services show distance references in unspecified units which are kilometre-based; the advent of the mobile phone has been instrumental in the British Department for Transport authorising the use of driver location signs to convey the distance reference information of location marker posts to road users should they need to contact the emergency services.
In the US, the National Highway System Designation Act of 1995 prohibits the use of federal-aid highway funds to convert existing signs or purchase new signs with metric units. The Executive Director of the US Federal Highway Administration, Jeffrey Paniati, wrote in a 2008 memo: "Section 205 of the National Highway System Designation Act of 1995 prohibited us from requiring any State DOT to use the metric system during project development activities. Although the State DOT's had the option of using metric measurements or dual units, all of them abandoned metric measurements and reverted to sole use of inch-pound values." The Manual on Uniform Traffic Control Devices since 2000 is published in both metric and American Customary Units. Some sporting disciplines feature 1000 m races in major events, but in other disciplines though world records are catalogued, the one kilometre event remains a minority event; the world records for various sporting disciplines are: Conversion of units, for comparison with other units of length Cubic metre Metric prefix Mileage Odometer Orders of magnitude Square kilometre Media related to Distance indicators at Wikimedia Commons
A day is the period of time during which the Earth completes one rotation around its axis. A solar day is the length of time which elapses between the Sun reaching its highest point in the sky two consecutive times. In 1960, the second was redefined in terms of the orbital motion of the Earth in year 1900, was designated the SI base unit of time; the unit of measurement "day", was symbolized d. In 1967, the second and so the day were redefined by atomic electron transition. A civil day is 86,400 seconds, plus or minus a possible leap second in Coordinated Universal Time, plus or minus an hour in those locations that change from or to daylight saving time. Day can be defined as each of the twenty-four-hour periods, reckoned from one midnight to the next, into which a week, month, or year is divided, corresponding to a rotation of the earth on its axis; however its use depends on its context, for example when people say'day and night','day' will have a different meaning. It will mean the interval of light between two successive nights.
However, in order to be clear when using'day' in that sense, "daytime" should be used to distinguish it from "day" referring to a 24-hour period. The word day may refer to a day of the week or to a calendar date, as in answer to the question, "On which day?" The life patterns of humans and many other species are related to Earth's solar day and the day-night cycle. Several definitions of this universal human concept are used according to context and convenience. Besides the day of 24 hours, the word day is used for several different spans of time based on the rotation of the Earth around its axis. An important one is the solar day, defined as the time it takes for the Sun to return to its culmination point; because celestial orbits are not circular, thus objects travel at different speeds at various positions in their orbit, a solar day is not the same length of time throughout the orbital year. Because the Earth orbits the Sun elliptically as the Earth spins on an inclined axis, this period can be up to 7.9 seconds more than 24 hours.
In recent decades, the average length of a solar day on Earth has been about 86 400.002 seconds and there are about 365.2422 solar days in one mean tropical year. Ancient custom has a new day start at either the setting of the Sun on the local horizon; the exact moment of, the interval between, two sunrises or sunsets depends on the geographical position, the time of year. A more constant day can be defined by the Sun passing through the local meridian, which happens at local noon or midnight; the exact moment is dependent on the geographical longitude, to a lesser extent on the time of the year. The length of such a day is nearly constant; this is the time as indicated by modern sundials. A further improvement defines a fictitious mean Sun that moves with constant speed along the celestial equator. A day, understood as the span of time it takes for the Earth to make one entire rotation with respect to the celestial background or a distant star, is called a stellar day; this period of rotation is about 4 minutes less than 24 hours and there are about 366.2422 stellar days in one mean tropical year.
Other planets and moons have solar days of different lengths from Earth's. A day, in the sense of daytime, distinguished from night time, is defined as the period during which sunlight directly reaches the ground, assuming that there are no local obstacles; the length of daytime averages more than half of the 24-hour day. Two effects make daytime on average longer than nights; the Sun has an apparent size of about 32 minutes of arc. Additionally, the atmosphere refracts sunlight in such a way that some of it reaches the ground when the Sun is below the horizon by about 34 minutes of arc. So the first light reaches the ground when the centre of the Sun is still below the horizon by about 50 minutes of arc. Thus, daytime is on average around 7 minutes longer than 12 hours; the term comes from the Old English dæg, with its cognates such as dagur in Icelandic, Tag in German, dag in Norwegian, Danish and Dutch. All of them from the Indo-European root dyau which explains the similarity with Latin dies though the word is known to come from the Germanic branch.
As of October 17, 2015, day is the 205th most common word in US English, the 210th most common in UK English. A day, symbol d, defined as 86 400 seconds, is not an SI unit, but is accepted for use with SI; the Second is the base unit of time in SI units. In 1967–68, during the 13th CGPM, the International Bureau of Weights and Measures redefined a second as … the duration of 9 192 631 770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium 133 atom; this makes the SI-based day last 794 243 384 928 000 of those periods. Due to tidal effects, the
Heidelberg is a university town in Baden-Württemberg situated on the river Neckar in south-west Germany. In the 2016 census, its population was 159,914, with a quarter of its population being students. Located about 78 km south of Frankfurt, Heidelberg is the fifth-largest city in the German state of Baden-Württemberg. Heidelberg is part of the densely populated Rhine-Neckar Metropolitan Region. Founded in 1386, Heidelberg University is Germany's oldest and one of Europe's most reputable universities. A scientific hub in Germany, the city of Heidelberg is home to several internationally renowned research facilities adjacent to its university, including four Max Planck Institutes. A former residence of the Electorate of the Palatinate, Heidelberg is a popular tourist destination due to its romantic cityscape, including Heidelberg Castle, the Philosophers' Walk, the baroque style Old Town. Heidelberg is in the Rhine Rift Valley, on the left bank of the lower part of the Neckar in a steep valley in the Odenwald.
It is bordered by the Gaisberg mountains. The Neckar here flows in an east-west direction. On the right bank of the river, the Heiligenberg mountain rises to a height of 445 meters; the Neckar flows into the Rhine 22 kilometres north-west in Mannheim. Villages incorporated during the 20th century stretch from the Neckar Valley along the Bergstraße, a road running along the Odenwald hills. Heidelberg is on European walking route E1. Since Heidelberg is among the warmest regions of Germany, plants atypical of the central-European climate flourish there, including almond and fig trees. Alongside the Philosophenweg on the opposite side of the Old Town, winegrowing was restarted in 2000. There is a wild population of African rose-ringed parakeets, a wild population of Siberian swan geese, which can be seen on the islands in the Neckar near the district of Bergheim. Heidelberg is a unitary authority within the Regierungsbezirk Karlsruhe; the Rhein-Neckar-Kreis rural district surrounds it and has its seat in the town, although the town is not a part of the district.
Heidelberg is a part of the Rhine-Neckar Metropolitan Region referred to as the Rhein-Neckar Triangle. This region consists of the southern part of the State of Hessen, the southern part of the State of Rhineland-Palatinate, the administrative districts of Mannheim and Heidelberg, the southern municipalities of the Rhein-Neckar-Kreis; the Rhein-Neckar Triangle became a European metropolitan area in 2005. Heidelberg consists of 15 districts distributed in six sectors of the town. In the central area are Altstadt and Weststadt; the new district will have 5,000–6,000 residents and employment for 7,000. Further new residential space for 10,000-15,000 residents was made available in Patrick Henry Village following the departure of the US Armed Forces; the following towns and communes border the city of Heidelberg, beginning in the west and in a clockwise direction: Edingen-Neckarhausen, Schriesheim, Schönau, Neckargemünd, Gaiberg, Sandhausen, Plankstadt and Mannheim. Heidelberg has an oceanic climate, defined by the protected valley between the Pfälzerwald and the Odenwald.
Year-round, the mild temperatures are determined by maritime air masses coming from the west. In contrast to the nearby Upper Rhine Plain, Heidelberg's position in the valley leads to more frequent easterly winds than average; the hillsides of the Odenwald favour precipitation. The warmest month is July, the coldest is January. Temperatures rise beyond 30 °C in midsummer. According to the German Meteorological Service, Heidelberg was the warmest place in Germany in 2009. Between 600,000 and 200,000 years ago, "Heidelberg Man" died at nearby Mauer, his jaw bone was discovered in 1907. Scientific dating determined his remains as the earliest evidence of human life in Europe. In the 5th century BC, a Celtic fortress of refuge and place of worship were built on the Heiligenberg, or "Mountain of Saints". Both places can still be identified. In 40 AD, a fort occupied by the 24th Roman cohort and the 2nd Cyrenaican cohort; the early Byzantine/late Roman Emperor Valentinian I, in 369 AD, built new and maintained older castra and a signal tower on the bank of the Neckar.
They built a wooden bridge based on stone pillars across it. The camp protected the first civilian settlements; the Romans remained until 260 AD. The local administrative center in Roman times was the nearby city of Lopodunum. Modern Heidelberg can trace its beginnings to the fifth century; the village Bergheim is first mentioned for that period in documents dated to 769 AD. Bergheim now lies in the middle of modern Heidelberg; the people converted to Christianity. In 863 AD, the monastery of St. Michael was founded on the Heiligenberg inside the double rampart of the Celtic fortress. Around 1130, the Neuburg Monastery was founded in the Neckar valley. At the same time, the bishopric of Worms extended its influence into the valley, founding Schönau Abbey in 1142. Modern He
Asteroids are minor planets of the inner Solar System. Larger asteroids have been called planetoids; these terms have been applied to any astronomical object orbiting the Sun that did not resemble a planet-like disc and was not observed to have characteristics of an active comet such as a tail. As minor planets in the outer Solar System were discovered they were found to have volatile-rich surfaces similar to comets; as a result, they were distinguished from objects found in the main asteroid belt. In this article, the term "asteroid" refers to the minor planets of the inner Solar System including those co-orbital with Jupiter. There exist millions of asteroids, many thought to be the shattered remnants of planetesimals, bodies within the young Sun's solar nebula that never grew large enough to become planets; the vast majority of known asteroids orbit within the main asteroid belt located between the orbits of Mars and Jupiter, or are co-orbital with Jupiter. However, other orbital families exist with significant populations, including the near-Earth objects.
Individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups: C-type, M-type, S-type. These were named after and are identified with carbon-rich and silicate compositions, respectively; the sizes of asteroids varies greatly. Asteroids are differentiated from meteoroids. In the case of comets, the difference is one of composition: while asteroids are composed of mineral and rock, comets are composed of dust and ice. Furthermore, asteroids formed closer to the sun; the difference between asteroids and meteoroids is one of size: meteoroids have a diameter of one meter or less, whereas asteroids have a diameter of greater than one meter. Meteoroids can be composed of either cometary or asteroidal materials. Only one asteroid, 4 Vesta, which has a reflective surface, is visible to the naked eye, this only in dark skies when it is favorably positioned. Small asteroids passing close to Earth may be visible to the naked eye for a short time; as of October 2017, the Minor Planet Center had data on 745,000 objects in the inner and outer Solar System, of which 504,000 had enough information to be given numbered designations.
The United Nations declared 30 June as International Asteroid Day to educate the public about asteroids. The date of International Asteroid Day commemorates the anniversary of the Tunguska asteroid impact over Siberia, Russian Federation, on 30 June 1908. In April 2018, the B612 Foundation reported "It's 100 percent certain we'll be hit, but we're not 100 percent sure when." In 2018, physicist Stephen Hawking, in his final book Brief Answers to the Big Questions, considered an asteroid collision to be the biggest threat to the planet. In June 2018, the US National Science and Technology Council warned that America is unprepared for an asteroid impact event, has developed and released the "National Near-Earth Object Preparedness Strategy Action Plan" to better prepare. According to expert testimony in the United States Congress in 2013, NASA would require at least five years of preparation before a mission to intercept an asteroid could be launched; the first asteroid to be discovered, was considered to be a new planet.
This was followed by the discovery of other similar bodies, with the equipment of the time, appeared to be points of light, like stars, showing little or no planetary disc, though distinguishable from stars due to their apparent motions. This prompted the astronomer Sir William Herschel to propose the term "asteroid", coined in Greek as ἀστεροειδής, or asteroeidēs, meaning'star-like, star-shaped', derived from the Ancient Greek ἀστήρ astēr'star, planet'. In the early second half of the nineteenth century, the terms "asteroid" and "planet" were still used interchangeably. Overview of discovery timeline: 10 by 1849 1 Ceres, 1801 2 Pallas – 1802 3 Juno – 1804 4 Vesta – 1807 5 Astraea – 1845 in 1846, planet Neptune was discovered 6 Hebe – July 1847 7 Iris – August 1847 8 Flora – October 1847 9 Metis – 25 April 1848 10 Hygiea – 12 April 1849 tenth asteroid discovered 100 asteroids by 1868 1,000 by 1921 10,000 by 1989 100,000 by 2005 ~700,000 by 2015 Asteroid discovery methods have improved over the past two centuries.
In the last years of the 18th century, Baron Franz Xaver von Zach organized a group of 24 astronomers to search the sky for the missing planet predicted at about 2.8 AU from the Sun by the Titius-Bode law because of the discovery, by Sir William Herschel in 1781, of the planet Uranus at the distance predicted by the law. This task required that hand-drawn sky charts be prepared for all stars in the zodiacal band down to an agreed-upon limit of faintness. On subsequent nights, the sky would be charted again and any moving object would be spotted; the expected motion of the missing planet was about 30 seconds of arc per hour discernible by observers. The first object, was not discovered by a member of the group, but rather by accident in 1801 by Giuseppe Piazzi, director of the observatory of Palermo in Sicily, he discovered a new star-like object in Taurus and followed the displacement of this object during several nights. That year, Carl Friedrich Gauss used these observations to calculate the orbit of this unknown object, found to be between the planets Mars and Jupiter.
Piazzi named it after Ceres, the Roman goddess of agriculture. Three other asteroids (2 Pallas, 3 Juno, 4 Ves