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
Orders of magnitude (length)
The following are examples of orders of magnitude for different lengths. To help compare different orders of magnitude, the following list describes various lengths between 1.6 × 10 − 35 metres and 10 10 10 122 metres. To help compare different orders of magnitude, this section lists lengths shorter than 10−23 m. 1.6 × 10−11 yoctometres – the Planck length. 1 ym – 1 yoctometre, the smallest named subdivision of the metre in the SI base unit of length, one septillionth of a metre 1 ym – length of a neutrino. 2 ym – the effective cross-section radius of 1 MeV neutrinos as measured by Clyde Cowan and Frederick Reines To help compare different orders of magnitude, this section lists lengths between 10−23 metres and 10−22 metres. To help compare different orders of magnitude, this section lists lengths between 10−22 m and 10−21 m. 100 ym – length of a top quark, one of the smallest known quarks To help compare different orders of magnitude, this section lists lengths between 10−21 m and 10−20 m. 2 zm – length of a preon, hypothetical particles proposed as subcomponents of quarks and leptons.
2 zm – radius of effective cross section for a 20 GeV neutrino scattering off a nucleon 7 zm – radius of effective cross section for a 250 GeV neutrino scattering off a nucleon To help compare different orders of magnitude, this section lists lengths between 10−20 m and 10−19 m. 15 zm – length of a high energy neutrino 30 zm – length of a bottom quark To help compare different orders of magnitude, this section lists lengths between 10−19 m and 10−18 m. 177 zm – de Broglie wavelength of protons at the Large Hadron Collider To help compare different orders of magnitude, this section lists lengths between 10−18 m and 10−17 m. 1 am – sensitivity of the LIGO detector for gravitational waves 1 am – upper limit for the size of quarks and electrons 1 am – upper bound of the typical size range for "fundamental strings" 1 am – length of an electron 1 am – length of an up quark 1 am – length of a down quark To help compare different orders of magnitude, this section lists lengths between 10−17 m and 10−16 m. 10 am – range of the weak force To help compare different orders of magnitude, this section lists lengths between 10−16 m and 10−15 m. 100 am – all lengths shorter than this distance are not confirmed in terms of size 850 am – approximate proton radius The femtometre is a unit of length in the metric system, equal to 10−15 metres.
In particle physics, this unit is more called a fermi with abbreviation "fm". To help compare different orders of magnitude, this section lists lengths between 10−15 metres and 10−14 metres. 1 fm – length of a neutron 1.5 fm – diameter of the scattering cross section of an 11 MeV proton with a target proton 1.75 fm – the effective charge diameter of a proton 2.81794 fm – classical electron radius 7 fm – the radius of the effective scattering cross section for a gold nucleus scattering a 6 MeV alpha particle over 140 degrees To help compare different orders of magnitude, this section lists lengths between 10−14 m and 10−13 m. 1.75 to 15 fm – Diameter range of the atomic nucleus To help compare different orders of magnitude, this section lists lengths between 10−13 m and 10−12 m. 570 fm – typical distance from the atomic nucleus of the two innermost electrons in the uranium atom, the heaviest naturally-occurring atom To help compare different orders of magnitude this section lists lengths between 10−12 and 10−11 m. 1 pm – distance between atomic nuclei in a white dwarf 2.4 pm – The Compton wavelength of the electron 5 pm – shorter X-ray wavelengths To help compare different orders of magnitude this section lists lengths between 10−11 and 10−10 m. 25 pm – approximate radius of a helium atom, the smallest neutral atom 50 pm – radius of a hydrogen atom 50 pm – bohr radius: approximate radius of a hydrogen atom ~50 pm – best resolution of a high-resolution transmission electron microscope 60 pm – radius of a carbon atom 93 pm – length of a diatomic carbon molecule To help compare different orders of magnitude this section lists lengths between 10−10 and 10−9 m. 100 pm – 1 ångström 100 pm – covalent radius of sulfur atom 120 pm – van der Waals radius of a neutral hydrogen atom 120 pm – radius of a gold atom 126 pm – covalent radius of ruthenium atom 135 pm – covalent radius of technetium atom 150 pm – Length of a typical covalent bond 153 pm – covalent radius of silver atom 155 pm – covalent radius of zirconium atom 175 pm – covalent radius of thulium atom 200 pm – highest resolution of a typical electron microscope 225 pm – covalent radius of caesium atom 280 pm – Average size of the water molecule 298 pm – radius of a caesium atom, calculated to be the largest atomic radius 340 pm – thickness of single layer graphene 356.68 pm – width of diamond unit cell 403 pm – width of lithium fluoride unit cell 500 pm – Width of protein α helix 543 pm – silicon lattice spacing 560 pm – width of sodium chloride unit cell 700 pm – width of glucose molecule 780 pm – mean width of quartz unit cell 820 pm – mean width of ice unit cell 900 pm – mean width of coesite unit cell To help compare different orders
The astronomical unit is a unit of length the distance from Earth to the Sun. However, that distance varies as Earth orbits the Sun, from a maximum to a minimum and back again once a year. Conceived as the average of Earth's aphelion and perihelion, since 2012 it has been defined as 149597870700 metres or about 150 million kilometres; the astronomical unit is used for measuring distances within the Solar System or around other stars. It is a fundamental component in the definition of another unit of astronomical length, the parsec. A variety of unit symbols and abbreviations have been in use for the astronomical unit. In a 1976 resolution, the International Astronomical Union used the symbol A to denote a length equal to the astronomical unit. In the astronomical literature, the symbol AU was common. In 2006, the International Bureau of Weights and Measures recommended ua as the symbol for the unit. In the non-normative Annex C to ISO 80000-3, the symbol of the astronomical unit is "ua". In 2012, the IAU, noting "that various symbols are presently in use for the astronomical unit", recommended the use of the symbol "au".
In the 2014 revision of the SI Brochure, the BIPM used the unit symbol "au". Earth's orbit around the Sun is an ellipse; the semi-major axis of this elliptic orbit is defined to be half of the straight line segment that joins the perihelion and aphelion. The centre of the Sun lies on this straight line segment, but not at its midpoint; because ellipses are well-understood shapes, measuring the points of its extremes defined the exact shape mathematically, made possible calculations for the entire orbit as well as predictions based on observation. In addition, it mapped out the largest straight-line distance that Earth traverses over the course of a year, defining times and places for observing the largest parallax in nearby stars. Knowing Earth's shift and a star's shift enabled the star's distance to be calculated, but all measurements are subject to some degree of error or uncertainty, the uncertainties in the length of the astronomical unit only increased uncertainties in the stellar distances.
Improvements in precision have always been a key to improving astronomical understanding. Throughout the twentieth century, measurements became precise and sophisticated, more dependent on accurate observation of the effects described by Einstein's theory of relativity and upon the mathematical tools it used. Improving measurements were continually checked and cross-checked by means of improved understanding of the laws of celestial mechanics, which govern the motions of objects in space; the expected positions and distances of objects at an established time are calculated from these laws, assembled into a collection of data called an ephemeris. NASA's Jet Propulsion Laboratory HORIZONS System provides one of several ephemeris computation services. In 1976, in order to establish a yet more precise measure for the astronomical unit, the IAU formally adopted a new definition. Although directly based on the then-best available observational measurements, the definition was recast in terms of the then-best mathematical derivations from celestial mechanics and planetary ephemerides.
It stated that "the astronomical unit of length is that length for which the Gaussian gravitational constant takes the value 0.01720209895 when the units of measurement are the astronomical units of length and time". Equivalently, by this definition, one AU is "the radius of an unperturbed circular Newtonian orbit about the sun of a particle having infinitesimal mass, moving with an angular frequency of 0.01720209895 radians per day". Subsequent explorations of the Solar System by space probes made it possible to obtain precise measurements of the relative positions of the inner planets and other objects by means of radar and telemetry; as with all radar measurements, these rely on measuring the time taken for photons to be reflected from an object. Because all photons move at the speed of light in vacuum, a fundamental constant of the universe, the distance of an object from the probe is calculated as the product of the speed of light and the measured time. However, for precision the calculations require adjustment for things such as the motions of the probe and object while the photons are transiting.
In addition, the measurement of the time itself must be translated to a standard scale that accounts for relativistic time dilation. Comparison of the ephemeris positions with time measurements expressed in the TDB scale leads to a value for the speed of light in astronomical units per day. By 2009, the IAU had updated its standard measures to reflect improvements, calculated the speed of light at 173.1446326847 AU/d. In 1983, the International Committee for Weights and Measures modified the International System of Units to make the metre defined as the distance travelled in a vacuum by light in 1/299792458 second; this replaced the previous definition, valid between 1960 and 1983, that the metre equalled a certain number of wavelengths of a certain emission line of krypton-86. The speed of light could be expressed as c0 = 299792458 m/s, a standard adopted by the IERS numerical standards. From this definition and the 2009 IAU standard, the time for light to traverse an AU is found to be
Wide-field Infrared Survey Explorer
Wide-field Infrared Survey Explorer is a NASA infrared-wavelength astronomical space telescope launched in December 2009, placed in hibernation mode in February 2011. It was re-activated in 2013. WISE discovered thousands of numerous star clusters, its observations supported the discovery of the first Y Dwarf and Earth trojan asteroid. WISE performed an all-sky astronomical survey with images in 3.4, 4.6, 12 and 22 μm wavelength range bands, over ten months using a 40 cm diameter infrared telescope in Earth orbit. After its hydrogen coolant depleted, a four-month mission extension called NEOWISE was conducted to search for near-Earth objects such as comets and asteroids using its remaining capability; the All-Sky data including processed images, source catalogs and raw data, was released to the public on March 14, 2012, is available at the Infrared Science Archive. In August 2013, NASA announced it would reactivate the WISE telescope for a new three-year mission to search for asteroids that could collide with Earth.
Science operations and data processing for WISE and NEOWISE take place at the Infrared Processing and Analysis Center at the California Institute of Technology in Pasadena. The mission was planned to create infrared images of 99 percent of the sky, with at least eight images made of each position on the sky in order to increase accuracy; the spacecraft was placed in a 525 km, polar, Sun-synchronous orbit for its ten-month mission, during which it has taken 1.5 million images, one every 11 seconds. The satellite orbited above the terminator, its telescope pointing always to the opposite direction to the Earth, except for pointing towards the Moon, avoided, its solar cells towards the Sun; each image covers a 47-arcminute field of view. Each area of the sky was scanned at least 10 times at the equator; the produced image library contains data on the local Solar System, the Milky Way, the more distant universe. Among the objects WISE studied are asteroids, dim stars such as brown dwarfs, the most luminous infrared galaxies.
Stellar nurseries, which are covered by interstellar dust, are detectable in infrared, since at this wavelength electromagnetic radiation can penetrate the dust. Infrared measurements from the WISE astronomical survey have been effective at unveiling undiscovered star clusters. Examples of such embedded star clusters are Camargo 18, Camargo 440, Majaess 101, Majaess 116. In addition, galaxies of the young Universe and interacting galaxies, where star formation is intensive, are bright in infrared. On this wavelength the interstellar gas clouds are detectable, as well as proto-planetary discs. WISE satellite was expected to find at least 1,000 of those proto-planetary discs. WISE was not able to detect Kuiper belt objects, it was able to detect any objects warmer than 70–100 K. A Neptune-sized object would be detectable out to 700 AU, a Jupiter-mass object out to 1 light year, where it would still be within the Sun's zone of gravitational control. A larger object of 2–3 Jupiter masses would be visible at a distance of up to 7–10 light years.
At the time of planning, it was estimated that WISE would detect about 300,000 main-belt asteroids, of which 100,000 will be new, some 700 near-Earth objects including about 300 undiscovered. That translates to about 1000 new main-belt asteroids per day, 1–3 NEOs per day; the peak of magnitude distribution for NEOs will be about 21–22 V. WISE would detect each typical Solar System object 10–12 times over about 36 hours in intervals of 3 hours. Construction of the WISE telescope was divided between Ball Aerospace & Technologies, SSG Precision Optronics, Inc. DRS and Rockwell, Lockheed Martin, Space Dynamics Laboratory; the program was managed through the Jet Propulsion Laboratory. The WISE instrument was built by the Space Dynamics Laboratory in Utah; the WISE spacecraft bus was built by Technologies Corp. in Boulder, Colorado. The spacecraft is derived from the Ball Aerospace RS-300 spacecraft architecture the NEXTSat spacecraft built for the successful Orbital Express mission launched on March 9, 2007.
The flight system has an estimated mass of 560 kg. The spacecraft is three-axis stabilized, with body-fixed solar arrays, it uses a high-gain antenna in the Ku band to transmit to the ground through the TDRSS geostationary system. Ball performed the testing and flight system integration. WISE surveyed the sky in four wavelengths of the infrared band, at a high sensitivity, its design specified as goals that the full sky atlas of stacked images it produced have 5-sigma sensitivity limits of 120, 160, 650, 2600 microjanskies at 3.3, 4.7, 12, 23 micrometers. WISE achieved at least 68, 98, 860, 5400 µJy 5-sigma sensitivity at 3.4, 4.6, 12, 22 micrometers for the WISE All-Sky data release. This is a factor of 1,000 times better sensitivity than the survey completed in 1983 by the IRAS satellite in the 12 and 23 micrometers bands, a factor of 500,000 times better than the 1990s survey by the Cosmic Background Explorer satellite at 3.3 and 4.7 micrometers. On the other hand, IRAS could observe 60 and 100 micron wavelengths.
Band 1 – 3.4 micrometers – broad-band sensitivity to stars and galaxies Band 2 – 4.6 micrometers – detect thermal radiation from the internal heat sources of sub-stell
Noricum is the Latin name for the Celtic kingdom or federation of tribes that included most of modern Austria and part of Slovenia. In the first century AD, it became a province of the Roman Empire, its borders were the Danube to the north and Vindelicia to the west, Pannonia to the east and southeast, Italia to the south. The kingdom was founded around 400 BC, had its capital at the royal residence at Virunum on the Magdalensberg. Around 800 BC, the region was inhabited by the people of the local Celtic Hallstatt culture. Around 450 BC, they merged with the people of the other core Celtic areas in the south-western regions of Germany and eastern France; the country is rich in iron and salt. It supplied material for the manufacturing of arms in Pannonia and northern Italy; the famous Noric steel was used in the making of Roman weapons. Gold and salt were found in considerable quantities; the plant called saliunca was used as a perfume according to Pliny the Elder. The Celtic inhabitants developed a culture rich in art, cattle breeding, salt mining and agriculture.
When part of the area became a Roman province, the Romans introduced water management and the vivid trade relations between the people north and south of the alps boosted - Noric steel was famous for its quality and hardness. Archaeological research in the cemeteries of Hallstatt, has shown that a vigorous Celtic civilization was in the area centuries before recorded history, but the Celtic Hallstatt civilization was a cultural manifestation prior to the other Celtic invasions, The Hallstatt graves contained weapons and ornaments from the Bronze Age, through the period of transition, up to the "Hallstatt culture", i.e. the developed older period of the Iron Age. The Noric language, a continental Celtic language, is attested in only fragmentary inscriptions, one from Ptuj and two from Grafenstein, neither of which provide enough information for any conclusions about the nature of the language; the kingdom of Noricum was a major provider of weaponry for the Roman armies from the mid-Republic onwards.
Roman swords were made of the best-quality steel available from this region, the chalybs Noricus. The strength of iron is determined by its carbon content; the wrought iron produced in the Greco-Roman world contained traces of carbon and was too soft for tools and weapons. It needed at least 1.5% carbon content. The Roman method of achieving this was to heat the wrought iron to a temperature of over 800 C and hammer it in a charcoal fire, causing the iron to absorb carbon from the charcoal; this technique developed empirically: there is no evidence ancient iron producers understood the chemistry. This rudimentary methods of carburisation made the quality of iron ore critical to the production of good steel; the ore needed to be rich in manganese, contain little or no phosphorus, which weakens steel. The ore mined in Carinthia fulfilled both criteria well; the Celts of Noricum discovered their ore made superior steel around 500 BC and built a major steel industry. At Magdalensberg, a major production and trading centre, specialised blacksmiths crafted metal products and weapons.
The finished arms were exported to Aquileia, a Roman colony founded in 180 BC. From 200 BC the Noricum tribes united into Celtic kingdom, known as the regnum Noricum, with its capital at a place called Noreia. Noricum became a key ally of the Roman Republic, providing high-quality weapons and tools in exchange for military protection; this was demonstrated in 113 BC. In response, the Roman consul Gnaeus Papirius Carbo led an army over the Alps to attack the Germanic tribes at the Noreia. Noricum was incorporated into the Roman Empire in 16 BC. For a long time the Noricans had enjoyed independence under princes of their own and carried on commerce with the Romans. In 48 BC they took the side of Julius Caesar in the civil war against Pompey. In 16 BC, having joined with the Pannonians in invading Histria, they were defeated by Publius Silius, proconsul of Illyricum. Thereafter, Noricum was called a province, although it was not organized as such and remained a kingdom with the title of regnum Noricum, yet under the control of an imperial procurator.
Under the reign of Emperor Claudius the Noricum Kingdom was incorporated into the Roman Empire without offering resistance. It was not until the reign of Antoninus Pius that the Second Legion, Pia was stationed in Noricum, the commander of the legion became the governor of the province. Under Diocletian, Noricum was divided into Noricum ripense, Noricum mediterraneum; the dividing line ran along the central part of the eastern Alps. Each division was under a praeses, both belonged to the diocese of Illyricum in the Praetorian prefecture of Italy, it was in this time that a Christian serving as a military officer in the province suffered martyrdom for the sake of his faith canonised as Saint Florian. The Roman colonies and chief towns were Virunum, Flavia Solva, Celeia in today's Slovenia, Ovilava, Lauriacum. Knowledge of Roman Noricum has been decisively expanded by the
In astronomy, magnitude is a unitless measure of the brightness of an object in a defined passband in the visible or infrared spectrum, but sometimes across all wavelengths. An imprecise but systematic determination of the magnitude of objects was introduced in ancient times by Hipparchus; the scale is logarithmic and defined such that each step of one magnitude changes the brightness by a factor of the fifth root of 100, or 2.512. For example, a magnitude 1 star is 100 times brighter than a magnitude 6 star; the brighter an object appears, the lower the value of its magnitude, with the brightest objects reaching negative values. Astronomers use two different definitions of magnitude: absolute magnitude; the apparent magnitude is the brightness of an object. Apparent magnitude depends on an object's intrinsic luminosity, its distance, the extinction reducing its brightness; the absolute magnitude describes the intrinsic luminosity emitted by an object and is defined to be equal to the apparent magnitude that the object would have if it were placed at a certain distance from Earth, 10 parsecs for stars.
A more complex definition of absolute magnitude is used for planets and small Solar System bodies, based on its brightness at one astronomical unit from the observer and the Sun. The Sun has an apparent magnitude of −27 and Sirius, the brightest visible star in the night sky, −1.46. Apparent magnitudes can be assigned to artificial objects in Earth orbit with the International Space Station sometimes reaching a magnitude of −6; the magnitude system dates back 2000 years to the Greek astronomer Hipparchus who classified stars by their apparent brightness, which they saw as size. To the unaided eye, a more prominent star such as Sirius or Arcturus appears larger than a less prominent star such as Mizar, which in turn appears larger than a faint star such as Alcor. In 1736, the mathematician John Keill described the ancient naked-eye magnitude system in this way: The fixed Stars appear to be of different Bignesses, not because they are so, but because they are not all distant from us; those that are nearest will excel in Bigness.
Hence arise the Distribution of Stars, according to their Order and Dignity, into Classes. For all the other Stars, which are only seen by the Help of a Telescope, which are called Telescopical, are not reckoned among these six Orders. Altho' the Distinction of Stars into six Degrees of Magnitude is received by Astronomers, and among those Stars which are reckoned of the brightest Class, there appears a Variety of Magnitude. For Example: The little Dog was by Tycho placed among the Stars of the second Magnitude, which Ptolemy reckoned among the Stars of the first Class: And therefore it is not either of the first or second Order, but ought to be ranked in a Place between both. Note that the brighter the star, the smaller the magnitude: Bright "first magnitude" stars are "1st-class" stars, while stars visible to the naked eye are "sixth magnitude" or "6th-class"; the system was a simple delineation of stellar brightness into six distinct groups but made no allowance for the variations in brightness within a group.
Tycho Brahe attempted to directly measure the "bigness" of the stars in terms of angular size, which in theory meant that a star's magnitude could be determined by more than just the subjective judgment described in the above quote. He concluded that first magnitude stars measured 2 arc minutes in apparent diameter, with second through sixth magnitude stars measuring 1 1⁄2′, 1 1⁄12′, 3⁄4′, 1⁄2′, 1⁄3′, respectively; the development of the telescope showed that these large sizes were illusory—stars appeared much smaller through the telescope. However, early telescopes produced a spurious disk-like image of a star, larger for brighter stars and smaller for fainter ones. Astronomers from Galileo to Jaques Cassini mistook these spurious disks for the physical bodies of stars, thus into the eighteenth century continued to think of magnitude in terms of the physical size of a star. Johannes Hevelius produced a precise table of star sizes measured telescopically, but now the measured diameters ranged from just over six seconds of arc for first magnitude down to just under 2 seconds for sixth magnitude.
By the time of William Herschel astronomers recognized that the telescopic disks of stars were spurious and a function of the telescope as well as the brightness of the stars, but still spoke in terms of a star's size more than its brightness. Well into the nineteenth century the magnitude system
The Infrared Astronomical Satellite was the first-ever space telescope to perform a survey of the entire night sky at infrared wavelengths. Launched on 25 January 1983, its mission lasted ten months; the telescope was a joint project of the United States, the Netherlands, the United Kingdom. Over 250,000 infrared sources were observed at 12, 25, 60, 100 micrometer wavelengths. Support for the processing and analysis of data from IRAS was contributed from the Infrared Processing and Analysis Center at the California Institute of Technology; the Infrared Science Archive at IPAC holds the IRAS archive. The success of IRAS led to interest in the 1985 Infrared Telescope mission on the Space Shuttle, the planned Shuttle Infrared Telescope Facility which transformed into the Space Infrared Telescope Facility, SIRTF, which in turn was developed into the Spitzer Space Telescope, launched in 2003; the success of early infrared space astronomy led to further missions, such as the Infrared Space Observatory and the Hubble Space Telescope's NICMOS instrument.
IRAS was the first observatory to perform an all-sky survey at infrared wavelengths. It mapped 96% of the sky four times, at 12, 25, 60 and 100 micrometers, with resolutions ranging from 30 arcseconds at 12 micrometers to 2 arcminutes at 100 micrometers, it discovered about 350,000 sources. About 75,000 of those are believed to be starburst galaxies, still enduring their star-formation stage. Many other sources are normal stars with disks of dust around them the early stage of planetary system formation. New discoveries included the first images of the Milky Way's core. IRAS's life, like that of most infrared satellites that followed, was limited by its cooling system. To work in the infrared domain, a telescope must be cooled to cryogenic temperatures. In IRAS's case, 73 kilograms of superfluid helium kept the telescope at a temperature of 2 K, keeping the satellite cool by evaporation; the on-board supply of liquid helium was depleted after 10 months on 21 November 1983, causing the telescope temperature to rise, preventing further observations.
The spacecraft continues to orbit the Earth. IRAS was designed to catalog fixed sources, so it scanned the same region of sky several times. Jack Meadows led a team at Leicester University, including John Davies and Simon Green, which searched the rejected sources for moving objects; this led to the discovery of three asteroids, including 3200 Phaethon, six comets, a huge dust trail associated with comet 10P/Tempel. The comets included 126P/IRAS, 161P/Hartley–IRAS, comet IRAS–Araki–Alcock, which made a close approach to the Earth in 1983. Out of the six comets IRAS found, four were long period and two were short period comets; the observatory made headlines with the announcement on 10 December 1983 of the discovery of an "unknown object" at first described as "possibly as large as the giant planet Jupiter and so close to Earth that it would be part of this solar system". Further analysis revealed that, of several unidentified objects, nine were distant galaxies and the tenth was "intergalactic cirrus".
None were found to be Solar System bodies. During its mission, IRAS detected odd infrared signatures around several stars; this led to the systems being targeted by the Hubble Space Telescope's NICMOS instrument between 1999 and 2006, but nothing was detected. In 2014, using new image processing techniques on the Hubble data, researchers discovered planetary disks around these stars. Several infrared space telescopes have continued and expanded the study of the infrared Universe, such as the Infrared Space Observatory launched in 1995, the Spitzer Space Telescope launched in 2003, the Akari Space Telescope launched in 2006. A next generation of infrared space telescopes began when NASA's Wide-field Infrared Survey Explorer launched on 14 December 2009 aboard a Delta II rocket from Vandenberg Air Force Base. Known as WISE, the telescope provided results hundreds of times more sensitive than IRAS at the shorter wavelengths. Diffuse Infrared Background Experiment, a infrared sky survey on COBE Infrared astronomy List of asteroid-discovering observatories List of largest infrared telescopes List of minor planet discoverers § Discovering dedicated institutions Category:IRAS catalogue objects Beichman, C. A..
Infrared Astronomical Satellite: Catalogs and Atlases. Volume 1: Explanatory Supplement. NASA Scientific and Technical Information Division. IRAS website by Caltech IRAS Minor Planet Survey archive by the Planetary Science Institute IRAS survey at WikiSky.org