1.
Comet
–
A comet is an icy small Solar System body that, when passing close to the Sun, warms and begins to evolve gasses, a process called outgassing. This produces an atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of 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 collections of ice, dust. The coma may be up to 15 times the Earths diameter, 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 observed and recorded since ancient times by many cultures, Comets usually have highly eccentric elliptical orbits, and they have a wide range of orbital periods, ranging from several years to potentially several millions of years. Short-period comets originate in the Kuiper belt or its associated scattered disc, 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, 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 coma and 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. As of November 2014 there are 5,253 known comets, 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. Roughly one comet per year is visible to the eye, though many of those are faint. Particularly bright examples are called Great Comets, the word comet derives from the Old English cometa from the Latin comēta or comētēs. That, in turn, is a latinisation of the Greek κομήτης, Κομήτης was derived from κομᾶν, which was itself derived from κόμη and was used to mean the tail of a comet. The astronomical symbol for comets is ☄, consisting of a disc with three hairlike extensions. The solid, core structure of a comet is known as the nucleus, cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen gases such as carbon dioxide, carbon monoxide, methane, and ammonia. As such, they are described as dirty snowballs after Fred Whipples model
2.
Gemini (constellation)
–
Gemini is one of the constellations of the zodiac. It was one of the 48 constellations described by the 2nd century AD astronomer Ptolemy and its name is Latin for twins, and it is associated with the twins Castor and Pollux in Greek mythology. Gemini lies between Taurus to the west and Cancer to the east, with Auriga and Lynx to the north and Monoceros, the Sun resides in the astrological sign of Gemini from June 20 to July 20 each year. By mid August, Gemini will appear along the horizon in the morning sky prior to sunrise. The best time to observe Gemini at night is overhead during the months of January and February, by April and May, the constellation will be visible soon after sunset in the west. The easiest way to locate the constellation is to find its two brightest stars Castor and Pollux eastward from the familiar “V” shaped asterism of Taurus and the three stars of Orion’s belt. Another way is to draw a line from the Pleiades star cluster located in Taurus. In doing so, you are drawing a line that is relatively close to the ecliptic. The constellation contains 85 stars visible to observation on Earth without a telescope, the brightest star in Gemini is Pollux, and the second brightest is Castor. Castors Bayer designation as Alpha is attributable to a mistake by Johann Bayer, α Gem is a sextuple star system 52 light-years from Earth, which appears as a magnitude 1.6 blue-white star to the unaided eye. Two spectroscopic binaries are visible at magnitudes 1.9 and 3.0 with a period of 470 years. A wide-set red dwarf star is also a part of the system, β Gem is an orange-hued giant star of magnitude 1.2,34 light-years from Earth. Pollux has an extrasolar planet revolving around it, as do two other stars in Gemini, HD50554, and HD59686, γ Gem is a blue-white hued star of magnitude 1.9,105 light-years from earth. δ Gem is a binary star 59 light-years from Earth. The primary is a star of magnitude 3.5. The period is over 1000 years, it is divisible in amateur telescopes. ε Gem, a star, includes a primary yellow supergiant of magnitude 3.1,900 light-years from Earth. The optical companion, of magnitude 9.2, is visible in binoculars and small telescopes
3.
Eridanus (constellation)
–
Eridanus is a constellation in the southern hemisphere. It is represented as a river and it was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations. It is the sixth largest of the modern constellations, the same name was later taken as a Latin name for the real Po River and also for the name of a minor river in Athens. At its southern end is the magnitude 0.5 star Achernar,144 light-years from Earth, it is a blue-white hued main sequence star whose traditional name means the rivers end. Achernar is a very peculiar star because it is one of the flattest stars known, observations indicate that its radius is about 50% larger at the equator than at the poles. This distortion occurs because the star is spinning extremely rapidly, there are several other noteworthy stars in Eridanus, including some double stars. Beta Eridani, traditionally called Cursa, is a star of magnitude 2.8,89 light-years from Earth. Its place to the south of Orions foot gives it its name, theta Eridani, called Acamar, is a binary star with blue-white components, distinguishable in small amateur telescopes and 161 light-years from Earth. The primary is of magnitude 3.2 and the secondary is of magnitude 4.3,32 Eridani is a binary star 290 light-years from Earth. The primary is a star of magnitude 4.8. 32 Eridani is divisible in amateur telescopes. 39 Eridani is a binary star divisible in small amateur telescopes,206 light-years from Earth. The primary is a giant star of magnitude 4.9. 40 Eridani is a star system consisting of an orange main-sequence star, a white dwarf. The orange main-sequence star is the primary of magnitude 4.4, the red dwarf, of magnitude 11, orbits the white dwarf every 250 years. The 40 Eridani system is 16 light-years from Earth, P Eridani is a binary star with two orange components,27 light-years from Earth. The magnitude 5.8 primary and 5.9 secondary have a period of 500 years. Epsilon Eridani is a star with one extrasolar planet similar to Jupiter and it is an orange-hued main-sequence star of magnitude 3.7,10.5 light-years from Earth
4.
Jet Propulsion Laboratory
–
The Jet Propulsion Laboratory is a federally funded research and development center and NASA field center in La Cañada Flintridge, California and Pasadena, California, United States. The JPL is managed by the nearby California Institute of Technology for NASA, the laboratorys primary function is the construction and operation of planetary robotic spacecraft, though it also conducts Earth-orbit and astronomy missions. It is also responsible for operating NASAs Deep Space Network and they are also responsible for managing the JPL Small-Body Database, and provides physical data and lists of publications for all known small Solar System bodies. The JPLs 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. Malinas 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, Malina, Parsons, Forman, Martin Summerfield, in 1943, von Kármán, Malina, Parsons, and Forman established the Aerojet Corporation to manufacture JATO motors. The project took on the name Jet Propulsion Laboratory in November 1943, during JPLs 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 and it also developed a number of other weapons system prototypes, such as the Loki anti-aircraft missile system, and the forerunner of the Aerobee sounding rocket. At various times, it carried out testing at the White Sands Proving Ground, Edwards Air Force Base. A lunar lander was developed in 1938-39 which influenced design of the Apollo Lunar Module in the 1960s. The team lost that proposal to Project Vanguard, and instead embarked on a project to demonstrate ablative re-entry technology using a Jupiter-C rocket. They carried out three successful flights in 1956 and 1957. Using a spare Juno I, the two organizations then launched the United States first satellite, Explorer 1, on February 1,1958, JPL was transferred to NASA in December 1958, becoming the agencys primary planetary spacecraft center. JPL engineers designed and operated Ranger and Surveyor missions to the Moon that prepared the way for Apollo, JPL also led the way in interplanetary exploration with the Mariner missions to Venus, Mars, 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 Earths orbit. JPL was early to employ women 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 their first woman engineer to work alongside male engineers as part of the Ranger and Mariner mission tracking teams, when founded, JPLs site was a rocky flood-plain just outside the city limits of Pasadena. Almost all of the 177 acres of the U. S, the city of La Cañada Flintridge, California was incorporated in 1976, well after JPL attained international recognition with a Pasadena address
5.
Saturn
–
Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with a radius about nine times that of Earth. Although it has only one-eighth the average density of Earth, with its larger volume Saturn is just over 95 times more massive, Saturn is named after the Roman god of agriculture, its astronomical symbol represents the gods sickle. Saturns interior is composed of a core of iron–nickel and rock. This core is surrounded by a layer of metallic hydrogen, an intermediate layer of liquid hydrogen and liquid helium. Saturn has a yellow hue due to ammonia crystals in its upper atmosphere. Saturns magnetic field strength is around one-twentieth of Jupiters, the outer atmosphere is generally bland and lacking in contrast, although long-lived features can appear. Wind speeds on Saturn can reach 1,800 km/h, higher than on Jupiter, sixty-two moons are known to orbit Saturn, of which fifty-three are officially named. This does not include the hundreds of moonlets comprising the rings, Saturn is a gas giant because it is predominantly composed of hydrogen and helium. It lacks a definite surface, though it may have a solid core, Saturns rotation causes it to have the shape of an oblate spheroid, that is, it is flattened at the poles and bulges at its equator. Its equatorial and polar radii differ by almost 10%,60,268 km versus 54,364 km, Jupiter, Uranus, and Neptune, the other giant planets in the Solar System, are also oblate but to a lesser extent. Saturn is the planet of the Solar System that is less dense than water—about 30% less. Although Saturns core is considerably denser than water, the specific density of the planet is 0.69 g/cm3 due to the atmosphere. Jupiter has 318 times the Earths mass, while Saturn is 95 times the mass of the Earth, together, Jupiter and Saturn hold 92% of the total planetary mass in the Solar System. On 8 January 2015, NASA reported determining the center of the planet Saturn, the temperature, pressure, and density inside Saturn all rise steadily toward the core, which causes hydrogen to transition into a metal in the deeper layers. Standard planetary models suggest that the interior of Saturn is similar to that of Jupiter, having a rocky core surrounded by hydrogen. This core is similar in composition to the Earth, but more dense, in 2004, they estimated that the core must be 9–22 times the mass of the Earth, which corresponds to a diameter of about 25,000 km. This is surrounded by a liquid metallic hydrogen layer, followed by a liquid layer of helium-saturated molecular hydrogen that gradually transitions to a gas with increasing altitude
6.
Neptune
–
Neptune is the eighth and farthest known planet from the Sun in the Solar System. In the Solar System, it is the fourth-largest planet by diameter, the planet. Neptune is 17 times the mass of Earth and is more massive than its near-twin Uranus. Neptune orbits the Sun once every 164.8 years at a distance of 30.1 astronomical units. It is named after the Roman god of the sea and has the astronomical symbol ♆, Neptune is not visible to the unaided eye and is the only planet in the Solar System found by mathematical prediction rather than by empirical observation. Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit was subject to perturbation by an unknown planet. Neptune was subsequently observed with a telescope on 23 September 1846 by Johann Galle within a degree of the predicted by Urbain Le Verrier. Its largest moon, Triton, was discovered shortly thereafter, though none of the remaining known 14 moons were located telescopically until the 20th century. The planets distance from Earth gives it a small apparent size. Neptune was visited by Voyager 2, when it flew by the planet on 25 August 1989, the advent of the Hubble Space Telescope and large ground-based telescopes with adaptive optics has recently allowed for additional detailed observations from afar. Neptunes composition can be compared and contrasted with the Solar Systems other giant planets, however, its interior, like that of Uranus, is primarily composed of ices and rock, which is why Uranus and Neptune are normally considered ice giants to emphasise this distinction. Traces of methane in the outermost regions in part account for the blue appearance. In contrast to the hazy, relatively featureless atmosphere of Uranus, Neptunes atmosphere has active, for example, at the time of the Voyager 2 flyby in 1989, the planets southern hemisphere had a Great Dark Spot comparable to the Great Red Spot on Jupiter. These weather patterns are driven by the strongest sustained winds of any planet in the Solar System, because of its great distance from the Sun, Neptunes outer atmosphere is one of the coldest places in the Solar System, with temperatures at its cloud tops approaching 55 K. Temperatures at the centre are approximately 5,400 K. Neptune has a faint and fragmented ring system. On both occasions, Galileo seems to have mistaken Neptune for a star when it appeared close—in conjunction—to Jupiter in the night sky, hence. At his first observation in December 1612, Neptune was almost stationary in the sky because it had just turned retrograde that day and this apparent backward motion is created when Earths orbit takes it past an outer planet. Because Neptune was only beginning its yearly cycle, the motion of the planet was far too slight to be detected with Galileos small telescope
7.
Zacatecas
–
Zacatecas, officially the Free and Sovereign State of Zacatecas, is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is divided in 58 municipalities and its capital city is Zacatecas City, Zacatecas is located in North-Central Mexico. The state is best known for its deposits of silver and other minerals, its colonial architecture. Its main economic activities are mining, agriculture and tourism, Zacatecas is located in the center-north of Mexico, and covers an area of 75, 284km2, the tenth largest state in the country. It borders the states of Jalisco, Aguascalientes, San Luis Potosí, Coahuila and Durango and is divided into fifty eight municipalities and 4,882, towns, cities and other communities. The state has an altitude of 2230 meters above sea level. The state has three geographical regions, the Sierra Madre Occidental in the west, the Mexican Plateau. Most of it is in the Sierra Madre Occidental with highly rugged with peaks of over 2,500 meters above sea level, the mountains of the southeast and northeast are lower but there large valleys such as the Juchipila and Tlaltenango. Most of the territory has only small mesas and other areas of flat land, in the center of the state, there is a small mountain chain called the Sierra de Fresnillo, from which much of the state’s mineral wealth comes from. In the extreme northwest, there is another important mountain chain called the Sierra de Sombrerete, marked by a mountain called Sombreretillo, near this chain is another called the Sierra de Órganos. No major rivers run through the state and most of the run only during the rainy season. The state belongs to two basins, the south east of the state belongs to the Lerma River basin, which eventually empties in the Pacific Ocean. Rivers belonging to this include the San Pedro, Juchipila, Jerez. The other basin is smaller and endorheic, and does not empty into any ocean, the state has eighty dams with a total capacity of 595,337 million cubic meters. The largest of these are the Leobardo Reynoso in Fresnillo, Miguel Aleman in Tlaltenango, much of the state’s water is underground divided into twenty hydraulic zones. These are accesses with over 5,800 wells mostly for agricultural use, most of the territory has a cool, dry climate, although areas in the south have more moisture, with most rain falling between June and September. The driest and coldest areas are in the northeast, known as the Salado because of its saltwater lakes, 75% of the state is arid or semi arid. 14% is arable and 79% is apt for the grazing of livestock, the average annual temperature is 16C with most of the state being temperate
8.
Comet nucleus
–
The nucleus is the solid, central part of a comet, popularly termed a dirty snowball or an icy dirtball. A cometary nucleus is composed of rock, dust, and frozen gases, when heated by the Sun, the gases sublimate and produce an atmosphere surrounding the nucleus known as the coma. The force exerted on the coma by the Suns radiation pressure and solar wind cause an enormous tail to form, a typical comet nucleus has an albedo of 0.04. This is blacker than coal, and may be caused by a covering of dust, comets, or their precursors, formed in the outer Solar System, possibly millions of years before planet formation. How and when formed is debated, with distinct implications for Solar System formation, dynamics. Three-dimensional computer simulations indicate the structural features observed on cometary nuclei can be explained by pairwise low velocity accretion of weak cometesimals. The currently favored creation mechanism is that of the nebular hypothesis, astronomers think that comets originate in both the Oort cloud and the scattered disk. Most cometary nuclei are thought to be no more than about 10 miles across, the largest comets that have come inside the orbit of Saturn are C/2002 VQ94, Hale–Bopp, 29P, 109P/Swift–Tuttle, and 28P/Neujmin. The potato-shaped nucleus of Halleys comet contains equal amounts of ice, during a flyby in September 2001, the Deep Space 1 spacecraft observed the nucleus of Comet Borrelly and found it to be about half the size of the nucleus of Halleys Comet. Borrellys nucleus was also potato-shaped and had a black surface. Like Halleys Comet, Comet Borrelly only released gas from small areas where holes in the crust exposed the ice to sunlight, the nucleus of comet Hale–Bopp was estimated to be 60 ±20 km in diameter. Hale-Bopp appeared bright to the eye because its unusually large nucleus gave off a great deal of dust. The nucleus of P/2007 R5 is probably only 100–200 meters in diameter, the largest centaurs are estimated to be 250 km to 300 km in diameter. Three of the largest would include 10199 Chariklo,2060 Chiron, known comets have been estimated to have an average density of 0.6 g/cm3. Below is a list of comets that have had estimated sizes, densities, about 80% of the Halleys Comet nucleus is water ice, and frozen carbon monoxide makes up another 15%. Much of the remainder is carbon dioxide, methane. Scientists think that comets are chemically similar to Halleys Comet. The nucleus of Halleys Comet is also a dark black
9.
Bird migration
–
Bird migration is the regular seasonal movement, often north and south along a flyway, between breeding and wintering grounds. Migration carries high costs in predation and mortality, including from hunting by humans and it occurs mainly in the northern hemisphere, where birds are funnelled on to specific routes by natural barriers such as the Mediterranean Sea or the Caribbean Sea. More recently, Johannes Leche began recording dates of arrivals of spring migrants in Finland in 1749, threats to migratory birds have grown with habitat destruction especially of stopover and wintering sites, as well as structures such as power lines and wind farms. The Arctic tern holds the long-distance migration record for birds, travelling between Arctic breeding grounds and the Antarctic each year, shorter migrations are common, including altitudinal migrations on mountains such as the Andes and Himalayas. The timing of migration seems to be controlled primarily by changes in day length, migrating birds navigate using celestial cues from the sun and stars, the earths magnetic field, and probably also mental maps. Records of bird migration were made as much as 3,000 years ago by the Ancient Greek writers Hesiod, Homer, Herodotus and Aristotle. The Bible also notes migrations, as in the Book of Job, the author of Jeremiah wrote, Even the stork in the heavens knows its seasons, and the turtle dove, the swift and the crane keep the time of their arrival. Aristotle noted that cranes traveled from the steppes of Scythia to marshes at the headwaters of the Nile, pliny the Elder, in his Historia Naturalis, repeats Aristotles observations. Aristotle however suggested that swallows and other birds hibernated and this belief persisted as late as 1878, when Elliott Coues listed the titles of no less than 182 papers dealing with the hibernation of swallows. It was not until the end of the century that migration as an explanation for the winter disappearance of birds from northern climes was accepted. Bewick then describes an experiment which succeeded in keeping alive in Britain for several years. He concludes, These experiments have since been confirmed by. Migration is the seasonal movement, often north and south. Bird movements include those made in response to changes in food availability, habitat, sometimes, journeys are not termed true migration because they are irregular or in only one direction. Migration is marked by its annual seasonality, non-migratory birds are said to be resident or sedentary. Approximately 1800 of the worlds 10,000 bird species are long-distance migrants, many bird populations migrate long distances along a flyway. The most common pattern involves flying north in the spring to breed in the temperate or Arctic summer, of course, in the southern hemisphere the directions are reversed, but there is less land area in the far south to support long-distance migration. The primary motivation for migration appears to be food, for example, also, the longer days of the northern summer provide extended time for breeding birds to feed their young
10.
Pisces (constellation)
–
Pisces is a constellation of the zodiac. Its name is the Latin plural for fish and it lies between Aquarius to the west and Aries to the east. The ecliptic and the equator intersect within this constellation and in Virgo. The vernal equinox is currently located in Pisces, due south of ω Psc, van Maanens Star, at 12.36 magnitude, is located in this constellation, along with others, such as HD222410, at 7.45 magnitude. It has many clusters of stars and the associated nebulae. It was discovered by Pierre Méchain, a French astronomer, in 1780, NGC488 is an isolated face-on prototypical spiral galaxy. NGC520 is a pair of colliding galaxies located 90 million lightyears away, CL 0024+1654 is a massive galaxy cluster that lenses the galaxy behind it, creating arc-shaped images of the background galaxy. 3C31 is a galaxy and radio source in Perseus located at a distance of 237 million light-years from Earth. Its jets, caused by the black hole at its center. Pisces originates from some composition of the Babylonian constellations Šinunutu4 the great swallow in current western Pisces, in the first-millennium BC texts known as the Astronomical Diaries, part of the constellation was also called DU. NU. NU. Pisces is associated with Aphrodite and Eros, who escaped from the monster Typhon by leaping into the sea, in order not to lose each other, they tied themselves together with rope. The Romans adopted the Greek legend, with Venus and Cupid acting as the counterparts for Aphrodite, the knot of the rope is marked by Alpha Piscium, also called Al-Rischa. Linum Boreum, χ – ρ,94 – VX – η – π – ο – α Psc, linum Austrinum, α – ξ – ν – μ – ζ – ε – δ –41 –35 – ω Psc. Piscis Austrinus, ω – ι – θ –7 – β –5 – κ,9 – λ – TX Psc. However the proposal was largely neglected by other astronomers with the exception of Admiral Smyth, who mentioned it in his book The Bedford Catalogue, and it is now obsolete. The Fishes are also associated with the German legend of Antenteh, who owned just a tub and they offered him a wish, which he refused. However, his wife begged him to return to the fish and this wish was granted, but her desires were not satisfied. She then asked to be a queen and have a palace, but when she asked to become a goddess, the tub in the story is sometimes recognized as the Great Square of Pegasus
11.
Coma (cometary)
–
The coma is the nebulous envelope around the nucleus of a comet, formed when the comet passes close to the Sun on its highly elliptical orbit, as the comet warms, parts of it sublimes. This gives a comet a fuzzy appearance when viewed in telescopes and distinguishes it from stars, the word coma comes from the Greek kome, which means hair and is the origin of the word comet itself. The coma is generally made of ice and comet dust, water dominates up to 90% of the volatiles that outflow from the nucleus when the comet is within 3-4 AU of the Sun. The parent molecule is destroyed primarily through photodissociation and to a smaller extent photoionization. The solar wind plays a role in the destruction of water compared to photochemistry. Larger dust particles are left along the orbital path while smaller particles are pushed away from the Sun into the comets tail by light pressure. Comas typically grow in size as comets approach the Sun, and they can be as large as the diameter of Jupiter, about a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun. The Great Comet of 1811 also had a coma roughly the diameter of the Sun, even though the coma can become quite large, its size can actually decrease about the time it crosses the orbit of Mars around 1.5 AU from the Sun. At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, Comets were found to emit X-rays in late-March 1996. This surprised researchers, because X-ray emission is associated with very high-temperature bodies. This ripping off leads to the emission of X-rays and far ultraviolet photons, with basic Earth-surface based telescope and some technique, the size of the Coma can be calculated. Called the drift method, one locks the telescope in position and that time multiplied by the cosine of comets declination, times.25 should equal the comas diameter in arcminutes. If the distance to the comet is known, then the apparent size of the coma can be determined. In 2015, it was noted that the ALICE instrument on the ESA Rosetta spacecraft to comet 67/P, detected hydrogen, oxygen, carbon and nitrogen in the Coma, which they also called the Comets atmosphere. Alice is a spectrograph, and it found that electrons created by UV light were colliding and breaking up molecules of water. OAO-2 discovered large halos of hydrogen gas around comets, space probe Giotto detected hydrogen ions at distance of 7.8 million km away from Halley when it did close flyby of the comet in 1986. A hydrogen gas halo was detected to be 15 times the diameter of Sun and this triggered NASA to point the Pioneer Venus mission at the Comet, and it was determined that the Comet emitting 12 tons of water per second. The hydrogen gas emission has not been detected from Earths surface because those wavelengths are blocked by the atmosphere, the process by which water is broken down into hydrogen and oxygen was studied by the ALICE instrument aboard the Rosetta spacecraft
12.
Apsis
–
An apsis is an extreme point in an objects orbit. The word comes via Latin from Greek and is cognate with apse, for elliptic orbits about a larger body, there are two apsides, named with the prefixes peri- and ap-, or apo- added to a reference to the thing being orbited. For a body orbiting the Sun, the point of least distance is the perihelion, the terms become periastron and apastron when discussing orbits around other stars. For any satellite of Earth including the Moon the point of least distance is the perigee, for objects in Lunar orbit, the point of least distance is the pericynthion and the greatest distance the apocynthion. For any orbits around a center of mass, there are the terms pericenter and apocenter, periapsis and apoapsis are equivalent alternatives. A straight line connecting the pericenter and apocenter is the line of apsides and this is the major axis of the ellipse, its greatest diameter. For a two-body system the center of mass of the lies on this line at one of the two foci of the ellipse. When one body is larger than the other it may be taken to be at this focus. Historically, in systems, apsides were measured from the center of the Earth. In orbital mechanics, the apsis technically refers to the distance measured between the centers of mass of the central and orbiting body. However, in the case of spacecraft, the family of terms are used to refer to the orbital altitude of the spacecraft from the surface of the central body. The arithmetic mean of the two limiting distances is the length of the axis a. The geometric mean of the two distances is the length of the semi-minor axis b, the geometric mean of the two limiting speeds is −2 ε = μ a which is the speed of a body in a circular orbit whose radius is a. The words pericenter and apocenter are often seen, although periapsis/apoapsis are preferred in technical usage, various related terms are used for other celestial objects. The -gee, -helion and -astron and -galacticon forms are used in the astronomical literature when referring to the Earth, Sun, stars. The suffix -jove is occasionally used for Jupiter, while -saturnium has very rarely used in the last 50 years for Saturn. The -gee form is used as a generic closest approach to planet term instead of specifically applying to the Earth. During the Apollo program, the terms pericynthion and apocynthion were used when referring to the Moon, regarding black holes, the term peri/apomelasma was used by physicist Geoffrey A. Landis in 1998 before peri/aponigricon appeared in the scientific literature in 2002
