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159-542: Comet Hale–Bopp (formally designated C/1995 O1 ) is a long-period comet that was one of the most widely observed of the 20th century and one of the brightest seen for many decades. Alan Hale and Thomas Bopp discovered Comet Hale–Bopp separately on July 23, 1995, before it became visible to the naked eye . It is difficult to predict the maximum brightness of new comets with any degree of certainty, but Hale–Bopp exceeded most predictions when it passed perihelion on April 1, 1997, reaching about magnitude −1.8. It

318-554: A "comet of the century" and turned out to be unspectacular. Hale–Bopp became visible to the naked eye in May 1996, and although its rate of brightening slowed considerably during the latter half of that year, scientists were still cautiously optimistic that it would become very bright. It was too closely aligned with the Sun to be observable during December 1996, but when it reappeared in January 1997 it

477-464: A binary nucleus. The discovery of the satellite was not confirmed by other observations. Also, while comets have been observed to break up before, no case had been found of a stable binary nucleus until the subsequent discovery of P/2006 VW 139 . In November 1996, amateur astronomer Chuck Shramek of Houston , Texas took a CCD image of the comet which showed a fuzzy, slightly elongated object nearby. His computer sky-viewing program did not identify

636-631: A car crash after photographing the comet. Gene died in the crash and his ashes were sent to the Moon aboard NASA's Lunar Prospector mission along with an image of Hale–Bopp, "the last comet that the Shoemakers observed together". Comet A comet is an icy, small Solar System body that warms and begins to release gases when passing close to the Sun , a process called outgassing . This produces an extended, gravitationally unbound atmosphere or coma surrounding

795-478: A collision between two objects in the Solar System. Other splitting comets include 3D/Biela in 1846 and 73P/Schwassmann–Wachmann from 1995 to 2006. Greek historian Ephorus reported that a comet split apart as far back as the winter of 372–373 BC. Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact. Comets 42P/Neujmin and 53P/Van Biesbroeck appear to be fragments of

954-727: A comet is known as the nucleus. Cometary nuclei are composed of an amalgamation of rock , dust , water ice , and frozen carbon dioxide , carbon monoxide , methane , and ammonia . As such, they are popularly described as "dirty snowballs" after Fred Whipple 's model. Comets with a higher dust content have been called "icy dirtballs". The term "icy dirtballs" arose after observation of Comet 9P/Tempel 1 collision with an "impactor" probe sent by NASA Deep Impact mission in July 2005. Research conducted in 2014 suggests that comets are like " deep fried ice cream ", in that their surfaces are formed of dense crystalline ice mixed with organic compounds , while

1113-433: A comet nucleus. Infrared imaging of Hartley 2 shows such jets exiting and carrying with it dust grains into the coma. Most comets are small Solar System bodies with elongated elliptical orbits that take them close to the Sun for a part of their orbit and then out into the further reaches of the Solar System for the remainder. Comets are often classified according to the length of their orbital periods : The longer

1272-541: A comet was seen or not. Using Edmond Halley 's records of comet sightings, however, William Whiston in 1711 wrote that the Great Comet of 1680 had a periodicity of 574 years and was responsible for the worldwide flood in the Book of Genesis , by pouring water on Earth. His announcement revived for another century fear of comets, now as direct threats to the world instead of signs of disasters. Spectroscopic analysis in 1910 found

1431-677: A crater on Comet Tempel 1 to study its interior, and the European Space Agency's Rosetta , which became the first to land a robotic spacecraft on a comet. The word comet derives from the Old English cometa from the Latin comēta or comētēs . That, in turn, is a romanization of the Greek κομήτης 'wearing long hair', and the Oxford English Dictionary notes that

1590-404: A distance of 1.315  au . As it passed perihelion on April 1, 1997, the comet developed into a spectacular sight. It shone brighter than any star in the sky except Sirius , and its dust tail stretched 40–45 degrees across the sky. The comet was visible well before the sky got fully dark each night, and while many great comets are very close to the Sun as they pass perihelion, comet Hale–Bopp

1749-493: A distance of about 180 km. This analysis was confirmed by observations in 1996 using Wide-Field Planetary Camera 2 of the Hubble Space Telescope which had taken images of the comet that revealed the satellite. Although observations using adaptive optics in late 1997 and early 1998 showed a double peak in the brightness of the nucleus, controversy still exists over whether such observations can only be explained by

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1908-519: A doughnut-shaped inner cloud, the Hills cloud, of 2,000–20,000 AU (0.03–0.32 ly). The outer cloud is only weakly bound to the Sun and supplies the long-period (and possibly Halley-type) comets that fall to inside the orbit of Neptune . The inner Oort cloud is also known as the Hills cloud, named after Jack G. Hills , who proposed its existence in 1981. Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as

2067-402: A faint sodium tail, only visible with powerful instruments with dedicated filters. Sodium emission had been previously observed in other comets, but had not been shown to come from a tail. Hale–Bopp's sodium tail consisted of neutral atoms (not ions ), and extended to some 50 million kilometres in length. The source of the sodium appeared to be the inner coma , although not necessarily

2226-779: A few genuinely hyperbolic (i.e. non-periodic) trajectories, but no more than could be accounted for by perturbations from Jupiter. Comets from interstellar space are moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of km per second). When such objects enter the Solar System, they have a positive specific orbital energy resulting in a positive velocity at infinity ( v ∞ {\displaystyle v_{\infty }\!} ) and have notably hyperbolic trajectories. A rough calculation shows that there might be four hyperbolic comets per century within Jupiter's orbit, give or take one and perhaps two orders of magnitude . The Oort cloud

2385-606: A group consisting of professional astronomers and citizen scientists in light curves recorded by the Kepler Space Telescope. After Kepler Space Telescope retired in October 2018, a new telescope called TESS Telescope has taken over Kepler's mission. Since the launch of TESS, astronomers have discovered the transits of comets around the star Beta Pictoris using a light curve from TESS. Since TESS has taken over, astronomers have since been able to better distinguish exocomets with

2544-466: A hyperbolic orbit (e > 1) when near perihelion that using a heliocentric unperturbed two-body best-fit suggests they may escape the Solar System. As of 2022 , only two objects have been discovered with an eccentricity significantly greater than one: 1I/ʻOumuamua and 2I/Borisov , indicating an origin outside the Solar System. While ʻOumuamua, with an eccentricity of about 1.2, showed no optical signs of cometary activity during its passage through

2703-515: A lifetime of about 10,000 years or ~1,000 orbits whereas long-period comets fade much faster. Only 10% of the long-period comets survive more than 50 passages to small perihelion and only 1% of them survive more than 2,000 passages. Eventually most of the volatile material contained in a comet nucleus evaporates, and the comet becomes a small, dark, inert lump of rock or rubble that can resemble an asteroid. Some asteroids in elliptical orbits are now identified as extinct comets. Roughly six percent of

2862-584: A mechanism for processing citizen inquiries and complaints. The U.S. Green Building Council (USGBC) has also incorporated a credit for reducing the amount of light trespass and sky glow into their environmentally friendly building standard known as LEED . Light trespass can be reduced by selecting light fixtures that limit the amount of light emitted more than 80° above the nadir. The IESNA definitions include full cutoff (0%), cutoff (10%), and semi-cutoff (20%). (These definitions also include limits on light emitted above 90° to reduce sky glow.) Over-illumination

3021-555: A meeting in 2009 on Circadian Disruption and Cancer. In different wavelengths of light, red light has the least inhibitory effect on melatonin. In June 2009, the American Medical Association developed a policy in support of control of light pollution. News about the decision emphasized glare as a public health hazard leading to unsafe driving conditions. Especially in the elderly, glare produces loss of contrast, obscuring night vision. A new 2021 study published in

3180-456: A paper was published that hypothesised the existence of a binary nucleus to fully explain the observed pattern of comet Hale–Bopp's dust emission observed in October 1995. The paper was based on theoretical analysis, and did not claim an observational detection of the proposed satellite nucleus, but estimated that it would have a diameter of about 30 km, with the main nucleus being about 70 km across, and would orbit in about three days at

3339-582: A parent comet. Numerical integrations have shown that both comets had a rather close approach to Jupiter in January 1850, and that, before 1850, the two orbits were nearly identical. Another group of comets that is the result of fragmentation episodes is the Liller comet family made of C/1988 A1 (Liller), C/1996 Q1 (Tabur), C/2015 F3 (SWAN), C/2019 Y1 (ATLAS), and C/2023 V5 (Leonard) . Some comets have been observed to break up during their perihelion passage, including great comets West and Ikeya–Seki . Biela's Comet

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3498-432: A periodic orbit (that is, all short-period comets plus all long-period comets), whereas others use it to mean exclusively short-period comets. Similarly, although the literal meaning of "non-periodic comet" is the same as "single-apparition comet", some use it to mean all comets that are not "periodic" in the second sense (that is, to include all comets with a period greater than 200 years). Early observations have revealed

3657-402: A very low albedo , making them among the least reflective objects found in the Solar System. The Giotto space probe found that the nucleus of Halley's Comet (1P/Halley) reflects about four percent of the light that falls on it, and Deep Space 1 discovered that Comet Borrelly 's surface reflects less than 3.0%; by comparison, asphalt reflects seven percent. The dark surface material of

3816-476: A very small fraction of the total potential comet population, as the reservoir of comet-like bodies in the outer Solar System (in the Oort cloud ) is about one trillion. Roughly one comet per year is visible to the naked eye , though many of those are faint and unspectacular. Particularly bright examples are called " great comets ". Comets have been visited by uncrewed probes such as NASA's Deep Impact , which blasted

3975-533: A very young A-type main-sequence star , in 1987. A total of 11 such exocomet systems have been identified as of 2013 , using the absorption spectrum caused by the large clouds of gas emitted by comets when passing close to their star. For ten years the Kepler space telescope was responsible for searching for planets and other forms outside of the solar system. The first transiting exocomets were found in February 2018 by

4134-446: Is a hormone that regulates photoperiodic physiology and behaviour. Some species of frogs and salamanders utilize a light-dependent "compass" to orient their migratory behaviour to breeding sites. Introduced light can also cause developmental irregularities, such as retinal damage, reduced juvenile growth, premature metamorphosis, reduced sperm production, and genetic mutation. Close to global coastal megacities (e.g. Tokyo, Shanghai),

4293-484: Is a little beyond the orbit of Neptune . Comets whose aphelia are near a major planet's orbit are called its "family". Such families are thought to arise from the planet capturing formerly long-period comets into shorter orbits. At the shorter orbital period extreme, Encke's Comet has an orbit that does not reach the orbit of Jupiter, and is known as an Encke-type comet . Short-period comets with orbital periods less than 20 years and low inclinations (up to 30 degrees) to

4452-523: Is a nine-level measuring system used to track how much light pollution there is in the sky. A Bortle scale of four or less is required to see the Milky Way whilst one is "pristine", the darkest possible. Inspection of the area surrounding Madrid reveals that the effects of light pollution caused by a single large conglomeration can be felt up to 100 km (62 mi) away from the center. Global effects of light pollution are also made obvious. Research in

4611-418: Is a public-health hazard—especially the older you become. Glare light scattering in the eye causes loss of contrast and leads to unsafe driving conditions, much like the glare on a dirty windshield from low-angle sunlight or the high beams from an oncoming car." In essence bright and/or badly shielded lights around roads can partially blind drivers or pedestrians and contribute to accidents. The blinding effect

4770-592: Is also used to refer to artificial light indoors. Adverse consequences are multiple; some of them may not be known yet. Light pollution competes with starlight in the night sky for urban residents, interferes with astronomical observatories , and, like any other form of pollution , disrupts ecosystems and has adverse health effects. Light pollution is a side-effect of industrial civilization. Its sources include building exterior and interior lighting, advertising, outdoor area lighting (such as car parks), offices, factories, streetlights, and illuminated sporting venues. It

4929-416: Is bound to the Sun with roughly a 92,600-year orbit because the eccentricity drops below 1 as it moves farther from the Sun. The future orbit of a long-period comet is properly obtained when the osculating orbit is computed at an epoch after leaving the planetary region and is calculated with respect to the center of mass of the Solar System . By definition long-period comets remain gravitationally bound to

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5088-449: Is caused by inefficient or unnecessary use of artificial light. Specific categories of light pollution include light trespass, over-illumination, glare, light clutter, and skyglow. A single offending light source often falls into more than one of these categories. Solutions to light pollution are often easy steps like adjusting light fixtures or using more appropriate light bulbs. Further remediation can be done with more efforts to educate

5247-417: Is caused in large part by reduced contrast due to light scattering in the eye by excessive brightness, or to the reflection of light from dark areas in the field of vision, with luminance similar to the background luminance. This kind of glare is a particular instance of disability glare, called veiling glare. (This is not the same as loss of accommodation of night vision which is caused by the direct effect of

5406-562: Is difficult. The nucleus of 322P/SOHO is probably only 100–200 meters (330–660 ft) in diameter. A lack of smaller comets being detected despite the increased sensitivity of instruments has led some to suggest that there is a real lack of comets smaller than 100 meters (330 ft) across. Known comets have been estimated to have an average density of 0.6 g/cm (0.35 oz/cu in). Because of their low mass, comet nuclei do not become spherical under their own gravity and therefore have irregular shapes. Roughly six percent of

5565-404: Is followed by its de-excitation into the ground state of the ion by the emission of X-rays and far ultraviolet photons. Bow shocks form as a result of the interaction between the solar wind and the cometary ionosphere, which is created by the ionization of gases in the coma. As the comet approaches the Sun, increasing outgassing rates cause the coma to expand, and the sunlight ionizes gases in

5724-407: Is formed as a result of the ionization by solar ultra-violet radiation of particles in the coma. Once the particles have been ionized, they attain a net positive electrical charge, which in turn gives rise to an "induced magnetosphere " around the comet. The comet and its induced magnetic field form an obstacle to outward flowing solar wind particles. Because the relative orbital speed of the comet and

5883-469: Is most severe in highly industrialized, densely populated areas of North America, Europe, and Asia and in major cities in the Middle East and North Africa like Tehran and Cairo , but even relatively small amounts of light can be noticed and create problems. Awareness of the harmful effects of light pollution began in the second half of the 19th century, but efforts to address its effects did not begin until

6042-400: Is natural. To precisely measure how bright the sky gets, night time satellite imagery of the earth is used as raw input for the number and intensity of light sources. These are put into a physical model of scattering due to air molecules and aerosoles to calculate cumulative sky brightness. Maps that show the enhanced sky brightness have been prepared for the entire world. The Bortle scale

6201-519: Is no replacement pollinator that would not be affected by the artificial light. This can lead to species decline of plants that are unable to reproduce, and change an area's longterm ecology . Among nocturnal insects, fireflies ( Coleoptera : Lampyridae, Phengodidae and Elateridae) are especially interesting study objects for light pollution, once they depend on their own light to reproduce and, consequently, are very sensitive to environmental levels of light. Fireflies are well known and interesting to

6360-574: Is not excessive in all developed countries. Amongst developed countries there are large variations in patterns of light use. American cities emit three to five times more light to space per capita compared to German cities. Over-illumination stems from several factors: Most of these issues can be readily corrected with available, inexpensive technology , and with the resolution of landlord/tenant practices that create barriers to rapid correction of these matters. Most importantly, public awareness would need to improve for industrialized countries to realize

6519-539: Is orchestrated by the natural diurnal patterns of light and dark, so disruption to these patterns impacts the ecological dynamics. Many species of marine plankton, such as Calanus copepods, can detect light levels as low as 0.1 μWm ; using this as a threshold a global atlas of marine Artificial Light at Night has been generated, showing its global widespread nature. Studies suggest that light pollution around lakes prevents zooplankton, such as Daphnia , from eating surface algae , causing algal blooms that can kill off

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6678-587: Is the excessive and unnecessary use of light. A large and overabundant amount of electricity is required to support light consumption in the United States. U.S homes consumed 81 billion kilowatthours (kWh) of electricity for lighting in 2020 according to the U.S. Energy Information Administration's (EIA). Additionally, the EIA reported that 208 billion kWh and 53 billion kWh of electricity were used for commercial and manufacturing buildings respectively in 2018. Light use

6837-418: Is the result of solar radiation getting scattered by the dust particles in the coma of the comet and depends on the nature of the grains. It further confirms that the dust grains in the coma of comet Hale–Bopp were smaller than inferred in any other comet. One of the most remarkable discoveries was that the comet had a third type of tail. In addition to the well-known gas and dust tails, Hale–Bopp also exhibited

6996-562: Is thought to occupy a vast space starting from between 2,000 and 5,000 AU (0.03 and 0.08 ly) to as far as 50,000 AU (0.79 ly) from the Sun. This cloud encases the celestial bodies that start at the middle of the Solar System—the Sun, all the way to outer limits of the Kuiper Belt. The Oort cloud consists of viable materials necessary for the creation of celestial bodies. The Solar System's planets exist only because of

7155-452: Is typical of all comets, this implies that although cometary impacts are thought to be the source of a significant amount of the water on Earth , they cannot be the only source. Deuterium was also detected in many other hydrogen compounds in the comet. The ratio of deuterium to normal hydrogen was found to vary from compound to compound, which astronomers believe suggests that cometary ices were formed in interstellar clouds , rather than in

7314-405: Is usually associated with very high-temperature bodies . The X-rays are generated by the interaction between comets and the solar wind: when highly charged solar wind ions fly through a cometary atmosphere, they collide with cometary atoms and molecules, "stealing" one or more electrons from the atom in a process called "charge exchange". This exchange or transfer of an electron to the solar wind ion

7473-550: The ALICE spectrograph on Rosetta determined that electrons (within 1 km (0.62 mi) above the comet nucleus ) produced from photoionization of water molecules by solar radiation , and not photons from the Sun as thought earlier, are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma. Instruments on the Philae lander found at least sixteen organic compounds at

7632-476: The IAU , fear that light pollution will increase significantly, one of many concerns reported in the media regarding satellite overcrowding. Public discourse surrounding the continuing deployment of satellite constellations includes multiple petitions by astronomers and citizen scientists, and has raised questions about which regulatory bodies hold jurisdiction over human actions that obscure starlight. Measuring

7791-489: The International Dark-Sky Association has developed a set of model lighting ordinances. The Dark-Sky Association was started to reduce the light going up into the sky which reduces the visibility of stars (see Skyglow below). This is any light that is emitted more than 90° above nadir . By limiting light at this 90° mark they have also reduced the light output in the 80–90° range which creates most of

7950-462: The Kreutz sungrazers . It would also be extremely bright, due to a combination of closeness to the Sun and nuclei size, potentially exceeding Halley's Comet in 837 AD. Due to the massive size of its nucleus, Comet Hale–Bopp was observed intensively by astronomers during its perihelion passage, and several important advances in cometary science resulted from these observations. The dust production rate of

8109-536: The Kuiper belt or its associated scattered disc , which lie beyond the orbit of Neptune . Long-period comets are thought to originate in the Oort cloud , a spherical cloud of icy bodies extending from outside the Kuiper belt to halfway to the nearest star. Long-period comets are set in motion towards the Sun by gravitational perturbations from passing stars and the galactic tide . Hyperbolic comets may pass once through

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8268-457: The Milky Way galaxy. Light trespass occurs when unwanted light enters one's property, for instance, by shining over a neighbour's fence. A common light trespass problem occurs when a strong light enters the window of one's home from the outside , causing problems such as sleep deprivation . A number of cities in the U.S. have developed standards for outdoor lighting to protect the rights of their citizens against light trespass. To assist them,

8427-489: The Solar cycle ) but given optimal conditions, the darkest possible sky has a brightness of about 22 magnitude/square arc second. If a full moon is present, the sky brightness increases to about 18 magnitude/sq. arcsecond depending on local atmospheric transparency, 40 times brighter than the darkest sky. In densely populated areas a sky brightness of 17 magnitude/sq. an arcsecond is not uncommon, or as much as 100 times brighter than

8586-807: The UK Schmidt Telescope in 1993 was found to show the then-unnoticed comet some 13 au from the Sun, a distance at which most comets are essentially unobservable. ( Halley's Comet was more than 100 times fainter at the same distance from the Sun.) Analysis indicated later that its comet nucleus was 60±20 kilometres in diameter, approximately six times the size of Halley's Comet . Its great distance and surprising activity indicated that comet Hale–Bopp might become very bright when it reached perihelion in 1997. However, comet scientists were wary – comets can be extremely unpredictable, and many have large outbursts at great distances only to diminish in brightness later. Comet Kohoutek in 1973 had been touted as

8745-464: The asteroid belt . Because their elliptical orbits frequently take them close to the giant planets, comets are subject to further gravitational perturbations . Short-period comets have a tendency for their aphelia to coincide with a giant planet 's semi-major axis, with the JFCs being the largest group. It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by

8904-460: The ionosphere of the planet Venus streams outwards in a manner similar to the ion tail seen streaming from a comet under similar conditions." Uneven heating can cause newly generated gases to break out of a weak spot on the surface of comet's nucleus, like a geyser. These streams of gas and dust can cause the nucleus to spin, and even split apart. In 2010 it was revealed dry ice (frozen carbon dioxide) can power jets of material flowing out of

9063-455: The near-Earth asteroids are thought to be the extinct nuclei of comets that no longer experience outgassing, including 14827 Hypnos and 3552 Don Quixote . Results from the Rosetta and Philae spacecraft show that the nucleus of 67P/Churyumov–Gerasimenko has no magnetic field, which suggests that magnetism may not have played a role in the early formation of planetesimals . Further,

9222-418: The nucleus . There are several possible mechanisms for generating a source of sodium atoms, including collisions between dust grains surrounding the nucleus, and "sputtering" of sodium from dust grains by ultraviolet light. It is not yet established which mechanism is primarily responsible for creating Hale–Bopp's sodium tail, and the narrow and diffuse components of the tail may have different origins. While

9381-420: The solar nebula . Theoretical modelling of ice formation in interstellar clouds suggests that comet Hale–Bopp formed at temperatures of around 25–45  kelvin . Spectroscopic observations of Hale–Bopp revealed the presence of many organic chemicals , several of which had never been detected in comets before. These complex molecules may exist within the cometary nucleus, or might be synthesised by reactions in

9540-404: The 1950s. In the 1980s a global dark-sky movement emerged with the founding of the International Dark-Sky Association (IDA). There are now such educational and advocacy organizations in many countries worldwide. About 83% of people, including 99% of Europeans and Americans, live under light-polluted skies that are more than 10% brighter than natural darkness. 80% of North Americans cannot see

9699-426: The 1982 perihelion passage, but a 1980 encounter with Jupiter accelerated the comet giving it the largest eccentricity (1.057) of any known solar comet with a reasonable observation arc. Comets not expected to return to the inner Solar System include C/1980 E1 , C/2000 U5 , C/2001 Q4 (NEAT) , C/2009 R1 , C/1956 R1 , and C/2007 F1 (LONEOS). Some authorities use the term "periodic comet" to refer to any comet with

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9858-484: The Central Bureau for Astronomical Telegrams through a Western Union telegram. Brian G. Marsden , who had run the bureau since 1968, laughed, "Nobody sends telegrams anymore. I mean, by the time that telegram got here, Alan Hale had already e-mailed us three times with updated coordinates." The following morning, it was confirmed that this was a new comet, and it was given the designation C/1995 O1. The discovery

10017-534: The CfDS handbook "Blinded by the Light?". Chapter 4, "Human health implications of light pollution" states that "...   light intrusion, even if dim, is likely to have measurable effects on sleep disruption and melatonin suppression. Even if these effects are relatively small from night to night, continuous chronic circadian, sleep and hormonal disruption may have longer-term health risks". The New York Academy of Sciences hosted

10176-514: The Solar System, such as Jupiter. An example of this is Comet C/1980 E1 , which was shifted from an orbit of 7.1 million years around the Sun, to a hyperbolic trajectory, after a 1980 close pass by the planet Jupiter. Interstellar comets such as 1I/ʻOumuamua and 2I/Borisov never orbited the Sun and therefore do not require a 3rd-body interaction to be ejected from the Solar System. Jupiter-family comets and long-period comets appear to follow very different fading laws. The JFCs are active over

10335-507: The Southern Economic Journal indicates that light pollution may increase by 13% in preterm births before 23 weeks of gestation. While light at night can be beneficial, neutral, or damaging for individual species , its presence invariably disturbs ecosystems. For example, some species of spiders avoid lit areas, while other species are happy to build their webs directly on lamp posts. Since lamp posts attract many flying insects,

10494-416: The Sun because this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory. On occasions—such as when Earth passes through a comet's orbital plane, the antitail , pointing in the opposite direction to the ion and dust tails, may be seen. The observation of antitails contributed significantly to the discovery of solar wind. The ion tail

10653-400: The Sun discovered by amateurs, with the largest well-measured cometary nucleus known after 95P/Chiron , and it was visible to the naked eye for twice as long as the previous record-holder. It was also brighter than magnitude 0 for eight weeks, longer than any other recorded comet. Carolyn Shoemaker and her husband Gene , co-discoverers of comet Shoemaker–Levy 9 , were involved in

10812-455: The Sun, outgassing of its icy components releases solid debris too large to be swept away by radiation pressure and the solar wind. If Earth's orbit sends it through that trail of debris, which is composed mostly of fine grains of rocky material, there is likely to be a meteor shower as Earth passes through. Denser trails of debris produce quick but intense meteor showers and less dense trails create longer but less intense showers. Typically,

10971-410: The Sun. The coma is generally made of water and dust, with water making up to 90% of the volatiles that outflow from the nucleus when the comet is within 3 to 4 astronomical units (450,000,000 to 600,000,000 km; 280,000,000 to 370,000,000 mi) of the Sun. The H 2 O parent molecule is destroyed primarily through photodissociation and to a much smaller extent photoionization , with

11130-459: The Sun. The comet likely made its previous perihelion approximately 4,200 years ago, roughly the year 2215 BC. The estimated closest approach to Earth was 1.4 au, and it may have been observed in ancient Egypt during the 6th dynasty reign of the Pharaoh Pepi II (Reign: 2247 – c. 2216 BC). Pepi's pyramid at Saqqara contains a text referring to an "nhh-star" as a companion of

11289-407: The Sun. At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, and in doing so enlarging the tail. Ion tails have been observed to extend one astronomical unit (150 million km) or more. Both the coma and tail are illuminated by the Sun and may become visible when a comet passes through the inner Solar System, the dust reflects sunlight directly while

11448-412: The Sun. For example, 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 had a coma roughly the diameter of the Sun. Even though the coma can become quite large, its size can decrease about the time it crosses the orbit of Mars around 1.5 astronomical units (220,000,000 km; 140,000,000 mi) from

11607-676: The Sun; those comets that are ejected from the Solar System due to close passes by major planets are no longer properly considered as having "periods". The orbits of long-period comets take them far beyond the outer planets at aphelia, and the plane of their orbits need not lie near the ecliptic. Long-period comets such as C/1999 F1 and C/2017 T2 (PANSTARRS) can have aphelion distances of nearly 70,000 AU (0.34 pc; 1.1 ly) with orbital periods estimated around 6 million years. Single-apparition or non-periodic comets are similar to long-period comets because they have parabolic or slightly hyperbolic trajectories when near perihelion in

11766-448: The U.S. Fish and Wildlife Service of the number of birds killed after being attracted to tall towers range from four to five million per year to an order of magnitude higher. The Fatal Light Awareness Program (FLAP) works with building owners in Toronto , Ontario , Canada and other cities to reduce mortality of birds by turning out lights during migration periods. Another study has found that

11925-517: The U.S. Sky quality in the U.S. ranges from pristine ( Capitol Reef National Park and Big Bend National Park ) to severely degraded ( Santa Monica Mountains National Recreation Area and Biscayne National Park ). The National Park Service Night Sky Program monitoring database is available online (2015). Light pollution in Hong Kong was declared the 'worst on the planet' in March 2013. In June 2016, it

12084-456: The alleged photo was an altered copy of one of their own comet images. Thirty-nine members of the Heaven's Gate cult died in a mass suicide , in March 1997 with the intention of teleporting to a spaceship which they believed was flying behind the comet. Nancy Lieder, who claims to receive messages from aliens through an implant in her brain, stated that Hale–Bopp was a fiction designed to distract

12243-428: The atmosphere The effect of sky glow can be harmful in astronomy and on the health of many organisms. It worsens the visibility of the stars, the Milky Way , and significantly increases the natural light levels at night. Also contributing to light pollution are artificial satellites . With increasing numbers of satellite constellations such as OneWeb and Starlink , members of the astronomical community, notably

12402-686: The blue band. In North America the situation is comparable. There is a significant problem with light pollution ranging from the Canadian Maritime Provinces to the American Southwest. The International Dark-Sky Association works to designate areas that have high-quality night skies. These areas are supported by communities and organizations that are dedicated to reducing light pollution (e.g. Dark-sky preserve ). The National Park Service Natural Sounds and Night Skies Division has measured night sky quality in national park units across

12561-431: The bow shocks already were fully developed. The Rosetta spacecraft observed the bow shock at comet 67P/Churyumov–Gerasimenko at an early stage of bow shock development when the outgassing increased during the comet's journey toward the Sun. This young bow shock was called the "infant bow shock". The infant bow shock is asymmetric and, relative to the distance to the nucleus, wider than fully developed bow shocks. In

12720-427: The case of Oort cloud objects) may throw one of these bodies into an elliptical orbit that takes it inwards toward the Sun to form a visible comet. Unlike the return of periodic comets, whose orbits have been established by previous observations, the appearance of new comets by this mechanism is unpredictable. When flung into the orbit of the sun, and being continuously dragged towards it, tons of matter are stripped from

12879-439: The coma. When the solar wind passes through this ion coma, the bow shock appears. The first observations were made in the 1980s and 1990s as several spacecraft flew by comets 21P/Giacobini–Zinner , 1P/Halley, and 26P/Grigg–Skjellerup . It was then found that the bow shocks at comets are wider and more gradual than the sharp planetary bow shocks seen at, for example, Earth. These observations were all made near perihelion when

13038-493: The comet approached the Sun, it continued to brighten, shining at 2nd magnitude in February, and showing a growing pair of tails , the blue gas tail pointing straight away from the Sun and the yellowish dust tail curving away along its orbit. On March 9, a solar eclipse in China, Mongolia and eastern Siberia allowed observers there to see the comet in the daytime. Hale–Bopp had its closest approach to Earth on March 22, 1997, at

13197-423: The comet formed beyond Neptune in the Kuiper belt region and then migrated outward to the Oort cloud . Comet Hale–Bopp's activity and outgassing were not spread uniformly over its nucleus, but instead came from several specific jets. Observations of the material streaming away from these jets allowed astronomers to measure the rotation period of the comet, which was found to be about 11 hours 46 minutes. In 1997

13356-403: The comet had remained visible without aid for 569 days, or about 18 + 1 ⁄ 2 months. The previous record had been set by the Great Comet of 1811 , which was visible to the naked eye for about 9 months. The comet continued to fade as it receded, but was still tracked by astronomers. In October 2007, 10 years after the perihelion and at a distance of 25.7 au from the Sun, the comet

13515-482: The comet passed within 0.77 au of Jupiter , close enough for its orbit to be measurably affected by the planet's gravity . The comet's orbit was shortened considerably to a period of roughly 2,399 years, and it will next return to the inner Solar System around the year 4385. Its greatest distance from the Sun ( aphelion ) will be about 354 au, reduced from about 525 au. The estimated probability of Hale–Bopp's striking Earth in future passages through

13674-443: The comet was very high (up to 2.0 × 10 kg/s), which may have made the inner coma optically thick . Based on the properties of the dust grains – high temperature, high albedo and strong 10 μm silicate emission feature – the astronomers concluded the dust grains are smaller than observed in any other comet. Hale–Bopp showed the highest ever linear polarization detected for any comet. Such polarization

13833-441: The comet's dust tail roughly followed the path of the comet's orbit and the gas tail pointed almost directly away from the Sun, the sodium tail appeared to lie between the two. This implies that the sodium atoms are driven away from the comet's head by radiation pressure . The abundance of deuterium in comet Hale–Bopp in the form of heavy water was found to be about twice that of Earth's oceans. If Hale–Bopp's deuterium abundance

13992-410: The comet's surface, four of which ( acetamide , acetone , methyl isocyanate and propionaldehyde ) have been detected for the first time on a comet. The streams of dust and gas thus released form a huge and extremely thin atmosphere around the comet called the "coma". The force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous "tail" to form pointing away from

14151-410: The comet. Hale–Bopp was the first comet where the noble gas argon was detected. Noble gases are chemically inert and vary from low to high volatility . Since different noble elements have different sublimation temperatures, and don't interact with other elements, they can be used for probing the temperature histories of the cometary ices. Krypton has a sublimation temperature of 16–20 K and

14310-498: The comets which greatly influence their lifetime; the more stripped, the shorter they live and vice versa. Long-period comets have highly eccentric orbits and periods ranging from 200 years to thousands or even millions of years. An eccentricity greater than 1 when near perihelion does not necessarily mean that a comet will leave the Solar System. For example, Comet McNaught had a heliocentric osculating eccentricity of 1.000019 near its perihelion passage epoch in January 2007 but

14469-406: The day, because direction of natural polarization of sun light and its reflection is a source of information for a lot of animals). This form of pollution is named polarized light pollution (PLP). Unnatural polarized light sources can trigger maladaptive behaviors in polarization-sensitive taxa and alter ecological interactions. Lights on tall structures can disorient migrating birds. Estimates by

14628-606: The day, its effects are magnified during the night with the contrast of the sky's darkness. It has been estimated that 83 percent of the world's people live under light-polluted skies and that 23 percent of the world's land area is affected by skyglow . The area affected by artificial illumination continues to increase. A major side effect of urbanization , light pollution is blamed for compromising health, disrupting ecosystems, and spoiling aesthetic environments. Studies show that urban areas are more at risk. Globally, it has increased by at least 49% from 1992 to 2017. Light pollution

14787-476: The density of the debris trail is related to how long ago the parent comet released the material. The Perseid meteor shower , for example, occurs every year between 9 and 13 August, when Earth passes through the orbit of Comet Swift–Tuttle . Halley's Comet is the source of the Orionid shower in October. Many comets and asteroids collided with Earth in its early stages. Many scientists think that comets bombarding

14946-551: The development and trial of new lighting technologies in the North Sea. In early 2007, the lights were installed on the Shell production platform L15. The experiment proved a great success since the number of birds circling the platform declined by 50 to 90%. Birds migrate at night for several reasons. Save water from dehydration in hot day flying and part of the bird's navigation system works with stars in some way. With city light outshining

15105-573: The direct lighting effect by adding softening filters or other solutions, though intensity will be reduced. Glare can be categorized into different types. One such classification is described in a book by Bob Mizon, coordinator for the British Astronomical Association's Campaign for Dark Skies, as follows: According to Mario Motta, president of the Massachusetts Medical Society , "...   glare from bad lighting

15264-513: The discovery of main-belt comets and active centaur minor planets has blurred the distinction between asteroids and comets . In the early 21st century, the discovery of some minor bodies with long-period comet orbits, but characteristics of inner solar system asteroids, were called Manx comets . They are still classified as comets, such as C/2014 S3 (PANSTARRS). Twenty-seven Manx comets were found from 2013 to 2017. As of November 2021 , there are 4,584 known comets. However, this represents

15423-424: The ecliptic are called traditional Jupiter-family comets (JFCs). Those like Halley, with orbital periods of between 20 and 200 years and inclinations extending from zero to more than 90 degrees, are called Halley-type comets (HTCs). As of 2023 , 70 Encke-type comets, 100 HTCs, and 755 JFCs have been reported. Recently discovered main-belt comets form a distinct class, orbiting in more circular orbits within

15582-416: The effect of sky glow on a global scale is a complex procedure. The natural atmosphere is not completely dark, even in the absence of terrestrial sources of light and illumination from the Moon. This is caused by two main sources: airglow and scattered light . At high altitudes, primarily above the mesosphere , there is enough UV radiation from the sun at very short wavelengths to cause ionization . When

15741-504: The estimated energy of the K-T impact event . Over many orbits, the cumulative effect of gravitational perturbations on comets with high orbital inclinations and small perihelion distances is generally to reduce the perihelion distance to very small values. Hale–Bopp has about a 15% chance of eventually becoming a sungrazing comet through this process. If such is the case, it could undergo huge mass loss, or break up into smaller pieces like

15900-679: The existence of tektites and australites . Fear of comets as acts of God and signs of impending doom was highest in Europe from AD 1200 to 1650. The year after the Great Comet of 1618 , for example, Gotthard Arthusius published a pamphlet stating that it was a sign that the Day of Judgment was near. He listed ten pages of comet-related disasters, including "earthquakes, floods, changes in river courses, hail storms, hot and dry weather, poor harvests, epidemics, war and treason and high prices". By 1700 most scholars concluded that such events occurred whether

16059-486: The gases glow from ionisation . Most comets are too faint to be visible without the aid of a telescope , but a few each decade become bright enough to be visible to the naked eye. Occasionally a comet may experience a huge and sudden outburst of gas and dust, during which the size of the coma greatly increases for a period of time. This happened in 2007 to Comet Holmes . In 1996, comets were found to emit X-rays . This greatly surprised astronomers because X-ray emission

16218-521: The general public (unlike many other insects) and are easily spotted by non-experts, and, due to their sensibility and rapid response to environmental changes, good bioindicators for artificial night lighting. Significant declines in some insect populations have been suggested as being at least partially mediated by artificial lights at night. A 2009 study also suggests deleterious impacts on animals and ecosystems because of perturbation of polarized light or artificial polarization of light (even during

16377-433: The gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined. These perturbations can deflect long-period comets into shorter orbital periods. Based on their orbital characteristics, short-period comets are thought to originate from the centaurs and the Kuiper belt/ scattered disc —a disk of objects in

16536-407: The higher layers and thus no appreciable ionization occurs. Apart from emitting light, the sky also scatters incoming light, primarily from distant stars and the Milky Way , but also the zodiacal light , sunlight that is reflected and backscattered from interplanetary dust particles. The amount of airglow and zodiacal light is quite varied (depending, amongst other things on sunspot activity and

16695-563: The highest levels of ALAN reported the highest number of cases of breast cancer. Seoul, which had the highest levels of light pollution, had 34.4% more cases of breast cancer than Ganwon-do, which had the lowest levels of light pollution. This suggested a high correlation between ALAN and the prevalence of breast cancer. It was also found that there was no correlation between other types of cancer such as cervical or lung cancer and ALAN levels. A more recent discussion (2009), written by Professor Steven Lockley, Harvard Medical School, can be found in

16854-992: The human body suggests that a variety of adverse health effects may be caused by light pollution or excessive light exposure, and some lighting design textbooks use human health as an explicit criterion for proper interior lighting. Health effects of over-illumination or improper spectral composition of light may include: increased headache incidence, worker fatigue , medically defined stress , decrease in sexual function and increase in anxiety. Likewise, animal models have been studied demonstrating unavoidable light to produce adverse effect on mood and anxiety. For those who need to be awake at night, light at night also has an acute effect on alertness and mood. Outdoor artificial light at night – exposure to contemporary types such as current types of street lighting – has been linked to risks for obesity, mental disorders, diabetes , and potentially other health issues by preliminary studies. In 2007, "shift work that involves circadian disruption"

17013-431: The inner Solar System before being flung to interstellar space. The appearance of a comet is called an apparition. 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. However,

17172-405: The inner Solar System in October 2017, changes to its trajectory—which suggests outgassing —indicate that it is probably a comet. On the other hand, 2I/Borisov, with an estimated eccentricity of about 3.36, has been observed to have the coma feature of comets, and is considered the first detected interstellar comet . Comet C/1980 E1 had an orbital period of roughly 7.1 million years before

17331-457: The inner Solar System is remote, about 2.5×10 per orbit. However, given that the comet nucleus is around 60 km in diameter, the consequences of such an impact would be apocalyptic. Weissman conservatively estimates the diameter at 35 km; an estimated density of 0.6 g/cm then gives a cometary mass of 1.3×10 g. At a probable impact velocity of 52.5 km/s, impact energy can be calculated as 1.9×10 ergs , or 4.4×10 megatons , about 44 times

17490-429: The inner Solar System. However, gravitational perturbations from giant planets cause their orbits to change. Single-apparition comets have a hyperbolic or parabolic osculating orbit which allows them to permanently exit the Solar System after a single pass of the Sun. The Sun's Hill sphere has an unstable maximum boundary of 230,000 AU (1.1 pc; 3.6 ly). Only a few hundred comets have been seen to reach

17649-429: The interior ice is colder and less dense. The surface of the nucleus is generally dry, dusty or rocky, suggesting that the ices are hidden beneath a surface crust several metres thick. The nuclei contains a variety of organic compounds, which may include methanol , hydrogen cyanide , formaldehyde , ethanol , ethane , and perhaps more complex molecules such as long-chain hydrocarbons and amino acids . In 2009, it

17808-472: The ions collide with electrically neutral particles they recombine and emit photons in the process, causing airglow . The degree of ionization is sufficiently large to allow a constant emission of radiation even during the night when the upper atmosphere is in the Earth's shadow. Lower in the atmosphere all the solar photons with energies above the ionization potential of N 2 and O 2 have already been absorbed by

17967-424: The lakes' plants and lower water quality. Light pollution may also affect ecosystems in other ways. For example, entomologists have documented that nighttime light may interfere with the ability of moths and other nocturnal insects to navigate. It can also negative impact on insect development and reproduction. Night-blooming flowers that depend on moths for pollination may be affected by night lighting, as there

18126-424: The large payoff in reducing over-illumination. In certain cases, an over-illumination lighting technique may be needed. For example, indirect lighting is often used to obtain a "softer" look, since hard direct lighting is generally found less desirable for certain surfaces, such as skin. The indirect lighting method is perceived as cozier and suits bars, restaurants, and living quarters. It is also possible to block

18285-444: The late 1990s showed that the entire area consisting of southern England, Netherlands, Belgium, West Germany, and northern France have a sky brightness of at least two to four times normal. The only places in continental Europe where the sky can attain its natural darkness are in northern Scandinavia and in islands far from the continent. The growth of light pollution on the green band has been 11% from 2012–2013 to 2014–2020, and 24% on

18444-407: The light itself on the eye.) Light clutter refers to excessive groupings of lights. Groupings of lights may generate confusion, distract from obstacles (including those that they may be intended to illuminate), and potentially cause accidents. Clutter is particularly noticeable on roads where the street lights are badly designed, or where brightly lit advertisements surround the roadways. Depending on

18603-533: The light trespass issues. U.S. federal agencies may also enforce standards and process complaints within their areas of jurisdiction. For instance, in the case of light trespass by white strobe lighting from communication towers in excess of FAA minimum lighting requirements the Federal Communications Commission maintains an Antenna Structure Registration database information which citizens may use to identify offending structures and provides

18762-456: The lights produced by the Post Tower has affected 25 bird species. As a result, they discovered that decreasing the use of excessive lights increased the survival rate of bird species. Similar disorientation has also been noted for bird species migrating close to offshore production and drilling facilities. Studies carried out by Nederlandse Aardolie Maatschappij b.v. (NAM) and Shell have led to

18921-425: The media meant that Hale–Bopp was probably the most-observed comet in history, making a far greater impact on the general public than the return of Halley's Comet in 1986, and certainly seen by a greater number of people than witnessed any of Halley's previous appearances. For instance, 69% of Americans had seen Hale–Bopp by April 9, 1997. Hale–Bopp was a record-breaking comet – the farthest comet from

19080-431: The motives of the person or organization that installed the lights, their placement and design can even be intended to distract drivers, and can contribute to accidents. Sky glow is the bright haze above cities that is produced from excessive artificial lighting at night. This type of light pollution is created from artificial light reflecting in the sky and bouncing around the different types of particles that reside in

19239-517: The natural illumination cycles provided by the moon in the marine environment are considerably disrupted by light pollution, with only nights around the full moon providing greater radiances, and over a given month lunar dosages may be a factor of 6 less than light pollution dosages. In September 2009, the 9th European Dark-Sky Symposium in Armagh, Northern Ireland had a session on the environmental effects of light at night (LAN). It dealt with bats, turtles,

19398-468: The near-Earth asteroids are thought to be extinct comet nuclei. The nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart. A significant cometary disruption was that of Comet Shoemaker–Levy 9 , which was discovered in 1993. A close encounter in July 1992 had broken it into pieces, and over a period of six days in July 1994, these pieces fell into Jupiter's atmosphere—the first time astronomers had observed

19557-802: The night sky, birds (and also about mammals) no longer navigate by stars. Sea turtle hatchlings emerging from nests on beaches are another casualty of light pollution. It is a common misconception that hatchling sea turtles are attracted to the moon. Rather, they find the ocean by moving away from the dark silhouette of dunes and their vegetation, a behavior with which artificial lights interfere. The breeding activity and reproductive phenology of toads, however, are cued by moonlight. Juvenile seabirds are also disoriented by lights as they leave their nests and fly out to sea, causing events of high mortality. Amphibians and reptiles are also affected by light pollution. Introduced light sources during normally dark periods can disrupt levels of melatonin production. Melatonin

19716-437: The nucleus may consist of complex organic compounds. Solar heating drives off lighter volatile compounds , leaving behind larger organic compounds that tend to be very dark, like tar or crude oil . The low reflectivity of cometary surfaces causes them to absorb the heat that drives their outgassing processes. Comet nuclei with radii of up to 30 kilometers (19 mi) have been observed, but ascertaining their exact size

19875-429: The nucleus, and sometimes a tail of gas and dust gas blown out from the coma. These phenomena are due to the effects of solar radiation and the outstreaming solar wind plasma acting upon the nucleus of the comet. Comet nuclei range from a few hundred meters to tens of kilometers across and are composed of loose collections of ice, dust, and small rocky particles. The coma may be up to 15 times Earth's diameter, while

20034-458: The object was simply the 8.5-magnitude star SAO141894. They noted that the star did not appear on Shramek's computer program because the user preferences were set incorrectly. Art Bell claimed to have obtained an image of the object from an anonymous astrophysicist who was about to confirm its discovery. However, astronomers Olivier Hainaut and David Tholen of the University of Hawaii stated that

20193-403: The outer Solar System , comets remain frozen and inactive and are extremely difficult or impossible to detect from Earth due to their small size. Statistical detections of inactive comet nuclei in the Kuiper belt have been reported from observations by the Hubble Space Telescope but these detections have been questioned. As a comet approaches the inner Solar System, solar radiation causes

20352-460: The outer halo; it is seen as a possible source of new comets that resupply the relatively tenuous outer cloud as the latter's numbers are gradually depleted. The Hills cloud explains the continued existence of the Oort cloud after billions of years. Exocomets beyond the Solar System have been detected and may be common in the Milky Way . The first exocomet system detected was around Beta Pictoris ,

20511-399: The outgassings of comet 67P, suggesting that the molecule may occur more often than had been thought, and thus less an indicator of life as has been supposed. It is suspected that comet impacts have, over long timescales, delivered significant quantities of water to Earth's Moon , some of which may have survived as lunar ice . Comet and meteoroid impacts are thought to be responsible for

20670-413: The period the more elongated the ellipse. Periodic comets or short-period comets are generally defined as those having orbital periods of less than 200 years. They usually orbit more-or-less in the ecliptic plane in the same direction as the planets. Their orbits typically take them out to the region of the outer planets ( Jupiter and beyond) at aphelion ; for example, the aphelion of Halley's Comet

20829-516: The pharaoh in the heavens, where " nhh " is the hieroglyph for long hair. Hale–Bopp may have had a near collision with Jupiter in 2215 BC, which probably caused a dramatic change in its orbit, and 2215 BC may have been its first passage through the inner Solar System from the Oort cloud . The comet's current orbit is almost perpendicular to the plane of the ecliptic , so further close approaches to planets will be rare. However, in April 1996

20988-498: The planetesimals (chunks of leftover space that assisted in the creation of planets) that were condensed and formed by the gravity of the Sun. The eccentric made from these trapped planetesimals is why the Oort Cloud even exists. Some estimates place the outer edge at between 100,000 and 200,000 AU (1.58 and 3.16 ly). The region can be subdivided into a spherical outer Oort cloud of 20,000–50,000 AU (0.32–0.79 ly), and

21147-492: The point where, at some distance along the ion tail, magnetic reconnection occurs. This leads to a "tail disconnection event". This has been observed on a number of occasions, one notable event being recorded on 20 April 2007, when the ion tail of Encke's Comet was completely severed while the comet passed through a coronal mass ejection . This event was observed by the STEREO space probe . In 2013, ESA scientists reported that

21306-521: The population from the coming arrival of "Nibiru" or "Planet X", a giant planet whose close passage would disrupt the Earth's rotation, causing global cataclysm. Her original date for the apocalypse was May 2003, which passed without incident, but various conspiracy websites continued to predict the coming of Nibiru, most of whom tied it to the 2012 phenomenon . Lieder and others' claims of the planet Nibiru have been repeatedly debunked by scientists. Its lengthy period of visibility and extensive coverage in

21465-401: The public in order to push legislative change. However, because it is a man-made phenomenon, addressing its impacts on humans and the environment has political, social, and economic considerations. Light pollution is the presence of anthropogenic artificial light in otherwise dark conditions. The term is most commonly used in relation to in the outdoor environment and surrounding, but

21624-651: The sky. Once he had established that the object was moving relative to the background stars, he emailed the Central Bureau for Astronomical Telegrams , the clearing house for astronomical discoveries. Bopp did not own a telescope . He was out with friends near Stanfield, Arizona , observing star clusters and galaxies when he chanced across the comet while at the eyepiece of his friend's telescope. He realized he might have spotted something new when, like Hale, he checked his star maps to determine if any other deep-sky objects were known to be near M70, and found none. He alerted

21783-429: The solar wind is supersonic, a bow shock is formed upstream of the comet in the flow direction of the solar wind. In this bow shock, large concentrations of cometary ions (called "pick-up ions") congregate and act to "load" the solar magnetic field with plasma, such that the field lines "drape" around the comet forming the ion tail. If the ion tail loading is sufficient, the magnetic field lines are squeezed together to

21942-443: The solar wind playing a minor role in the destruction of water compared to photochemistry . Larger dust particles are left along the comet's orbital path whereas smaller particles are pushed away from the Sun into the comet's tail by light pressure . Although the solid nucleus of comets is generally less than 60 kilometers (37 mi) across, the coma may be thousands or millions of kilometers across, sometimes becoming larger than

22101-425: The spectroscopic method. New planets are detected by the white light curve method which is viewed as a symmetrical dip in the charts readings when a planet overshadows its parent star. However, after further evaluation of these light curves, it has been discovered that the asymmetrical patterns of the dips presented are caused by the tail of a comet or of hundreds of comets. As a comet is heated during close passes to

22260-508: The spiders that tolerate the light gain an advantage over the spiders that avoid it. This is a simple example of the way in which species frequencies and food webs can be disturbed by the introduction of light at night. Light pollution poses a serious threat in particular to nocturnal wildlife, having negative impacts on plant and animal physiology. It can confuse animal navigation , alter competitive interactions, change predator-prey relations, and cause physiological harm. The rhythm of life

22419-455: The star, so Shramek called the Art Bell radio program Coast to Coast AM to announce that he had discovered a "Saturn-like object" following Hale–Bopp. UFO enthusiasts, such as remote viewing proponent and Emory University political science professor Courtney Brown , soon concluded that there was an alien spacecraft following the comet. Several astronomers, including Alan Hale, stated that

22578-517: The tail may stretch beyond one astronomical unit . If sufficiently close and bright, a comet may be seen from Earth without the aid of a telescope and can subtend an arc of up to 30° (60 Moons) across the sky. Comets have been observed and recorded since ancient times by many cultures and religions. 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

22737-566: The term ( ἀστὴρ ) κομήτης already meant 'long-haired star, comet' in Greek. Κομήτης was derived from κομᾶν ( koman ) 'to wear the hair long', which was itself derived from κόμη ( komē ) 'the hair of the head' and was used to mean 'the tail of a comet'. The astronomical symbol for comets (represented in Unicode ) is U+2604 ☄ COMET , consisting of a small disc with three hairlike extensions. The solid, core structure of

22896-414: The toxic gas cyanogen in the tail of Halley's Comet, causing panicked buying of gas masks and quack "anti-comet pills" and "anti-comet umbrellas" by the public. If a comet is traveling fast enough, it may leave the Solar System. Such comets follow the open path of a hyperbola, and as such, they are called hyperbolic comets. Solar comets are only known to be ejected by interacting with another object in

23055-489: The trans-Neptunian region—whereas the source of long-period comets is thought to be the far more distant spherical Oort cloud (after the Dutch astronomer Jan Hendrik Oort who hypothesized its existence). Vast swarms of comet-like bodies are thought to orbit the Sun in these distant regions in roughly circular orbits. Occasionally the gravitational influence of the outer planets (in the case of Kuiper belt objects) or nearby stars (in

23214-448: The volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail called the type II or dust tail. At the same time, the ion or type I tail, made of gases, always points directly away from

23373-446: The young Earth about 4 billion years ago brought the vast quantities of water that now fill Earth's oceans, or at least a significant portion of it. Others have cast doubt on this idea. The detection of organic molecules, including polycyclic aromatic hydrocarbons , in significant quantities in comets has led to speculation that comets or meteorites may have brought the precursors of life—or even life itself—to Earth. In 2013 it

23532-409: Was already bright enough to be seen by anyone who looked for it, even from large cities with light-polluted skies. The Internet was a growing phenomenon at the time, and numerous websites that tracked the comet's progress and provided daily images from around the world became extremely popular. The Internet played a large role in encouraging the unprecedented public interest in comet Hale–Bopp. As

23691-535: Was announced in International Astronomical Union circular 6187. Hale–Bopp's orbital position was calculated as 7.2  astronomical units (au) from the Sun, placing it between Jupiter and Saturn and by far the greatest distance from Earth at which a comet had been discovered by amateurs. Most comets at this distance are extremely faint, and show no discernible activity, but Hale–Bopp already had an observable coma . A precovery image taken at

23850-400: Was confirmed that the amino acid glycine had been found in the comet dust recovered by NASA's Stardust mission . In August 2011, a report, based on NASA studies of meteorites found on Earth, was published suggesting DNA and RNA components ( adenine , guanine , and related organic molecules) may have been formed on asteroids and comets. The outer surfaces of cometary nuclei have

24009-480: Was estimated that one third of the world's population could no longer see the Milky Way, including 80% of Americans and 60% of Europeans. Singapore was found to be the most light-polluted country in the world. Over the past 21 years, China's provincial capital cities have seen a major increase in light pollution, with hotspots along the eastern coastline region. Medical research on the effects of excessive light on

24168-438: Was found to be depleted more than 25 times relative to the solar abundance, while argon with its higher sublimation temperature was enriched relative to the solar abundance. Together these observations indicate that the interior of Hale–Bopp has always been colder than 35–40 K, but has at some point been warmer than 20 K. Unless the solar nebula was much colder and richer in argon than generally believed, this suggests that

24327-567: Was listed as a probable carcinogen by the World Health Organization's International Agency for Research on Cancer. (IARC Press release No. 180). Multiple studies have documented a correlation between night shift work and the increased incidence of breast and prostate cancer. One study which examined the link between exposure to artificial light at night (ALAN) and levels of breast cancer in South Korea found that regions which had

24486-608: Was one significant example when it broke into two pieces during its passage through the perihelion in 1846. These two comets were seen separately in 1852, but never again afterward. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A minor meteor shower, the Andromedids , occurs annually in November, and it is caused when Earth crosses the orbit of Biela's Comet. Light pollution Although this type of pollution can exist throughout

24645-505: Was still active as indicated by the detection of the CO-driven coma. Herschel Space Observatory images taken in 2010 suggest comet Hale–Bopp is covered in a fresh frost layer. Hale–Bopp was again detected in December 2010 when it was 30.7 au away from the Sun, and in 2012, at 33.2 au from the Sun. The James Webb Space Telescope observed Hale–Bopp in 2022, when it was 46.2 au from

24804-475: Was suggested that impacts between rocky and icy surfaces, such as comets, had the potential to create the amino acids that make up proteins through shock synthesis . The speed at which the comets entered the atmosphere, combined with the magnitude of energy created after initial contact, allowed smaller molecules to condense into the larger macro-molecules that served as the foundation for life. In 2015, scientists found significant amounts of molecular oxygen in

24963-462: Was tracking known comets from his driveway in New Mexico when he chanced upon Hale–Bopp just after midnight. The comet had an apparent magnitude of 10.5 and lay near the globular cluster M70 in the constellation of Sagittarius . Hale first established that there was no other deep-sky object near M70, and then consulted a directory of known comets, finding that none were known to be in this area of

25122-436: Was visible all night to Northern Hemisphere observers. After its perihelion passage, the comet moved into the southern celestial hemisphere . The comet was much less impressive to southern hemisphere observers than it had been in the northern hemisphere, but southerners could see the comet gradually fade from view during the second half of 1997. The last naked-eye observations were reported in December 1997, which meant that

25281-503: Was visible to the naked eye for a record 18 months, due to its massive nucleus size. This is twice as long as the Great Comet of 1811 , the previous record holder. Accordingly, Hale–Bopp was dubbed the Great Comet of 1997 . The comet was discovered independently on July 23, 1995, by two observers, Alan Hale and Thomas Bopp , both in the United States. Hale had spent many hundreds of hours searching for comets without success, and

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