The Leonids (play /ˈliːənɪdz/ lee-ə-nidz) is a prolific meteor shower associated with the comet Tempel-Tuttle. The Leonids get their name from the location of their radiant in the constellation Leo: the meteors appear to radiate from that point in the sky. They tend to peak in November. Earth moves through the meteoroid stream of particles left from the passages of a comet. The stream comprises solid particles, known as meteoroids, ejected by the comet as its frozen gases evaporate under the heat of the Sun when it is close enough – typically closer than Jupiter's orbit. The Leonids are a fast moving stream which come close to or cross the path of the Earth and impact the Earth at 72 km/s.[1] Leonids in particular are well known for having bright meteors or fireballs which may be 9 mm across and have 85 g of mass and punch into the atmosphere with the kinetic energy of a car hitting at 60 mph. An annual Leonid shower may deposit 12 or 13 tons of particles across the entire planet. Sometimes these trails of meteoroids cause meteor showers and sometimes meteor storms. The meteoroids left by the comet are organized in trails in orbits similar to though different from that of the comet. They are differentially disturbed by the planets, in particular Jupiter[2] and to a lesser extent by radiation pressure from the sun, the Poynting–Robertson effect, and the Yarkovsky effect.[3] Old trails are spatially not dense and compose the meteor shower with a few meteors per minute. In the case of the Leonids, that tends to peak around November 17, but some are spread through several days on either side and the specific peak changing every year.[4] Conversely, young trails are spatially very dense and the cause of meteor storms when the Earth enters one. Usual counts during a storm exceed 1000 meteors per hour,[5] to be compared to the annual background (1 to 2 meteors per hour) and the shower background (a few per hour). History The Leonids are famous because their meteor showers, or storms, can be, and have been in a few cases, among the most spectacular. Because of the superlative storm of 1833 and the recent developments in scientific thought of the time (see for example the identification of Halley's Comet) the Leonids have had a major effect on the development of the scientific study of meteors which had previously been thought to be atmospheric phenomena. The meteor storm of 1833 was of truly superlative strength. One estimate is over one hundred thousand meteors an hour,[6] but another, done as the storm abated, estimated in excess of two hundred thousand meteors an hour[7] over the entire region of North America east of the Rocky Mountains. It was marked by the Native Americans,[8][9] slaves like Harriet Tubman and slave-owners[10][11] and others.[12] Near Independence, Missouri, it was taken as a sign to push the growing Mormon community out of the area.[13] The leader of the Mormon community, Joseph Smith, noted in his journal that this event was a literal fullfillment of the word of God and a sure sign that the coming of Christ is close at hand.[14] Denison Olmsted explained the event most accurately. After spending the last weeks of 1833 collecting information he presented his findings in January 1834 to the American Journal of Science and Arts, published in January–April 1834, and January 1836. He noted the shower was of short duration and was not seen in Europe, and that the meteors radiated from a point in the constellation of Leo and he speculated the meteors had originated from a cloud of particles in space.[15] Accounts of the 1866 repeat of the Leonids counted hundreds per minute/a few thousand per hr in Europe.[16] The Leonids were again seen in 1867, when moonlight reduced the rates to 1000 per hour. Another strong appearance of the Leonids in 1868 reached an intensity of 1000 per hour in dark skies. It was in 1866–67 that information on Comet Tempel-Tuttle was gathered pointing it out as the source of the meteor shower.[15] When the storms failed to return in 1899, it was generally thought that the dust had moved on and storms were a thing of the past. Then, in 1966 a spectacular storm was seen over the Americas.[17] Historical notes were gathered thus noting the Leonids back to 900AD.[18] Radar studies showed the 1966 storm included a relatively high percentage of smaller particles while 1965's lower activity had a much higher proportion of larger particles. In 1981 Donald K. Yeomans of the Jet Propulsion Laboratory reviewed the history of meteor showers for the Leonids and the history of the dynamic orbit of Comet Tempel-Tuttle.[19] A graph from it was adapted and re-published in Sky and Telescope from Comet 55P/Tempel-Tuttle and the Leonid Meteors(1996, see p. 6.) It showed relative positions of the Earth and Tempel-Tuttle and marks where Earth encountered dense dust. This showed that the meteoroids are mostly behind and outside the path of the comet, but paths of the Earth through the cloud of particles resulting in powerful storms were very near paths of nearly no activity. But overall the 1998 Leonids were in a favorable position so interest was rising. Leading up to the 1998 return, an airborne observing campaign was organized to mobilize modern observing techniques by Peter Jenniskens at NASA Ames Research Center.[20] There were also efforts to observe impacts of meteoroids, as an example of transient lunar phenomenon, on the Moon in 1999. A particular reason to observe the Moon is that our vantage from a location on Earth sees only meteors coming into the atmosphere relatively close to us while impacts on the Moon would be visible from across the Moon in a single view.[21] A sodium tail of the Moon tripled just after the 1998 Leonid shower which was composed of larger meteoroids (which in the case of the Earth was witnessed as fireballs.)[22] However in 1999 the sodium tail of the Moon did not change from the Leonid impacts. Research by Kondrat'eva, Reznikov and colleagues[23] at Kazan University had shown how meteor storms could be accurately predicted but for some years the worldwide meteor community remained largely unaware of these results. The work of David J. Asher, Armagh Observatory and Robert H. McNaught, Siding Spring Observatory[2] and independently by Esko Lyytinen[24][25] in 1999, following on from the Kazan research, is considered by most meteor experts as the breakthrough in modern analysis of meteor storms. Whereas previously it was hazardous to guess if there would be a storm or little activity, the predictions of Asher and McNaught timed bursts in activity down to ten minutes by narrowing down the clouds of particles to individual streams from each passage of the comet, and their trajectories amended by subsequent passage near planets. However, whether a specific meteoroid trail will be primarily composed of small or large particles, and thus the relative brightness of the meteors, was not understood. But McNaught did extend the work to examine the placement of the Moon with trails and saw a large chance of a storm impacting in 1999 from a trail while there were less direct impacts from trails in 2000 and 2001 (successive contact with trails through 2006 showed no hits.)[22] Viewing campaigns resulted in spectacular footage from the 1999, 2001 and 2002, storms producing up to 3,000 Leonid meteors per hour.[20] Predictions for the Moon's Leonid impacts also noted that in 2000 the side of the Moon facing the stream was away from the Earth but that impacts should be in number enough to raise a cloud of particles kicked off the Moon by impacts would cause a detectable increase in the sodium tail of the Moon.[22] Research using the explanation of meteor trails/streams have explained the storms of the past. The 1833 storm was not due to the recent passage of the comet, but from a direct impact with the previous 1800 dust trail.[26] The meteoroids from the 1733 passage of Comet Tempel-Tuttle resulted in the 1866 storm[27] and the 1966 storm was from the 1899 passage of the comet.[28] The double spikes in Leonid activity in 2001 and in 2002 were due to the passage of the comet's dust ejected in 1767 and 1866.[29] This ground breaking work was soon applied to other meteor showers – for example the 2004 June Bootids. Peter Jenniskens has published predictions for the next 50 years.[30] However, a close encounter with Jupiter is expected to perturb the comet's path, and many streams, making storms of historic magnitude unlikely for many decades.[31] Recent work tries to take into account the roles of differences in parent bodies and the specifics of their orbits, ejection velocities off the solid mass of the core of a comet, radiation pressure from the sun, the Poynting–Robertson effect, and the Yarkovsky effect on the particles of different sizes and rates of rotation to explain differences between meteor showers in terms of being predominantly fireballs or small meteors.[3] "There are no guarantees in meteor work ... observers should be alert as often as conditions allow throughout the shower, in case something unexpected happens." See also List of meteor showers References ^ Space.com The Power of a Shooting Star
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