The annual Leonids Meteor Shower for 2021 occurred between November 6 and November 30. The peak activity was on November 17, 2021 when bits of the cosmic debris appeared to viewers from Earth like a display of fireworks in the sky.
Leonids Meteor Showers
The Leonids refer to a prolific meteor shower that is among the most spectacular, viewed from Earth. The showers occur in November when the Earth passes through the debris left by Comet 55P/Tempel-Tuttle, which takes around 33 years to make one orbit around the Sun. The bits of comet debris are said to have been formed when Sun condensed out of a cloud of hydrogen, helium, and some dust.
The shower appears as fireballs with bright colour light streak close to the horizon. The light is a result of the friction between the meteorite and the molecules present in Earth’s atmosphere because of which it burns. Sometimes, large Leonid meteors have reportedly generated loud whistling or buzzing sound before they appear as glowing fireballs in the sky. Occasionally, they can fall on Moon.
The Leonids are called fireballs because of their bright colours, and earthgazers, as they streak close to the horizon.
Speed The showers feature the fastest meteors—typically travelling at speeds of 71 km per second though rates may be as low as 15 meteors an hour. Unlike other meteoroids, the Leonid bits orbit Sun in the opposite direction as Earth; so they strike Earth’s upper atmosphere at a higher relative speed (i.e., 72 km/sec compared to which a typical bullet, at blinding speed, travels at one km, per second).
Size and energy The faintest meteor that becomes visible to the average viewer on Earth is typically about 0.6 mm across, less than one-tenth of an inch or about the size of a sand grain. The energy it produces could light a 100-watt light bulb for about 2.5 seconds.
Bright fireballs can be produced by chunks the size of a marble, about 9–10 mm in diameter with a mass of half a gram. The power of these bright meteors exceeds one million joules (about the same punch as a small car moving at 60 mph).
An annual Leonid shower may deposit 12 or 13 tonnes of particles across Earth.
The heat created by a meteor vaporises most of the bits high up in the air, including large space rocks (up to the size of a basketball); a handful of dense material may survive. Largely, the stuff of the Leonids is ‘fluffy’ and disintegrates easily, as Leonids start disintegrating in the thin air, 140 km above Earth.
The Leonids are named after the location of their radiant in the constellation Leo (appearing to radiate from the head or ‘sickle’ of the constellation).
The Leonids also produce meteor storms (very large outbursts), during which activity exceeds 1,000 meteors per hour, with some events exceeding 1,00,000 meteors per hour, in contrast to the sporadic background (5 to 8 meteors per hour) and the shower background (several meteors per hour).
Viewing the showers The Leonid meteors are visible in all parts of the sky on any cloudless night when Moon is not very bright. They are best viewed at midnight and preferably to be seen from a wide, open space (like parks) where one can scan the night sky with the naked eye. The point in the sky from which they appear to radiate is called the radiant point, which is an optical illusion.
One may only look at the constellation Leo to view the Leonids, which are visible throughout the night sky usually until day break.
Because of the storm of 1833 and other developments such as identification of Halley’s Comet, the Leonids have an important place in the scientific study of meteors, which had earlier been thought to be atmospheric phenomena.
History Leonid meteor showers and storms seem to have been noted in ancient times.
In modern history, the meteor storm of 1833 had an estimated peak rate of over one hundred thousand meteors an hour; another estimate was in excess of 2,40,000 meteors during the nine hours of the storm over the entire region of North America, east of the Rocky Mountains. The shower was of short duration and was not seen in Europe.
Leonid storms were then noted in 1866 (hundreds per minute/a few thousand per hour in Europe); in 1867, when moonlight reduced the rates to 1,000 meteors per hour; and in 1868 when they reached an intensity of 1,000 meteors per hour in dark skies. Comet Tempel-Tuttle was identified as the source of the meteor shower/storms at this time.
A major sighting came much later in 1966 (the expected 1899 and 1933 storms were disappointing), the interval resulting in a notion that the dust had moved on and the storms were a thing of the past.
The 1999–2001 storms produced about 3,000 meteors per hour.
Comet Tempel-Tuttle
Comet Tempel-Tuttle (or 55P/Tempel-Tuttle ) is a periodic comet with an orbital period of 33 years. The parent body of the Leonid meteor shower, Comet Tempel-Tuttle fits the classical definition of a Halley-type comet with a period of between 20 and 200 years. Its discovery is traced to Wilhelm Tempel (December 19, 1865) and Horace Parnell Tuttle (January 6, 1866).
The orbit of 55P/Tempel-Tuttle intersects that of Earth nearly exactly; so, streams of material ejected from the comet during perihelion passes (when closest to Sun) do not have to spread out over time to encounter Earth. Streams from the comet at perihelion are still dense when they encounter Earth, resulting in the 33-year cycle of Leonid meteor storms.
Meteor Showers and Storms
A meteor shower is considered a storm when more than 1,000 meteors are rained down per hour; the Leonids deliver some 10 to 15 meteors per hour in average years.
Meteor showers happen when Earth moves through the meteoroid stream of particles left from the passages of a comet. The meteoroids are ejected by the comet as its frozen gases evaporate due to Sun’s heat when it is close enough. The meteoroids left by the comet are in trails.
The light of the meteors is seen when it burns from the friction between the meteorite and the molecules present in Earth’s atmosphere. Meteors leave behind a trail of ionised gas.
Meteor, Meteoroid, and Meteorite
A meteoroid refers to a small bit of comet (comet debris) or asteroid that floats through interplanetary space (outside of Earth’s atmosphere).
A meteor is the streak of light in the sky when a small piece of comet or asteroid material enters the atmosphere at high speed and burns up because of the frictional heating from its collision with the atoms and molecules in the atmosphere.
Most meteoroids that enter Earth’s atmosphere burn up completely (meteors). The meteoroid chunk that does not completely burn up and makes it to Earth’s surface is called a meteorite.
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