Monday 21 December 2020

The Ursid Meteor Shower.

The Ursid Meteors are expected to peak early in the morning of Tuesday 22 December this year, with the shower being potentially visible to some extent between Thursday 17 and Saturday 26 December. The extent of the shower is variable, some years producing over 100 meteors per hour at its peak, others less than 10; this years shower falls rouhgly half way between the New Moon on 14 December and the Full Moon on 30 December, so viewing should be fairly good, though not as good as in years when the peak coincides with the New Moon, as glare from the Moon can hinder the viewing of meteors. The meteor shower gets its name from the constellation of Ursa Minor, in which it appears to originate.
The radiant (apparent point of origin) of the Ursid Meteors. NASA/JPL/Caltech.
 Meteor streams are thought to come from dust shed by comets as they come close to the Sun and their icy surfaces begin to evaporate away. Although the dust is separated from the comet, it continues to orbit the Sun on roughly the same orbital path, creating a visible meteor shower when the Earth crosses that path, and flecks of dust burn in the upper atmosphere, due to friction with the atmosphere.
The Earth passing through a stream of comet dust, resulting in a meteor shower. Not to scale. Astro Bob.
The Urdid Meteor Shower is caused by the Earth passing through the tail of Comet 8P/Tuttle, and encountering dust from the tail of this comet. The dust particles strike the atmosphere at speeds of over 200 000 km per hour, burning up in the upper atmosphere and producing a light show in the process. The Earth does not need to pass close to Comet 8P/Tuttle for the meteor shower to occur, it simply passes through a trail of dust from the comet's tail that is following the same orbital path. Comet 8P/Tuttle visits the Inner Solar System once every 13.6 years, last doing so in 2008.
How the passage of the Earth through a meteor shower creates a radiant point from which they can be observed. In The Sky.
Comet 8P/Tuttle was discovered by Horace Parnell Tuttle on 5 January 1858. The designation 8P/Tuttle indicates that it was the eighth comet discovered (people have known about comets for thousands of years, but it was only realised that they were objects orbiting the Sun, which could be repeatedly observed and predicted, in the mid-eighteenth century), that it is a Periodic Comet (comet with an orbital period of less than 200 years) and that it was discovered by Horace Parnell Tuttle.
Comet 8P/Tuttle imaged from Lexington, Ohio, on 26 December 2007. The comet is the point at the centre of the image, the elongate dotted lines are stars, which have moved over the course of the thirteen 180 second exposures used to make this composite image, during which the telescope was focused on the comet. Hunter Wilson/Sky and Telescope.

Comet 8P/Tuttle has an orbital period of 4972 days (13.6 years) and a highly eccentric orbit tilted at an angle of 55.0° to the plain of the Solar System, that brings it from 1.03 AU from the Sun at closest perihelion (103% of the distance between the Earth and the Sun) to 10.4 AU from the Sun at aphelion (10.4 times as far from the Sun as the Earth or slightly outside the orbit of the planet Saturn). As a comet with a period of less than 20 years, 8P/Tuttle is considered to be a Jupiter Family Comet.

The calculated orbit and position on 21 December 2020 of 8P/Tuttle.  JPL Small Body Database.

This means that 8P/Tuttle has occasional close encounters with the Earth, with the last thought to have happened in January 2008 and the next predicted in December 2048. The comet also has occasional close encounters with the planets Jupiter, which it last came close to in December 1995 and is next predicted to pass in September 2078, and Saturn, which it last came close to in February 1930 and is expected to pass again in February 2107. Objects which make close passes to multiple planets are considered to be in unstable orbits, and are often eventually knocked out of these orbits by these encounters, either being knocked onto a new, more stable orbit, dropped into the Sun, knocked out of the Solar System or occasionally colliding with a planet. 

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