Tuesday, 6 October 2020

The Draconid Meteors.

The Draconids are one of the most notable annual meteor showers, in some years producing several thousand meteors per hour (like most meteor showers the number of Draconids varies from year to year). The shower is visible between 6 and 10 October each year, and is expected to peak on Thursday 8 October 2020, with best viewing in the evening (this will be the same wherever you are on Earth), though visibility may be somewhat hampered this year, as the meteor shower peaks a week after the Full Moon on Thursday 1 October, so that the meteors may be obscured by the brightness of the Moon. The Draconids take their name from the constellation of Draconis, with the meteors appearing to radiate from the mouth of the Dragon, between the stars Eltanin and Rastaban. Since this constellation is very high in the northern sky, the Draconids are an almost exclusively Northern Hemisphere phenomenon, which sightings from south of the equator being quite rare.

The radiant point (point from which the meteors appear to radiate) of the Draconid Meteor Shower. PA.

Meteor showers are thought to be largely composed of material from the tails of comets. Comets are composed largely of ice (mostly water and carbon dioxide), and when they fall into the inner Solar System the outer layers of this boil away, forming a visible tail (which always points away from the Sun, not in the direction the comet is coming from, as our Earth-bound experience would lead us to expect). Particles of rock and dust from within the comet are freed by this melting (strictly sublimation, transforming directly from a solid to a gas due to the low pressure on it's surface) of the comet into the tail and continue to orbit in the same path as the comet, falling behind over time. 

The Earth passing through a stream of comet dust, resulting in a meteor shower. Not to scale. Astro Bob.

The Draconid Meteor Shower is caused by the Earth passing through the trail of the Comet 21P/Giacobini-Zinner, and encountering dust from the tail of this comet (for this reason the shower is sometimes known as the 'Giacobinid Meteors'). 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 name 21P/Giacobini-Zinner implies that the comet was the 21st Periodic Comet discovered (a Periodic Comet has an orbital period of less than 200 years) and that it was discovered by Michel Giacobini, who first observed the comet from the Nice Observatory in France in 1900, and Ernst Zinner who observed the comet from the Remeis Observatory in Bamberg, Germany, in 1913.

Image of 21P/Giacobini-Zinner captured by the Kitt Peak 0.9-m telescope on 31 October 1998. North is up with east to the left. NA Sharp/NASA/NOAO/AURA/NSF.

The Earth does not need to pass close to  21P/Giacobini-Zinner 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  21P/Giacobini-Zinner itself visits the Inner Solar System once every 6.6 years, last doing so in 2018, when it reached about 58 434 00 km (0.39 AU) from Earth on 11 September. The comet has an eccentric orbit tilted at 31.9° to the plane of the Solar System, that takes it from 1.03 AU from the Sun (103% of the average distance at which the Earth orbits the Sun) to 6.00 AU from the Sun (6 times as far from the Sun as the Earth,slightly outside the orbit of Jupiter). 
The calculated orbit and position on 6 October 2020 of Comet 21P/Giacobini-Zinner.  JPL Small Body Database.
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