Saturday, 9 December 2017

The Gemenid Meteors.

The Geminid Meteor Shower is expected to peak on Wednesday 13 December this year (2017) with potentially up to 120 meteors per hour being visible in areas of the Northern Hemisphere with a clear sky. This year peak activity for the shower coincides with a waning Crescent Moon, with the New Moon falling on Sunday 17 December, so viewing should be fairly good. The meteors appear to radiate from a point in the constellation of Gemini, hence their name.

The relative positions of the radiant point of the Gemenid Meteors (i.e. the point from which the meteors radiate) at 9.00 pm on 13 December 2017. Sky & Telescope.

Oddly for a meteor shower, the Geminids do not appear to be related to a comet, but instead are associated with an object called 3200 Phaethon, which is classed as an Apollo Asteroid (an asteroid with an orbit that crosses that of the Earth). 3200 Phaethon has a highly elliptical orbit, which takes it in as close as 0.14 Au (14% of the distance between the Earth and the Sun, more than twice as close as Mercury) and out as far as 2.4 AU (2.4 times as far from the Sun as the Earth or 1.6 times as far as Mars). 3200 Phaethon does not appear to produce any sort of halo (a cloud of material produced by the evaporation of gas ice from the surface of a comet, thought to be the source of most meteor showers); rather it appears dark in colour an is classed as a B-type Carbonaceous Asteroid, thought to have a surface covering of  anhydrous silicates, hydrated clay minerals, organic polymers, magnetite, and sulphides.

Asteroid 3200 Phaethon is a 5 km body with a highly eccentric orbit similar to that of a comet, which takes it closer to the Sun than any other named Asteroid. It appears to be the parent body of the Geminid Meteors, which share essentially the same orbit as it, as well as a group of larger bodies known as the Phaethon-Geminid Complex. Such meteor showers typically form from the tail of a comet; as the comet approaches its perihelion (the closest point in its orbit to the Sun), ice at the surface sublimates away (turns directly from a solid to a gas - liquids do not form in a vacuum), releasing particles of silica trapped in the ice, which continue to follow essentially the same path as the comet, creating a meteor shower every time the Earth passes through this stream. However, 3200 Phaethon, which has a 1.43 year orbital period in which it reaches 0.14 AU from the Sun (14% of the distance between the Earth and the Sun, or less than half the distance at which Mercury orbits) is thought to regularly suffer surface temperatures in excess of 1000K, making it highly unlikely that it has ice on its surface, which calls its potential role as the parent body to the Geminid Meteors into question.

Image of 3200 Phaethon taken on 20 November 2017 with the Pearl Telescope at the Tenagra Observatory in Arizona The asteroid is the point in the centre of the picture, indicated by the two red lines. The longer lines are stars, their elongation being caused by the telescope tracking the asteroid over the length of the exposure, in this case five exposures, each of 180 seconds. Gianluca Masi/Virtual Telescope/Michael Schwartz/Tenagra Observatory.
In a paper published on the arXiv online database at Cornell University Library on 17 June 2013, David Jewitt of the Department of Earth and Space Sciences and Department of Physics and Astronomy at the University of California Los AngelesJing Li of the Department of Earth and Space Sciences at the University of California Los Angeles, and Jessica Agarwal of the Max Planck Institute for Solar System Research, describe the results of a study of 3200 Phaeton using the NASA STEREO Spacecraft.

Jewitt et al. observed two successive perihelions of 3200 Phaeton, in June 2009 and May 2012. On both occasions they were able to observe a faint comet-like dust tail emerging from the body, even though it was apparently reaching temperatures that would rapidly destroy an icy comet. This tail grew rapidly, reaching a length of over 250 000 km within a day of first appearing, and appeared to represent material being lost from the parent body at a rate of about 3 kg per second.

Composite images of 3200 Phaethon in 2009 (top row) and 2012 (bottom row) compared with the projected sun- comet line (white). The Sun is to the upper right in each panel. Insets are 49000 square and show eld stars near to Phaethon to demonstrate the point spread function of the data. Each panel has North to the top, East to the left and shows the median of 30 images taken over a 1 day period. Jewitt et al. (2013).

Jewitt et al. suggest that at it's perihelion 3200 Phaethon is being heated to such a degree that hydrated minerals at its surface could be thermally fractured and desiccated, leading to the ejection of dust particles.

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