Sunday, November 13, 2011

Fire from the skies - Leonids Meteor Shower

Fire from the skies
Leonids Meteor Shower
18th Nov, 2011

Come November and all amateur astronomers around the world start looking up to the skies for an event which we call the “leonids meteor shower”. One can see trains of shooting stars coming from a particular area in the sky at an amazing rate. Only Geminids shower can rival this event. You all must have seen a shooting star whizzing past in the skies. This phenomenon can be seen almost any day of the year but a shower of shooting star is something that happens only few times. Leonids is one of these and the most prominent one.

Leonids are a prolific meteor shower associated with the comet Tempel-Tuttle. The meteor shower is visible every year around November 17 when the Earth moves through the Leonid meteor stream. The stream comprises solid particles, known as meteoroids, ejected by the comet as it passes by the Sun. It’s like truck carrying garbage littering the road with it while it moves. A typical particle is no bigger than fine dust, and rapidly vaporizes emitting a streak of light as it hits Earth's atmosphere at tremendous speed (71 km/s).  As the entire meteor streaks are parallel, because of effect of perspective they appear to originate from a single point in the sky (just like railway tracks appear to meet at some distance) and as a result Leonids get their name from the location of their radiant in the constellation Leo. The Leonids are famous because their meteor showers, or storms, can be among the most spectacular. In most years the rate at which meteors are likely to be seen is not great; however, years of highly elevated rates tend to follow a 33 year cycle, associated with the 33 year orbit of Comet Tempel-Tuttle. Storms in peak years featured thousands of meteors per hour; notable events were observed in 1698, 1799, 1832, 1833, 1866, 1966, 1999, 2001 and 2002. During a strong storm in dark viewing conditions the sky can appear to be "raining stars".

More information about the Leonids:

Most visible Leonids are between 1 mm and 1 cm in diameter. For example, a Leonid meteor barely visible with the naked eye in a dark sky, is caused by a meteoroid of 0.5 mm in diameter and weights only 0.00006 gram.

Just before they enter the Earth's atmosphere, Leonid meteoroids travel at 71 kilometers per second, or 213 times as fast as speed of sound.

Source of light - When meteoroids enter the Earth's atmosphere, they collide with air molecules. Those collisions sputter away the outer layers of the particle, creating a vapor of sodium, iron and magnesium atoms. Electrons are knocked into larger orbits from the nucleus of the atoms. When the electrons fall back to their rest positions, light is emitted. This is the same process as in gas discharge lamps.

Sounds - Meteors do not normally cause audible sounds. Hence, they will pass by unnoticed if not seen. A sonic boom is sometimes heard for very bright Leonid meteors, called fireballs

Leonid meteor shower date and timings:

It can be seen from Nov 14 till Nov 21. The peak usually occurs around 17 November This time its spread over 2 days. This year Moon will mar the observations but still its worth observing. Details are given below:

In 2011 usual Leonid return is expected with overall maximum ZHR up to 15. However, there is a possibility of three local peaks, two of which will be caused by interaction with trails and the third is the background one.

19:58 UT on 16 November (01:28 IST 17th November)

The first local peak will be caused by 1800 trail. Its own ZHR is 4-5, and adding to the background activity this should give ZHR ~10 around 19:58 UT on 16 November (01:28 IST 17th November).

The encountering parts of 1800 trail have low meteor brightness around this submaximum time and possibility of higher activity on radio. Then some decrease in activity is expected with local minimum of ZHR=7 around 3:24 UT on 17 November.

3:34 UT on 18 November 2011 (09:04 IST on 18th November)
The next maximum will be the background one. By IMO's data, it occurs at 3:34 UT on 18 November 2011 (09:04 IST on 18th November). We expect that its intensity will be not high, with ZHR of ~10.Then some minor decrease in activity to ZHR=9 is possible around 17:56 UT 18 November (23:26 IST November 18th). Though such weak variations in activity could hardly be detected by visual observations, so between second and third peaks activity will likely sustain at ZHR=9-10.

23:25 UT on 18 November (04:55 IST 19th November)

Finally, the third and most intensive among non-prominent Leonids 2011 peaks will be an enhancement from 1567 trail. It is expected to occur around 23:25 UT on 18 November (04:55 IST 19th November), and ZHR for a short period should reach 15. This peak will be notably sharper that previous two, significant changes in meteor brightness are not expected.

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