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Fireball Reports from 2013 February and Chelyabinsk Fireball

February tends to be a quiet month for fireball reports : the background meteor rates are generally low in the first half of the year, the UK weather is often unhelpful (many parts of the UK had a rather cloudy February 2013) and the low outside temperatures encourage people to stay indoors.

Only two UK fireballs were reported during February :

Steve Caress (Grantham, Lincs) reported a mag -5 white fireball seen in the NNW sky while driving at 04:31 GMT on Feb 6.

Philip Symons (Stanwell, Surrey, near Heathrow) reported a possible mag -7 fireball seen at 18:44 GMT on Feb 15. A check of the University of Hertfordshire all-sky cameras found that the Niton (Isle of Wight) camera wasn't operating at the time and the Bayfordbury (Hertfordshire) camera was clouded out. However, there is something bright seen through the cloud in the image that starts at  18:37:43 (see star.herts.ac.uk/allsky/imageget.php ) . 

The reported duration of 35 seconds was unusually long for a fireball - the lack of other reports of this object probably indicates that this was not a fireball, but a more local that strongly resembled a fireball

Any further sightings of these or different fireballs (meteors of magnitude -3 and brighter) would be welcomed by the Meteor Section. 

Advice on what to record and where to report to  can be found at : www.popastro.com/meteor/fireballs/makingobservations/index.php

Chelyabinsk fireball of 2013 Feb 15 

This fireball appeared at 09:20 local time (03:20 GMT) over the city of Chelyabinsk, which lies close to Russia's border with Kazakhstan. Unlike the mini-asteroid 2008 TC3 (which entered the atmosphere over Sudan in October 2008), the object wasn’t detected in advance due to its “small” size and due to its final approach to the Earth being from a direction within 15 degrees of the Sun. 

Given the magnitude of the event it soon became worldwide news, especially with attention already being directed to the predicted close encounter later that day with asteroid 2012 DA14. Despite some wild claims that the Chelyabinsk object was a fragment of 2012 DA14, the two objects were not related. 2012 DA14 was approaching the Earth from the south (effectively from a radiant close to declination -81 degrees) and so was well below the horizon from Chelyabinsk (Lat 55 N). The object that produced the Chelyabinsk fireball was, in contrast, approaching from the east (from the constellation of Pegasus) and so two orbits were very different. 

There are many on-line reports of the Chelyabinsk fireball, including

www.nasa.gov/mission_pages/asteroids/news/asteroid20130215.html

rt.com/news/meteorite-crash-urals-chelyabinsk-283/

www.bbc.co.uk/news/world-europe-21468116

apod.nasa.gov/apod/ap130223.html

Some early reports referred to this event as a "meteor shower". This is inaccurate as it was a single object entering the atmosphere. 

Some other early reports even claimed that the Russian military had shot it down. However, given that the event lasted only around 30 seconds after entering the Earth's atmosphere, it would have required an unbelievably rapid response to launch a missile to intercept it is such a short time !  

There were also suggestions that had the object arrived several hours later that it would have occurred over northern England. This is not true of course as by then the Earth would have moved further around its orbit and therefore the object would have missed the Earth. 

Making magnitude estimates of “routine” fireballs can be tricky. For bright objects such as this it is much harder. However, inspections of shadows in videos suggested that the object was brighter than the Sun.

In order to work out the atmospheric trajectory of the fireball and the former solar system orbit of the parent object, it is necessary to have access to images or videos of the fireball from several well-separated locations. We were fortunate therefore in that the fireball appeared in the daytime over a populated area and that in this area many drivers have video cameras fitted that show the view through their car windscreens. It is also straightforward nowadays to upload images to social media such as Facebook. Had the fireball occurred 20 years earlier, such images would have been less readily available. Had the event occurred at the height of the Cold War (or in 2013 over Korea), we have to wonder about how the event could have been misinterpreted ...  Fortunately, the Cold War came to an end and some of the most useful information about the object actually came from a network of infrasound sensors operated by the Comprehensive Test Ban Treaty Organization (CTBTO).

Early analyses soon showed that the fireball achieved its maximum brightness just south of Chelyabinsk, Russia, at an altitude of 23.3 km (14.5 miles) approx 13 seconds after entering the atmosphere and was travelling at a velocity of 18.6 km/s (11.6 miles/sec) at the time. Infrasound data indicates that the event, from atmospheric entry to the meteor's airborne disintegration took 32.5 seconds. The estimated size of the object, prior to entering Earth's atmosphere, was approx 17 metres (55 feet). Its estimated mass was 10,000 tons and the energy released during the event was equivalent to nearly 500 kilotons of TNT, making it the biggest event since that of Tunguska in June 1908. It was fortunate in that the object entered the atmosphere at a fairly shallow (16-20 deg) angle and therefore much of its energy was absorbed without it penetrating to a lower altitude. Nevertheless, over a thousand people needed medical attention, mostly resulting from the blast wave which arrived several minutes after the fireball. By this time people were looking out through windows and were consequently hit by fragments of glass when the blast wave shattered the windows.

Recovering meteorite fragments that have reached the Earth’s surface is usually not straightforward – most black rocks lying on the ground are terrestrial rocks rather than meteorites. However, in this case, the ground was covered in snow and the smaller fragments were either lying on top of the snow or encased within it, whereas terrestrial black rocks would be buried under the snow.

Added by:  Tracie Heywood