* Astronomy

Last July, people in Spain, Portugal and France watched the brilliant fireball produced by a boulder crashing down through the Earth's atmosphere. In a paper to be published in the journal Monthly Notices of the Royal Astronomical Society, astronomers Josep M. Trigo-Rodríguez (Institute of Space Sciences, CSIC-IEEC, Spain), José M. Madiedo (University of Huelva-CIECEM, Spain) and Iwan P. Williams (Queen Mary, University of London) present dramatic images of this event.
The scientists go on to explain how the boulder may originate from a comet which broke up nearly 90 years ago and suggest the tantalising possibility that chunks of the boulder (and hence pieces of the comet) are waiting to be found on the ground.
Fireballs (or bolides) are the name given by astronomers to the brightest meteors (popularly referred to as 'shooting stars'). On 11th July 2008, at 2117 GMT, a brilliant fireball was recorded. At maximum intensity, the object was more than 150 times as bright as the full Moon. It was first picked up at a height of 98.3 km and disappeared from view 21.5 km above the surface of the Earth, tracked by three stations of the Spanish Fireball Network (SPMN) above Bejar, near Salamanca in Spain. At the same time a professional photographer took a picture of the bolide from the north of Madrid.

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Title: Observations of a very bright fireball and its likely link with comet C/1919 Q2 Metcalf
Authors: Josep Ma Trigo-Rodríguez, José M. Madiedo, Iwan P. Williams, Alberto J. Castro-Tirado, Jordi Llorca, Stanislav Vítek and Martin Jelínek

A very bright fireball called Béjar (SPMN110708), with a maximum brightness of -18, was observed over much of Spain as well as parts of Portugal and France on 2008 July 11 at 21:17:39 utc. Fortuitously, it flew over many of the instruments that are part of the Spanish Meteor and Fireball Network so that accurate measurements of its properties were recorded. We describe these observations and make deductions from them regarding the nature and origin of the body that gave rise to this fireball. The bolide first became visible at a height of 98.3 km, attained its maximum brightness at a height of 26 km and finished at a height of 21.5 km. These values are very much in line with other well-known fireballs producing meteorites. Standard calculations based on the meteoroids' ability to survive in the atmosphere suggest a strength for the remnant that survived to this height of about 14 MPa, similar to those for meteorite-dropping bolides. So far, this fireball looks typical and one might well expect to find meteorites on the ground in due course. The heliocentric orbit of the meteoroid determined from the observations had a perihelion essentially at the Earth's orbit and an eccentricity of 0.775, so that the orbit extends far beyond Jupiter, nearly reaching Saturn's heliocentric distance and is a typical orbit for a member of the Jupiter family of comets. This is unlike other bright fireballs, where aphelion is within the asteroidal belt and clearly points to an asteroidal origin. The orbit is also very similar to the mean orbit of the Omicron Draconid meteor shower, which is an additional pointer to this fireball being of cometary origin. If the parent was indeed a comet, this has implications for the internal structure of comets in that significant-sized non-icy inclusions must exist there. This is not surprising, but this is probably the first time that direct evidence has been found showing that this is the case. Further, such chunks can only be released through the catastrophic breakup of the nucleus. Remarkably, a candidate for the parent of the Omicron Draconid meteor shower is comet C/1919 Q2 (Metcalf) which suffered a catastrophic breakup in the early decades of the last century.

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