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RAS PN 09/5: How a cometary boulder lit up the Spanish sky

Last Updated on Monday, 29 March 2010 22:26
Published on Friday, 13 February 2009 00:00

Image: J. Madiedo, University of Huelva-CIECEM, Spain

In a forthcoming paper in  Monthly Notices of the Royal Astronomical Society, an international team of astronomers link a brilliant fireball seen in 2008 to the breakup of a comet in 1920.

Date: 13th February 2009
For Immediate Release
Ref.: PN 09/5

Issued by:
Dr Robert Massey
Press and Policy Officer
Royal Astronomical Society
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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), Iwan P. Williams (Queen Mary, University of London) and Alberto J. Castro-Tirado (Instituto de Astrofísica de Andalucia) 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.

From these images, the astronomers can deduce the trajectory and properties of the incoming boulder. The team think it was a dense object, about 1 metre across and with a mass of 1.8 tonnes, large enough that some fragments probably survived intact and fell to the ground as meteorites.

The astronomers demonstrate that before its fiery demise, the boulder travelled on an unusual orbit around the Sun, on a path which took it from beyond the orbit of Jupiter to the vicinity of the Earth. This orbit is very similar to that of a cloud of dusty particles (meteoroids) known as the Omicron Draconids, which on rare occasions produces a minor meteor shower and probably originates from the breakup of Comet C/1919 Q2 Metcalf in 1920. This strongly suggests that the boulder was once embedded in the nucleus of that comet.

In the mid-1980s the astronomers Tamas I. Gombosi and Harry L.F. Houpis first suggested that the nuclei of comets consist of relatively large boulders cemented together by a ‘glue’ of smaller particles and ice. If the rocky and icy nucleus of a comet disintegrates, then these large boulders are set loose into space. If the Bejar bolide was formed in this way, then it confirms the glue model for at least some comets.

And if fragments of the bolide can be recovered, then for the first time, scientists will be able to study large pieces from a comet in a laboratory. Dr Trigo-Rodríguez comments “If we are right, then by monitoring future encounters with other clouds of cometary debris, we have the chance to recover meteorites from specific comets and analyse them in a lab. Handling pieces of comet would fulfil the long-held ambitions of scientists - it would effectively give us a look inside some of the most enigmatic objects in the Solar System."


Dr Josep M. Trigo-Rodriguez
Inst de Ciencies de l’Espai
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Images of the incoming fireball and an animation tracing its path from Comet Metcalf to Earth can be found at


The paper will be published in Monthly Notices of the Royal Astronomical Society. The abstract is available online at



Comet C/1919 Q2 Metcalf was discovered by Joel H. Metcalf from Vermont in August 1919 and was visible until 3rd February 1920. The orbit was not well determined and no subsequent appearances are known. The Omicron Draconids meteor stream was discovered to be following a similar orbit to this comet by Allan F. Cook in 1973. This stream characteristically produces bright fireballs and rare meteor outbursts.


The Royal Astronomical Society (RAS), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organizes scientific meetings, publishes international research and review journals, recognizes outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3000 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others.


The SPNM project was initiated in 2002 with the aim of studying meteors and fireballs. The project brings together scientists from several Spanish universities and research centres. Since 2004 SPMN has used high-resolution CCD and video cameras to achieve complete coverage of fireballs travelling through the Earth’s atmosphere above the Iberian Peninsula.


The International Year of Astronomy (IYA 2009) celebrates the 400th anniversary of Galileo’s use of the telescope for astronomy. IYA2009 is endorsed by UNESCO and is now supported by 136 countries under the leadership of the International Astronomical Union (IAU). In the UK IYA2009 is backed by the Royal Astronomical Society, the Institute of Physics and the Science and Technology Facilities Council.