Space rock risk underreported, researchers argue

 A small but growing number of astronomers are arguing that the risks of comets or
 meteors hitting Earth are much higher than past estimates suggest.

 Some of these objects may be going unnoticed in space, the researchers say, and
 scientists may need to begin new studies tailored to finding them.

 But advocates of the earlier estimates are shooting back that the evidence doesn't
 warrant revising the figures drastically.

 The traditional estimates, based on sky surveys and other techniques, vary. 

 But in general, they conflict with the number of objects actually found to have
 visited Earth's neighborhood, according to David J. Asher of the Armagh Observatory
 in Northern Ireland and three colleagues.

 These numbers may provide better estimates, they wrote in the October issue of the
 research journal Observatory, because they "represent observational `ground truth'
 so far as impact statistics are concerned."

 To re-estimate risks from these limited data, the team made a simple - but to some
 critics, questionable - assumption.

 Basically, if a given type of object struck once in the past two centuries, they
 assumed it hits once every two centuries on average. A somewhat longer, but
 essentially similar calculation translated known near-misses into impact
 probabilities.

 They analyzed in this way three well-studied types of object either seen to have
 passed near Earth or believed to have hit recently. They also took into account
 other flybys whose occurrence they deduced from meteorites.

 Their revised estimates in the Observatory paper were as follows:

* Impacts greater than 10 megatons (about 500 Hiroshima bombs), like a 1908 explosion
 in remote Siberia widely attributed to a space object, would occur at least every 300
 years. That's almost tenfold the rate generally accepted estimates suggest. The blast
 in Tunguska, Siberia, felled an estimated 60 million trees.

* Strikes of a hundredfold greater destructive energy or more would happen every 3,000
 years or so, more than 10 times as often as prevailing estimates.

* Objects known as active comets packing a 100-million-megaton punch - an event like what
 may have killed off the dinosaurs along with more than half the Earth's other species -
 would strike once every few tens of millions of years. That's about 100 times more often
 than a conventional figure of every 3 billion years. The team didn't present revised
 estimates for another class of comets, inactive comets, thought to hit every 150 million
 years or so.

 "We're not trying to make a precise estimate," Asher said. "We're trying to point out
 that if you accept all the current estimates, then the three or four single objects we've
 mentioned, these together don't fit prevailing views."

 The work was in part a follow-up on earlier findings by researchers at Cardiff University
 in Cardiff, U.K., one of whom also co-authored the Observatory paper. 

 The earlier study, which appeared in the Nov. 2004 issue of the research journal Monthly
 Notices of the Royal Astronomical Society, proposed that most potentially dangerous
 comets are unseen because they're too dark.

 March 21, 2006
 Special to World Science

http://www.world-science.net/exclusives/060320_impactfrm1.htm

-----------------------------------------------

 Early extinction
 www.perceptions.couk.com/thumlate.txt

 Meteor Impact Database
 www.unb.ca/passc/ImpactDatabase/

 Terrestrial Impact Craters
 www.solarviews.com/eng/tercrate.htm

 Reclassification
 www.perceptions.couk.com/uef/thump.txt

 Reclassification
 www.perceptions.couk.com/uef/thump2.txt

 Reclassification
 www.perceptions.couk.com/uef/thump3.txt

 A plea
 www.perceptions.couk.com/uef/complasens.txt

 Calculate the mega-tonnage odds per year
 www.perceptions.couk.com/chance.txt

-----------------------------------------------

Our crowded Solar System

http://cfa-www.harvard.edu/iau/lists/OuterPlot.html

http://cfa-www.harvard.edu/iau/lists/InnerPlot.html

http://cfa-www.harvard.edu/iau/lists/InnerPlot2.html

-----------------------------------------------
 
              FURTHER REFERENCES

 GO - "search perceptions" - in SEARCH-ENGINE


file-ID www.perceptions.couk.com/uef/thump7.txt