02 August 2008

What about the Damacloids?

The global inaction on the known and solvable problem of asteroids is bad enough.

It is worse that the results of SpaceGuard seem to be making people complacent.
See http://www.nature.com/news/2008/080625/full/4531164a.html Fewer people are searching for near-Earth asteroids, astronomer David Morrison said in the 1990s, than work a shift in a small McDonalds. But that group — a little larger now — has over the past two decades discovered a host of happily harmless rocks, and in doing so reduced the risk of an unknown asteroid blighting civilization. [Note, SpaceGuard has not taken any rocks out of the sky, and has found less than 1% of the estimated population of Tunguska size impactors, and is still missing 21% of the expected civilization ending Asteroid population]

But what about the Damacloids?


The Dark Comets?

In this paper http://astrobiology.cf.ac.uk/MNPAPER.pdf authors W. M. Napier, J. T. Wickramasinghe and N. C. Wickramasinghe argue:

Dynamical balance arguments that involve the capture of long-period comets from the Oort
cloud imply that there should be ∼1000 times more Halley-type objects than are actually
...Here we demonstrate that the surfaces of inactive comets, if composed of loose, fluffy organic material like cometary meteoroids, develop reflectivities that are vanishingly small in visible light. The near-Earth objects may therefore be dominated by a population of fast, multi-kilometre bodies too dark to be seen with current near-Earth object surveys. Deflection strategies that assume decades or centuries of warning before impact are inapplicable to this hazard.

We propose, as a solution to the paradox, that the surfaces of inert comets become extremely dark. An immediate consequence of the above solution of the ‘missing comet’ problem is that there exists a large population of extremely dark comets in Earth-crossing orbits, which are undetectable with current near-Earth object (NEO) search programmes but are nevertheless impact hazards (cf. Bailey & Emel’yanenko 1998)...the mean interval between impacts of such bodies is ∼0.67 Myr, with impact energies 1.5 × 106 Mt, for a mean impact speed ∼60 km s−1 (Jeffers et al. 2001). Impacts of at least 1.5 × 107 Mt energy are expected at mean intervals ∼2.3 Myr...Rickman et al. (2001), on the other hand, find that comets yield a large, perhaps dominant, contribution to km-sized impactors, estimating for example a terrestrial impact rate of about one Jupiter family comet (active or dormant) per Myr. While the uncertainties are large, it seems unavoidable that, if the present hypothesis is correct, a dormant Halley population represents a major if not dominant global impact hazard at the present time...The Oort cloud is demonstrably sensitive to Galactic perturbers of various sorts – stars, nebulae and tides (Napier & Staniucha 1982; Byl 1986, etc.). Nurmi, Valtonen & Zheng (2001) find that the flux of comets from the Oort cloud, and hence the impact rate, may fluctuate by an order of magnitude arising from the motion of the Sun with respect to the Galactic mid-plane. As we are at present passing through the plane of the Galaxy, it is expected that the current impact rate is several times higher than that deduced from the lunar cratering record, which is time-averaged over one or two Gyr. The dark Halleys, on this hypothesis, are a link in the chain between Galactic disturbances and huge impacts. That passages of the Sun through the spiral arms of the Galaxy might induce terrestrial disturbances has been discussed by a number of authors (e.g. McCrea 1975). Napier & Clube (1979) specifically proposed that bombardment episodes might occur during such passages, leading to mass extinctions and other trauma. Leitch & Vasisht (1998) have shown that, indeed, the Sun was passing through a spiral arm during the Cretaceous-Tertiary and Permo-Triassic extinctions of 65 and 250 million years ago, which also coincided with the Deccan and Siberian trap flood basalts. On this model, each of these was caused, not by a stray asteroid, but by an episode of cometary bombardment. For at least some of the great mass extinctions, multiple impacts coupled with climatic trauma appear to have been involved (e.g. McGhee 1996; Keller 2002). Current detection and deflection strategies involve the assumption that decades or centuries of warning will be available following the discovery of a threat asteroid. However if the major impact hazard indeed comes from this essentially undetectable population, the warning time of an impact is likely to be at most a few days. A typical Halley-type dormant comet spends 99 per cent of its time beyond the orbit of Mars and so a full mapping of this population is beyond current technology.

[Lesson: We need to get "out there" with IR telescopes an deflection capabilities, not sit on our complacent hands trusting we are several rolls of the dice away from a double six.]


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