Another Expert Agrees With Dark Comet Theory

Astronomer David Asher (from Armagh University) has agreed with Bill Napier and Janaki Wickramasinghe (Cardiff University) that “dark comets” are real and dangerous.
The following quotes are from a paper by Napier and Asher published in Astronomy & Geophysics.
http://star.arm.ac.uk/preprints/2009/539.pdf

We know that about one bright comet (of absolute magnitude as bright as 7, comparable to Halley’s Comet) arrives in the visibility zone (perihelion q<5AU, say) each year from the Oort cloud. It seems to be securely established that ~1–2% of these are captured into Halleytype (HT) orbits. The dynamical lifetime of a body in such an orbit can be estimated, from which the expected number of HT comets is perhaps ~3000. The actual number of active HT comets is ~25. This discrepancy of at least two powers of 10 in the expected impact rate from comets as deduced from this theoretical argument on the one hand, and observations on the other, is an aspect of the well-known fading problem of cometary dynamics. A similar problem holds with regard to Jupiter family comets (orbital periods <20 years): many more dormant comets should exist in such orbits than are observed.

They present three possibilities, and I’ve abbrevaited the first two, which they dismiss:
1. They might turn to dust – but the problem with this is that we should see visible comets disintegrating as they round the Sun on a regular basis, and we don’t.
2. They could just lose their ice. “The problem here is that even for albedos pz0.04, characteristic of the inactive surfaces on comets, the Spaceguard surveys should by now have detected ~400 dark comets >2.5km across.”

Thirdly, the comets may develop super-dark mantles, with albedos p<0.01. This is possible if the comet nucleus becomes covered with organic grains ~10–5cm comprising a bird’s nest structure with porosity ~0.7 or more, consistent with that observed in Brownlee particles of probable cometary origin. If sublimation of ices leaves such a structure, then vanishingly small albedos become possible. The nucleus of Comet 19P/Borrelly has developed patches of albedo ~0.008, blacker than anything on Earth outside of nanoengineered surfaces, and if the entire nucleus became this dark we would probably not know that the comet existed.