The story seems garbled to me.
The story above says:
Instead of the balance that nature demands, the interactions are mainly left-handed, leading Foot and other physicists to argue the mathematical probability of mirror particles, invisible and unable to interact with ordinary particles but providing essential symmetry.
1) If everything in nature was balanced, there'd be as much anti-matter as matter. There's lots of "broken symmetry" in nature. The universe we know wouldn't exist otherwise.
2) If "mirror matter" is unable to interact with ordinary particles, then why would it be in a comet? What would attract ordinary matter to it to cause it to clump up in a comet? It doesn't make sense.
"The ordinary matter gets burned off and you're left with this mirror matter core, which is virtually invisible." This could explain the destruction of 2100 sq km of forest around Tunguska, Siberia, in 1908, generally attributed to an asteroid strike.
So this invisible, non-interacting core is going to be visible and interact with the Earth to destroy a large area of forest???
My recollection is that the [link|http://www-th.bo.infn.it/tunguska/|Tunguska] incident was commonly explained by a fast comet that vaporized before hitting the ground, thus a huge amount of energy was dissipated in the atmosphere but no crater was left. The damage was done by the sound waves, fires, etc. But an [link|http://www.usm.maine.edu/~planet/tung.html|asteroid] is a competing theory as well, as discussed in the above links. It hasn't been resolved yet.
In short, I think Foot is grasping at straws for evidence of something that may exist (on very short length or time scales - just as other strange bits of the universe exist there). I don't think it has anything to do with missing comets or Tunguska.
My $0.02.
Cheers,
Scott.