It’s basically a biological equivalent to a Von Neumann self-replicating probe; a constructed living organism which would feed upon interstellar matter and use the waste as propellant. See the essay, “Engineers’ Dreams” from Dyson’s Infinite In All Directions.
Stranger
Well, but that doesn’t answer why the Astrochicken crossed the Milky Way, now, does it?
Going back to the OP and energy yields, it seems to me that the speck is by weight quite up to the task of matching a nuclear weapon in destructive power.
That is, if it would be possible to scale down an A or H bomb to sand grain size, the yield wouldn’t be quite as high as the speck traveling at ludicrous speed.
Not so surprising–the percentage of mass in an A-bomb that’s actually converted to energy is pretty small. Relativistic effects notwithstanding, the energy contained within a dust speck traveling near the speed of light is still same order of magnitude as mc[sup]2[/sup], where m is tae mass of the entire speck.
Because it could?
If the speck passes completely through the spacecraft, leaving a pinhole, what proportion of the KE is actually transferred to the craft?
RR
Only as much as it needs to punch through the structure. If there is no effective momentum transfer, there is no kinetic energy transmitted. The same is true with radiation; very high energy gamma rays like those found in cosmic radiation, for instance, can often pass through soft tissue without doing any damage. No viable transportation spacecraft is going to be able to allow holes to be punched through it; however, a small “whispy” micromodular probe (think the big blobby cloud monsters that always figure into Star Trek plots) could accept this kind of damage and keep going.
Stranger
Actually, by the time we have the technology to accelerate to a percent of light speed and try going to other stars, having holes punched in a ship might not be a problem. They’re already experimenting with ideas that could yield a strong yet rubbery, self-regenerating material for use in just about anything. The example application was in cars, where they might automatically repair themselves after a fender-bender. In a spaceship, it could be designed to encourage penetration and then to seal the tiny holes before any significant atmosphere is lost. We’re decades away from materials like that, but we might be closer to those than we are to interstellar travel.
As inspiring as Dyson was, using waste as propellant, invoking nasty chickenology to boot, probably isnt inspiring in my minds eye the same thing that Dyson envisioned 
Exactly!
I agree the dust has to be relatively thin though. They have pics of galaxies IIRC 14 billion light years away. That means enough photons for the galaxy to be seen happened to be heading our way, and traveled (using google calc) 822,989,974,000,000,000,000 miles without running into anything that’d block them.
I got annoyed when my car died and I had to bike a few lousy miles to work. Kinda of puts things in persepective.