Justin,

A Whipple shield with an outer wall that was an inch thick would probably not completely disintegrate a rock that was a foot in diameter. The physics of the disintegration process would be something that a nuclear weapons designer would readily understand. For example a 1/2 inch rock striking a one inch thick outer wall at 0.1 C would instantly vaporize into plasma at the contact point between the rock and wall. A hellish shockwave would pass through the rock's material (like the shockwave running down a stick of lithium deuteride in an H-bomb). The shockwave would vaporize everything it passed through until the shockwave attenuated away. This attenuation process would limit the maximum radius of the shockwave that propagated in the outer wall's material. The original contact point between the rock and wall would expand as the rock penetrated the wall. The rate of penetration would be much faster than the speed of the shockwave so the rock's internal shockwave would actually initiate across the entire front face of the rock instead of at a single point. For the Whipple shield to work, the entire rock must be processed into plasma by its shockwave. It's obvious that there is a maximum size of rock that a given thickness of outer wall could defend against. The worst case rock impact would set the standoff distance between the inner and outer walls. The inner wall should have a highly reflective ablating heatshield material on it to take care of the plasma's heat. Justin incorrectly said the plasma's radiation would be infra-red. The plasma's color would depend upon it's temperature which would depend upon the rock's initial diameter and velocity (kinetic energy = 1/2 M v^2). The temperature would be very hot. The rule-of-thumb for spacecraft heat shield shocklayers (which is almost certainly wrong for this case) is 1000 deg.K for 1 km/sec entry velocity. If we assumed 30^4 km/sec then the rule-of-thumb says the shocklayer temperature would be 30 million deg.K (no way this is right but the plasma would probably be a fully electron stripped like in the core of a nuclear weapon's explosion). The temperature "color" would probably be X-rays. It might be desireable to have several sheets of plastic foam with vacuum spaces between the inner and outer walls. Each sheet of plastic foam would absorb and re-radiate the X-rays from the outer wall (like the secondary section of a thermonuclear weapon).

Now the Whipple shield would only be part of the package. For example what do you do about big rocks (1 foot in diameter or larger)? Obviously the front end of the starship needs a phased array radar on it that could look ahead of the starship for at least a million miles. When the radar saw something big then the autopilot would fire off some solid rockets on the side of the starship so it could dodge the rock. Another issue is the front of the starship would be quite hot due to collision with interstellar gas (0.1 cm^3). This heat would be a "blessing in disguise" because it could provide power for the ship while it was in interstellar space with it's propulsion system shutdown. The wake of the starship would be at cryogenic temperature if no heat was flowing to it. A practical design would require heat to flow from the shield to radiators in the starship's wake. This heat flow could drive an energy generation system for powering the autopilot and radar system. A starship shield concept is workable but there would be lots of fiddly aspects to a practical design.

Gary