The biggest problem with Whipple shields with relativistic projectiles is that the projectiles literally don't have time to react. Partly because of the speed i

The biggest problem with Whipple shields with relativistic projectiles is that the projectiles literally don't have time to react. Partly because of the speed itself, partly because of time dilation. You need to throw enough at it that a large chunk of the nuclei in the impactor are hit by nuclei of the atoms you are throwing at it - not an insignificant feat. Small moon may work. Anything less and yes, it's rapidly expanding into a cloud of gas, but nowhere near fast enough. It'll hit the target well before it dissipates significantly if you're not careful.