The Chelyabinsk meteroite is estimated to have been equivalent to about 500 kt TNT and mass over 10,000 metric tons. If you arranged the trajectory to be vertical down throught eh atmosphere you could probably get more bang for your bucks, but I suspect most of the energy of a 50 kt scale event would be expended into the upper atmosphere. Also, at those kinds of velocities it would be picked up and intercepted by a planetry defence systm without too much trouble, after all it would be essentially indistinguishable from a pretty minor natural meteorite.
To defeat a planetary defence system you need to get up to a significant francion of the speed of light, not so much to increase the impact energy because you could get the same energy froma slower moving bigger object, but so they get the absolute minimum warning that the object is coming.
The problem is this takes quite a while. To get to 0.9C at 2G thrust would take about 160 days. That means if a tracking system picked up the projectile at 10 AU range (around the orbit of Saturn), it would reach earth in about 80 minutes, but the radio or laser comms signal carrying the warning would take 72 minutes to reach Earth, so they'd only have 8 minutes before the impact to do anything about it.
You wouldn't need to use a scout though. If you dropped off a 30 dton (say 10 metric ton) ship's boat, at 6G it would reach 0.9c in 53 days. The impact energy would be about 90,000 megatons. That's 450x Krakatoa, but only about 1,000th of the Chicxulub impact thought to have wiped out the dinosaurs.
Simon Hibbs