NASA is hot to high-energy impact a comet. When I ran the numbers I got 4.2x10^10 joules or almost exactly 10 tonnes of TNT equivalent. (Assuming the English units are correct, do the metric conversions yourself. NASA creative bookkeeping strikes again.) NASA wants to spurt up a bunch of vaparized comet to do spectroscopy and see what is in there. However...

Comets, by whatever model you choose, are not well consolidated. An 820 pound impactor a yard across packing 10 tonnes of TNT equivalent of kinetic energy is the ever so sharp tip of an icepick screaming into a block of dirty ice at 6.6 miles/second. Upon impact it will transform into a directed dense hypersonic plasma jet as it passes through a few times its own mass of comet - still going forward. That is how a shaped explosive penetrates armor. The icepick tip is now a buried hollow point bullet. It will bloom and suddenly deposit energy deeply subsurface.

The Official Truth

The most likely scenario of a small dense body hypersonically (6.6 miles/second, Mach 33) impacting a low density loosely consolidated weakly gravitationally bound 3.7-mile diameter body is to burst it. Check out a high velocity small caliber hollow point bullet hitting a watermelon. SPLAT! One sees the same behavior in a hollow point bullet entering a transparent block of ballistic gelatin: a thin penetration line terminates in a huge internal cavity suddenly billowing. Comets are brittle not elastic.

NASA says they will dig a crater seven stories deep and 300 feet wide at the surface. (The resulting thrust changing the comet's orbit is, ah, ignored.) Uncle Al predicts NASA will shatter comet Tempel 1 and suddenly have all sorts of good reasons to begin frantic no-budget-is-too-large studies. The party is PERTed for birth on 04 July 2005. Knocking a big chip off the old block with a coincidentally lethal Earth-bound trajectory would do it.
_________________________
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/qz3.pdf