So I'm starting to GM a RT campaign in the near future, having run Deathwatch, Dark Heresy and a little Only War. Looking through the various books, I'd noticed that the blast radius for a Nova Cannon (1VU from where the shell detonates) is considerably wider than most earth-sized planets (2VU across is about 20,000km - Terra's only 12,000km across). Obviously, this means that any shell detonating on, or near, a planet swathes it in fire. My question is, therefore, what happens on the ground?
How much damage should a Nova Cannon do to a planet hit by the shell? Should it really cover the whole surface of the planet in fire? If so, why aren't fleets armed with Nova Cannons all that's needed for Exterminatus? How do I prevent my players from simply wiping out any planets they feel are hostile once they, inevitably, get their paws on one of these **** things? Any suggestions would be most gratefully appreciated.