I don't follow this: "Imagine that a killer asteroid is heading straight for Earth. With sufficient effort and ingenuity, humanity could work to deflect it. But no-one bothers. Everybody dies. This is clearly not a great outcome, even if no-one has done anything morally wrong (since no-one has done anything at all). This scenario poses a challenge to the adequacy of traditional morality, with its focus on moral prohibitions, or “thou shalt nots”. "

What is the challenge? Plainly it's impermissible to ignore the child drowning in the pond (/to do nothing while the child drowns in the pond—if you're reifying the idea of "doing nothing"). And plainly it's also impermissible to ignore an asteroid flying towards the planet if you're in a position to stop the collision. (One reason to be sceptical of giving weight to this idea of "doing nothing" is that it's very hard to cash out in a sensible way. Does napping count? Standing still? Ignoring someone? Trying hard to ignore someone? Etc. etc.) Unless I'm missing something, I don't see why this would be a challenge for anyone—deontologists included.