Now that Comet C/2012 S1 ISON has fallen apart into a sad pile of gravel and dust, the fear merchants are at it again. With headlines like "COMET ISON'S KILLER Debris Wave Heading for EARTH" (I'm not going to link to the various pages and give them the page views, but you can google it yourself if you want to kill a few brain cells) the fear merchants are trying to scare people into thinkg a massive strom of rocks is headed our way.
But it isn't.
ISON's predicted orbit takes it well away from us, the nearest the pile of gravel formerly known as ISON comes to us is 0.43 AU (64,000,000 km), that's a bit under half the distance form Earth to the Sun, or the distance from Earth to Venus's Orbit.
i.e. Nowhere near us. I have previously checked ISON's orbit as it came out from its close approach to the Sun in the SOHO LASCO C3 images, it was right on track. I've done it again for ISON is the high resolution STEREO H1A imager images. This is more tricky as STEREO-A does not have the same field of view as Earth, so I needed to make a special ephemeris using JPL horizons based on the STEREO-A spacecraft's position.
They match to within 8 arc seconds. This means that ISON is on track in it's predicted orbit, and will come no closer to Earth than 0.43 AU. The rather compact debris trail will be high above Earth's orbit heading out to space. Oh yes, and the leftover material will not be "city sized". ISON itself was less than 1 Km in diameter (more like 600 meters), the biggest bits in the leftover gravel will be meter sized, at the outside as big as the Chelyabinsk meteor. If the debris cloud did come near us, we would get a pretty meteor display, but nothing more.
But of course, they will not come anywhere near us.