It’s unlikely to happen — some say we’ll never see it, some say it’s inevitable and right around the corner. Either way — if a comet does in fact hit the United States and, more specifically, Manhattan, it is expected to cause $1 trillion in damage and 3.2 million deaths.
This is according to Risk Management Solutions, the Newark, California-based catastrophe modeling company. They based their study on the 1908 asteroid explosion that rocked Siberia. The Tunguska Event, as it’s often referred to, occurred near the Tunguska River and knocked over an estimated 80 million trees covering 830 square miles. Several amazing eyewitness accounts of the event are listed here.
The entire RMS report is available here. I highly recommend it as it includes a history of the Tunguska event, along with probabilities of such an event happening again and an in-depth analysis of insurance implications for this type of catastrophe.