The book may be a work of fiction, but the premise is not that far fetched. Today, astronomers are aware of a Potentially Hazardous Asteroid (PHA) labeled 2012 DA14 which will miss the earth by less than 25,000 miles or approximately one tenth of the distance between the earth and the moon. Granted, this PHA is not a comet and is a rather small asteroid measuring about 60 yards wide. If it were to hit the earth the impact would be equivalent to a 2.4 megaton nuclear bomb. The resulting crater could be about a mile wide and 500 feet deep. The date of the upcoming close encounter is February 15, 2013. Hypothetically speaking, what is the possibility that if it were to hit the earth, scientists would have the faintest idea exactly where or when the impact would occur? Even if they knew (or guessed) the location of ground zero, what are the odds the local or federal governments would have any ability to prepare for the event or react to the devastation after the fact? I'm not confident.
Not hypothecially speaking... the last time a large enough object was detected early enough, it was filmed from aircraft as it fell, and picked up after it fell.
Them math guys... They know when and where stuff meets atmosphere.
/johnny