I was always told to be wary of any comet that doesn't show a tail. That's because the tail is streaming out directly behind it, opposite to its direction of travel, so you just can't see it.
Of course, that may not be the only reason for the absence of an observable tail.
Comet Holmes orbits the Sun once every 7 years at a distance of about 200 million miles (compared to Earth's 93-million-mile orbit). As a result, it was re-observed in 1899 and 1906 before being lost for nearly six decades. Based on a prediction by Brian Marsden, of the Minor Planet Centre, the comet was recovered in 1964.
It just so happens that there was a large impact near Tunguska, in Northern Siberia, around the time of the second observation in the early 20th Century, which coincided with a similar outburst from this very comet.
Given that the resources required for tracking and compiling trajectories for these objects are mostly in the hands organisations and/or institutions which are capable of withholding calamitous information from the general public, the question must be asked:-
Would 'They' tell us if it was going to hit?