Timing is everything, and you have to applaud the guys at NASA... when it comes to public relations, these guys are real rocket scientists!
With the Deep Impact mission, not only did they perform the equivalent of hitting a moving bullet with a BB at a range of 1000 miles, they also timed it beautifully. Making the actual collision between the probe and the comet occur on a holiday weekend ensured that
Having worked in news for 16 years, I came to know the calendar very well. There are just certain weeks and extended weekends where there is little news to cover. When you've got the time to think and plan, you can take advantage of slow news days to extend the value of the coverage you get. The week between Christmas and New Year's Day is among the worst. So is the weekend after Thanksgiving, which is why merchants and retailers want to focus the world on shopping. (By the way -- the day after Thanksgiving is not the biggest shopping day of the year... but a lot of people still buy into that hype.)