Corrected entry: When the Messiah mission withdraws from the Comet to remote-detonate its nukes, the implication is that the nukes must be let off NOW to have any chance of success, and the Messiah is thus caught in the blast and damaged, conveniently putting it out of communication with Earth for the duration of its journey home. They are months away from impact and there would be time to fire up the main drive and make the return burn before detonating the nukes, thus putting a safe distance between themselves and the blast. The only argument against this is that if the attempt failed they might go back to the comet with the remaining nukes for a second try - this is never suggested, so why not get out of there?
Correction: It may be that they had to be within a certain range in order to send the signal to the detonators.
Corrected entry: In the scene where the comet hits the ocean, there is a massive traffic jam just 5 miles from the beach. Since everyone has known about the comet for over a year, wouldn't they have headed away from the ocean a few days/weeks/months earlier?
Correction: There was only reason to move away from the ocean after the President announced, quite close to impact time, that the smaller portion of the comet would cause tidal waves. Before, with the comet as a whole, it wouldn't have mattered how close you were to the coast.
Correction: There still was the plan with the missiles, so many people probably were hoping that would do the trick. The message from the president that the missiles had failed was only several hours before the impact.
Corrected entry: The astronauts' communication with their loved ones was without delay, when in fact it should have been a couple of seconds.
Correction: Just like with the entry of the tidal wave taking 45 minutes to reach shore (with the correction being it would be too boring to wait the 45 minutes) it would be boring to have to sit through the seconds of delay as well.
Correction: It was said that the delay was because of the distance from Earth. When they say goodbye to their loved ones, they are much closer to Earth (only a few hours before impact), so there no longer is any (noticeable) delay on the line.
Correction: Actually, the reason they have to leave the comet is because of a very low fuel tank. They only had enough to get to the comet, land, get off and get back. If they were to land on it a second time they wouldn't had enough fuel to return home, and most of us would remember the scene where they decide to sacrifice themselves? They didn't plan a second launch until the suicide launch.