Why are there so many nuclear mushroom clouds on the horizon? On television, anyway (and let’s hope it stays that way).

“24” kicked off its most recent season with Jack Bauer not saving the day. At the end of the fourth hour, the terrorist villains managed to set off one of the five “suitcase nukes” they possess, destroying an entire suburb of Los Angeles, if not more.

Meanwhile NBC’s popular “Heroes,” “24’s” time-slot competitor, has got a nuke – or possible nuke – of its own: The superhero drama is unfurling a story line in which Peter Petrelli sees a vision of New York City going up in a nuclear blast.

As if that’s not enough, CBS’ “Jericho,” which returns Feb. 21, started off the fall season with a nuclear bang. Residents of the small Kansas town in the title have spent the last few months trying to figure out why the bomb they saw on the horizon was set off and how widespread the damage across America is.

Last summer, who would have thought that a show with that kind of grim premise would be one of the few serialized dramas to find a regular audience?

Perhaps the most pertinent question is, why is today’s escapist entertainment being invaded by such terrifying imagery? Maybe because television is able, in the case of these particular shows, to take the unthinkable and make it look less threatening. A little.

In “24,” it was “only” a suitcase nuke that went off, and the damage appears to be contained in one part of Los Angeles. Executive producer Howard Gordon told reporters the day after the Season 6 premiere that the show wouldn’t spend much time in or near the center of the blast or the fallout zone.

“We sort of are at the edges of (the zone), but it does create a kind of baseline anxiety,” Gordon said.

No kidding.

Perhaps these blasts are a sign that it’s even more difficult for “24” to raise the stakes yet again and create an even bigger crisis for Jack Bauer to deal with. It’s hard to see how the Fox drama will top itself this time, but we’ve said that before.

“Heroes” has a more comic-book approach, so Peter’s visions don’t seem quite as threatening as the events on “24.” which can seem far-fetched at times but are rooted in a recognizable, nitty-gritty reality. But the “Godsend” episode of “Heroes” also featured a comical interlude in which Hiro stole what he thought was a mythical sword from a museum – only to find that it was a fake.

Still, those mushroom clouds are pretty dark stuff, and they have to be an obvious sign of our national unease. Since Sept. 11, some part of our collective unconscious has been waiting for more death and destruction to arrive. And it has, at least on these fictional dramas – which, perhaps not surprisingly, give us hope that we can endure or prevent such nightmarish scenarios.

All three shows hold out the hope that these kinds of terrifying crises would provide a chance for well-intentioned people to find a solution together, and that there is a solution. “24” lets us believe that, whatever the crisis, efficient government operatives in a shiny, high-tech facility will figure it all out on our behalf. It’ll all be solved by tomorrow – that’s the show’s reassuring fantasy.

“Jericho,” the show that deals most heavily with the post-nuclear scenario, is, strangely enough, often the most optimistic of the three shows. At one point last fall, residents of Jericho set up an impromptu post-blast barbecue in the town square (the local grocery store’s supplies would have otherwise rotted).

Would we all act so normally and eat pork chops and burgers with our neighbors if we’d just seen a nuclear blast over our homes? I hardly think so. I tend to think it would be more “Lord of the Flies” than “Our Town.”

But then, we want our TV characters to be, for the most part, a little more noble than we are. Having seen endless seasons of “Survivor,” we know that real people marooned on islands get nasty once the food runs out. Tell that to the inhabitants of “Lost,” who’ve rarely fought over so much as a stale cracker.

Truth is, there’s a lot of wishful thinking amid all the television darkness, but then, maybe we’re a nation in need of that.

We want to believe we could survive the unthinkable, deal with a nuclear crisis and remain united. Whether we actually could or not – well, maybe that’s a topic for an HBO show.