A couple of years ago, two volcano movies erupted onto the nation's screens just months apart (remember "Dante's Peak" and "Volcano?"). This year, moviegoers are threatened by tandem space rocks hurtling madly toward our multiplexes. In Hollywood, these days, disasters come in twos.

Later this summer, expect "Armageddon,'' with Bruce Willis saving the world from a hulking, earthbound asteroid -- that is, if Earth survives this weekend's arrival of "Deep Impact,'' a film that has Robert Duvall on a collision course with a big chunk of celestial fallout.

"Deep Impact'' is a star-studded affair directed by the smart, tough Mimi Leder (who raced pulses in last year's "Peacemaker'') from an ambitious script by Michael Tolkin ("The Player'') and Bruce Joel Rubin ("Ghost''), and its arrival marks the first significant explosion of the summer blockbuster season.

But, as blockbusters go, "Deep Impact'' is more metaphysical than most. Not only is it concerned with big-bang pyrotechnics and monumental special effects, it's also interested in how impending disaster affects its characters' lives (the, ahem, deep impact of such catastrophes, a la "On the Beach'').

But, oddly, that approach, which should be its greatest strength, turns out to be its greatest failing. For while "Deep Impact'' deserves credit for trying to do more than the average disaster flick -- simply heaping death and destruction down on a lot of cardboard-cutout characters -- it's so packed with people, incident and narrative detail that it practically collapses under its own weight.

The overpacked story is related through the lives of three characters:

There's Jenny Lerner (Tea Leoni), a hot-shot MSNBC reporter who thinks she's stumbled onto a bimbo- gate story about presidential infidelity with someone named Ellie. Turns out, her tip concerns a top-secret government project dubbed E.L.E. -- for "Extinction Level Event.'' That's a comet the size of Mt. Everest that's racing toward Earth, and when it lands in one year it could do to us what a previous event did to the dinosaurs.

Anyway, while Jenny is pursuing the story of her career, she's also struggling with the wrenching breakup of her aging parents' marriage (Venessa Redgrave and Maximilian Schell).

There's also Leo Biederman (Elijah Wood), the brainy high school astronomy student who first sights the coming comet and, with the end of the world in sight, pursues a star- crossed romance with the girl next door, Sarah (LeeLee Sobieski).

And there's Spurgeon Tanner (Duvall), a veteran NASA moonwalker who's assigned to a team of young, cocky rocket jockeys who will man a Russian-American space shuttle designed as Earth's first line of defense. The plan: the space craft, armed with six nuclear-warhead moles, will land on the comet, plant the bombs and blow the big rock to smithereens -- well before it enters Earth's atmosphere.

But folks looking for lots of dazzling special effects might feel shortchanged. Other than a few good on- the-comet effects and a big tidal- wave-swallowing-Manhattan sequence at the end, this film is a relatively low-keyed affair.

Leder and company spend inordinate amounts of time playing out the little psychodramas of these people's lives, occasionally ratcheting up the tension as the comet races nearer. Problem is, with so many players and so much to do, she really doesn't have time to develop any the characters to a satisfying degree.

There are some nicely modulated performances that help cover the story's essential thinness. Morgan Freeman lends gravity to the film as the quietly paternal U.S. president. And the always-reliable Duvall has a couple of magic moments, especially when he sits down to read passages of "Moby Dick" to a gravely injured comrade.

But, for the most part, what we get here are emotions delivered in shorthand and a lot of mechanical, tearjerker moments that pass for human drama -- all topped off by one pretty spectacular special effect. As a result, "Deep Impact" seems mildly engaging, but, well, rather shallow.