Critics of a U.S. national missile defense are trying to use the recent test failure as "proof" the technology is unworkable. They're wrong, but the debate threatens to eclipse a more fundamental question: If the United States needs to protect itself from the growing arsenal of missiles around the world, what's the best way to do it?

We think the president's efforts to limit the United States to a ground-based missile defense alone are a mistake. A sea-based missile defense could be built more quickly and for less money than the administration's system. Best of all, it would be more effective.

Certainly, the president deserves credit for recognizing the need (albeit belatedly) for some type of national missile defense. We live in an unsettled world, with an increasing number of unpredictable outlaw nations working overtime to beg, borrow, build or steal the kinds of missiles and warheads that can threaten both their neighbors - Israel, Saudi Arabia, Turkey, Japan, the Philippines, South Korea - and our neighborhoods. We need to be able to defend ourselves against this growing threat.

And the decision to proceed with any type of national missile defense will implicitly acknowledge that the 1972 Anti-Ballistic Missile (ABM) Treaty is a Cold War-era relic. That treaty with the former Soviet Union was based on the premise that neither the United States nor the U.S.S.R. would be the first to use nuclear weapons, because such a misstep would trigger a nuclear holocaust. The treaty thus prohibited national missile defenses for both sides.

Though the White House is still trying to tip-toe around this sensitive issue, any effort to build a national missile defense recognizes that the ABM Treaty (which legally died when the Soviet Union dissolved in 1991) has outlived its usefulness. The world in which we live requires new thinking, new tactics and new technology. In short, we can no longer rely on the deterrent power of Mutually Assured Destruction.

The president favors a single missile-defense site in Alaska where 100 interceptors (estimated cost: $17 billion) would be based by 2007. This is enough, the president contends, to protect the United States from the type of limited attack it might face in the future.

We do not oppose land-based defenses. But they should be America's last line of defense, not its first or only.

Our plan calls for a mobile, sea-based missile defense, based on the Navy's 20-year-old Aegis system, which defends the U.S. fleet against aircraft and cruise missiles. An upgraded Aegis could protect the United States, our friends and allies, and our troops and military bases overseas. It could be deployed in less than five years, at an initial cost of $3 billion. Soon after, space based interceptors could be added, at an extra $5 billion. Since the interceptors would be positioned globally - and could be repositioned as the threat changes - they would be able to destroy target missiles during the "boost" phase of flight, when a missile is more vulnerable and before decoys or mock warheads have been released.

In other words, we would kill the target early, when it is moving more slowly. The president would try to kill it on the way down, when it is traveling much more quickly and is surrounded by decoys.

Our plan would cost less because the United States already has invested some $50 billion in Aegis ships and technology, with 22 Aegis cruisers and destroyers currently deployed. Aegis is tried and true. Today, Aegis interceptors can knock cruise missiles out of the sky. The system easily could be upgraded to protect against ballistic missiles. Adding a space component would improve the targeting, tracking and kill capabilities even more. For all these reasons, it's the best way to go.

And apparently the Pentagon agrees. A still unpublished Pentagon study supports a sea-based system. An earlier Department of Defense study, made public last year, also endorsed the feasibility of the proposal by The Heritage Foundation's Commission on Missile Defense.

The Navy already has started upgrading Aegis. Known as the Navy Theater-Wide program, NTW is intended to defend U.S. forces against shorter-range missiles. Under the Heritage proposal, the Navy would place 650 anti-missile interceptors on the 22 Aegis ships and improve NTW's capabilities to make it effective against long-range missiles.

It's too early to tell whether President Clinton will give a "limited green light" on his missile-defense system before leaving office. But defining the parameters of that system could be the most important decision the next president will make.

Heritage Foundation Vice President Kim R. Holmes is Vice President of Foreign and Defense Policy Studies and Director of the Kathryn and Shelby Cullom Davis Institute for International Studies at The Heritage Foundation. Baker Spring is a Heritage  senior defense policy analyst.