There's a consequential debate in Washington about a single word that has guided America's missile-defense policy and spending for almost two decades. That word, "limited," has been a thorn in the side of congressional missile-defense advocates, led by former presidential hopeful Sen. Ted Cruz (R-Texas). And they are using the Senate's version of the 2017 defense-authorization bill to excise it from the National Missile Defense Act of 1999.

According to that law, it is U.S. policy to "deploy as soon as is technologically possible an effective National Missile Defense system capable of defending the territory of the United States against limited ballistic missile attack."

The goal of trying to protect against such a limited attack has not noticeably limited spending on the Ballistic Missile Defense System and its components. Funding for the Missile Defense Agency has hovered between $7.5 billion and $9 million annually for the past ten years. The agency has requested $7.5 billion for fiscal year 2017.

Tests of the Ground-Based Midcourse Defense system, the terrestrial arm of the architecture, have seen mixed results over the past years, and critics argue that the test conditions don't come close a real-world intercept scenario – limited or not.

The White House earlier this month rejected the Senate bill’s language, arguing in a statement that striking "limited" from the U.S. policy would raise the expectation that the system is effective against nuclear missiles from Russia and China, which it is not. Atomic weapons from major global players are best kept in check by way of deterrence, which means maintaining a "modern and robust" nuclear arsenal, White House officials contend.

Besides those geopolitical arguments, the technical obstacles of an unshackled, limitless ballistic missile-defense program should give reason for pause. There is a dedicated constituency in Congress already chomping at the bit over the prospect of increased spending and branching out into technology areas that previously consumed billions of dollars without much to show for.

Without the moderating effect of the word "limited," ideas like the creation of a space-based missile defense layer, as advocated by Rep. Trent Franks, R-Ariz, would surely gain greater weight and, if implemented, eat up large sums of scarce defense dollars with an uncertain return on investment.

Congressional appetite for greater BMD spending was on full display at a hearing of the House Armed Services Committee in April. Franks urged the witnesses to talk up the chances of fielding a space-based capability, only to be reminded by MDA Director Vice Adm. James Syring of the "overwhelming" costs such a project would bring.

"I have serious concerns about the technical feasibility of the interceptors in space, and I have serious concerns about the long-term affordability of a program like that," said Syring.

Also on the table with an unrestricted mandate for more missile defense –  or even the perception of one – also on the table could be the full-fledged revival of a program to shoot down enemy ballistic missiles during their boost phases with an airborne laser. While the Defense Department had to terminate an ill-fated, multibillion-dollar attempt to that end, officials are now pursuing the concept on a much smaller, experimental scale.

The cost of that effort is $23 million in fiscal year 2017, and $278 million over the next five years.

Congress should let the science play out on missile defense before rushing for an expansion.

More In Intercepts