There's simply no pleasing some people.

In the 1990s, Congress decided it was time for the United States to build a missile-defense system. This was a reasonable -- even overdue -- step. After all, we'd been completely defenseless against any sort of missile attack since the missile had been invented.

But not everybody liked the idea.

For example, in 2000 the Union of Concerned Scientists issued a report that questioned the technical feasibility of a Ground-Based Midcourse Defense system. The report claimed it would be extremely difficult to hit an incoming missile, especially since our enemies would likely employ countermeasures designed to confuse our defense systems.

"Past U.S. missile-defense tests against missiles using 'countermeasures' did not demonstrate that defenses could defeat such countermeasures," the scientists insisted. Since "an attacker could deploy such countermeasures before even the first phase of the NMD system was operational," they added, "it makes no sense to begin deployment."

To give these scientists their due, it is indeed difficult to "hit a bullet with a bullet," which is what a mid-flight missile interceptor must do. But what they failed to take into account was American ingenuity.

Last month, about 75 miles above the Pacific Ocean, a "kill vehicle" from our bare-bones missile-defense system did what these scientists said wouldn't be possible in practice. It destroyed the mock warhead of a long-range missile. That's especially heartening because similar technology is used in three other missile-defense systems. It wasn't the first time UCS experts were proven wrong. Over the last seven years these four programs have passed their tests -- "done the impossible" -- roughly 80 percent of the time.

The dire predictions of the Union of Concerned Scientists have been, well, shot down repeatedly. The system works. Not perfectly (which is why we must keep testing and perfecting the technology), but far better than expected.

That hasn't stopped the naysayers. They've merely shifted gears. Instead of insisting missile defense can't work, they now declare it works too well. You see, our missile-defense screen isn't static. It's growing, slowly but surely. We have working outposts in Alaska and California, and the military's Missile Defense Agency aims to place others in Eastern Europe, to protect both our allies and ourselves from possible missile attack.

The MDA says this proposed system wouldn't keep Russian missiles out, but some scientists disagree. An Associated Press story last month quoted six scientists who "are skeptical that the U.S. missile-defense system can work." Yet, strangely, "they believe that in terms of raw speed, U.S. interceptors in Poland could catch a Russian ICBM launched from western Russia at any part of the continental United States."

The question Americans ought to ask is, "Why is that a bad thing?" These scientists are undermining their own past arguments. They now insist this technology can protect us, even against threats it's not intended to thwart. That would make missile defense one of the few federal programs to deliver more than it promised.

Many of us have insisted for decades that the United States needs missile defense. A handful of others, including the Union of Concerned Scientists, have long insisted that's not true.

But even they say the evidence shows the system is improving every year. It's better than they ever thought it could be. Better, even, than they think it should be.

Apparently they won't be satisfied unless the U.S. is defenseless against incoming missiles. But the rest of us shouldn't rest until a robust missile-defense system is in place. It's coming along very well. Just ask its staunchest critics.

Ed Feulner is president of The Heritage Foundation (heritage.org).