The Sept. 11 attacks proved that even vast oceans can no longer protect the American homeland from a determined enemy. Terrorists are eager to kill civilians any way they can, and we must think creatively to counter them.

Stepped-up airport security procedures, combined with passengers and crew on high alert, make any future attempts to hijack airplanes unlikely. But that's not the only way terrorists could attack from above.

More than two dozen countries now have ballistic missile technology, including international pariahs Iran and North Korea. Some of those missiles could be topped by the most dangerous technology mankind has developed: nuclear weapons. In 1972 there were only five "nuclear nations." Today there are 10, including North Korea and Pakistan -- the birthplace of A.Q. Khan.

Khan stole nuclear know-how from the Dutch and built Pakistan's bombs. He has admitted reselling nuclear technology to Iran, Libya and North Korea. There's no telling how many other nations or groups (al Qaeda has deep pockets, if nothing else) might have obtained Khan's information, either from him or from one of his clients.

The frightening truth is that rogue nations and terrorist groups won't be deterred by the "Mutually Assured Destruction" policy that kept the Soviet Union at bay for decades. North Korea's Kim Jung-il, for example, has allowed his people to starve while he developed nuclear weapons. He's irrational, so there's no diplomatic way to deter him from launching a weapon.

To protect itself, the U.S. needs to be able to shoot down incoming missiles.

We do have a handful of interceptors on the ground in Alaska and California and missiles on some Navy vessels. Further, Poland may agree to host more batteries that can protect the U.S. and Europe. They should be in place by 2013 and would provide our allies some protection against long-range Iranian missiles.

These ground-based interceptors are a start. They've passed several tests proving it's possible to "hit a bullet with a bullet." But we need to set our sights higher.

Since ballistic missiles fly through space, that's the best place to put our defensive weapons. The Pentagon should be working to develop and deploy space-based interceptors. This wouldn't represent the "weaponization of space" that some fear, because space has already been weaponized (ballistic missiles pass through it en route to their targets). Besides, it would be far better to destroy offensive missiles while they're safely above the atmosphere.

This approach would be decidedly defensive. Missile defense is never offensive, and thus represents no threat to traditional nuclear powers such as Russia and China.

Indeed, although the Russians officially oppose the interceptors slated for Poland, they seem to understand we're determined to deploy it and appear resigned. "Since the U.S. is going to carry this out," Russian Foreign Minister Sergey Lavrov told reporters after a recent summit, "those proposals that we are expecting to receive on paper today seemed to us, as I said, important and useful for the minimization of our concerns."

No defense is perfect, and no one is claiming that missile defense will make us impervious to attack. But we can -- and should -- take every reasonable step possible to protect ourselves from foreseeable and growing threats, such as ballistic missiles.

Twenty five years ago, President Ronald Reagan spoke to the American people, asking what if "we could intercept and destroy strategic ballistic missiles before they reached our soil or that of our allies?"

The answer was simple: The world would be a better and safer place for peace-loving people. There's still plenty of work to do, but Reagan's Strategic Defense Initiative put us on the path to fielding missile defenses -- which are steadily making us safer.

Ed Feulner is president of The Heritage Foundation.