The spectacular hit marks a definitive turn in the debate concerning missile defense, from whether it's technically possible to whether it's ethically desirable. Many of the same people who'd argued for years that missile defense couldn't be done now will complain that it constitutes a nefarious "weaponizing of space."

The United States normally isn't in the business of shooting down satellites. It took out the dead National Reconnaissance Office satellite because it had a full, 1,000-pound tank of toxic rocket fuel that there was some slim chance could fall on a populated area when it re-entered the atmosphere in a few weeks. Now, the hydrazine fuel appears to have burned up in an explosion in space, and small pieces of the 5,000-pound satellite will fall harmlessly to Earth.

The satellite wasn't a missile launched with just minutes warning, but hitting it is still a major success for our missile-defense system. The window for a successful strike was about 30 seconds, the speeds involved were mind-boggling, and all the same technologies that would be deployed against a missile - a Standard Missile 3 rocket launched from an Aegis-class cruiser and a battery of radar and sensors - were in play.

China immediately lashed out. A Foreign Ministry spokesman said that they'll continue to monitor "the possible harm caused by the US action to outer space security and relevant countries." The Chinese commitment to "outer-space security" was recently exemplified by their shooting down an aging weather satellite with no warning, then denying that they had done it for two weeks, and doing it at an orbit so high that 1,600 pieces of space debris will clutter Earth's orbit for years.

The Chinese test - of a system that is explicitly designed to target satellites - didn't produce much outrage from arms controllers. It's long been an axiom of arms control that whatever the United States does is dangerous and a provocation to other countries, while our adversaries are merely forced into hostile or irresponsible acts by our recklessness. But the US position on space - like our position on the high seas - is that everyone should have full and free access to it for peaceful purposes.

What we have resisted is getting pushed into an unenforceable treaty against weapons in space that could hamper our ability to address threats in the future. So many weapons can be transformed instantly into "space weapons" if they are used against targets above the Earth's atmosphere - as we've seen with the SM-3 missile - that banning them is impossible. The real agenda of the Russians and the Chinese is to keep us from ever putting missile-defense interceptors in space. That would enhance our capability against their intercontinental ballistic missile (ICBM) arsenals.

This is how deep the Chinese/Russian commitment to the peacefulness of space runs: They want the option of launching ICBMs into space, where they'll travel undisturbed until they re-enter Earth's atmosphere en route to visiting untold devastation on a target. China could make a genuine gesture toward peace in space by ending its rapid ICBM buildup, but its true interest is in preventing us from checking its missile threat to us and our allies.

Space has been weaponized at least since the Nazis launched V-2 rockets against Britain. Today, we use satellites not just for commercial purposes, but for intelligence and military command and control. That's why the Chinese are so keen to be able to shoot them down.

Space isn't a pristine last frontier unsullied by human competitiveness and ferocity, but an extension of our flawed world down here below. It can be dangerous, which is why it's a comfort that we're building defenses against threats more serious than a tank of hydrazine.