WE ALL KNEW that an exchange of nuclear missiles between the United States and the Soviet Union was bound to happen, and it has. The Soviet SS-20 landed right here on the Mall; our Pershing II was dead on target in Moscow.

The precise point of impact of the Russian rocket was the Milestones of Flight Hall of the National Air and Space Museum, where it was handed over with a handshake by Vladimir A. Kartamyshev, the Soviet Embassy counselor. A similar ceremony marked our donation of a Pershing II for display in Moscow.

In this case of course the 54-foot Soviet missile stands as a milestone of a flight that didn't happen. The SS-20 and the Pershing II are the first casualties of the peace that has broken out between the former cold warriors. All are to be scrapped except for a few disarmed specimens saved for display. "Let us hope for the day when the only missiles in the world will be museum pieces," Kartamyshev said.

The Smithsonian's SS-20 is flanked by a Pershing II, and accompanied by an exhibit on the progress and procedures of arms reduction. Displayed without fanfare is a Soviet spy satellite photo of Washington, something secret agents might have died for not so long ago. Ironically, there's nothing in the space reserved for a copy of the "verification document" certifying the status and whereabouts of the Russian rocket. It hasn't yet been declassified by the American government.

So blase have we become about glasnost that the missile exchange caused hardly a ripple. But at least one child of the cold war, trained to duck and cover in kindergarten, found himself happily dazed by the experience. And a visitor who wandered by after the ceremony stood before the Soviet missile with tears running down her smiling face.

"I have a son in the Army," she said.