ARE THE visitors gone yet? Well, it won't be long before they vacate for the summer, and we have room to maneuver in the Air & Space Museum again.

When you revisit it, you'll see that the space shuttle exhibit -- "America's Space Truck" -- has been updated. Besides the usual 16-foot model of Columbia, there are photographs and space tools and a video screen on which you can call up profiles of shuttle missions and crews. You'll meet Julie (for Joint Utilization of Laser Integrated Experiments). It was a shuttle payload or "getaway special," sponsored by a hospital in Milwaukee for a modest fee. The canister rode in a cargo bay, where its contents -- aspirin, penicillin and surgical suture material -- were subject to cosmic radiation, laser beams and weightlessness. Shuttle payloads even have their own uniform badges.

You can't look at the exhibit without thinking at length on the Challenger disaster. A plaque here commemorates the lost astronauts. Ironically, one photo shows Judy Resnick learning how to make a parachute landing during survival training. And some of the things in the shuttle exhibit we already know, having learned them through news accounts. There is something excruciatingly familiar about the parts of the model -- the orbiter, the external tank containing propellants for the main engine, and the solid-fuel boosters, now being redesigned. Contributing factors to the accident -- "a flawed decision-making process leading to permission to launch and insufficient attention by the agency to safety" -- are duly noted.

It is a small, restrained exhibit, about hindsight. AMERICA'S SPACE TRUCK --

At the Air & Space Museum indefinitely.