Surprising NASA physicians, astronaut Norman Thagard briskly walked off the shuttle Atlantis when it landed at the Kennedy Space Center today, looking none the worse for wear after four months of weightlessness aboard theMIR space station. Having spent the same 115 days in orbit, the two cosmonauts who hitched a ride back to earth aboard Atlantis were carried from the spacecraft, feeling a bit wobbly.Congratulating the Russian and American space teams for flawlessly executing their mission, President Clinton quipped: "The next time we have any problem between American and Russian officials, I'm going to send them intospace. I think I now know how to solve all international problems." While researchers hope to learn more about the effects of prolongedspace travelby studying the blood and other physical samples from Thagard, NASA concedes that it already has learned one very valuable psychological lesson. Thagard complained of acute "cultural isolation." Astronauts trained to cope with long space flights may need additional preparation for international missions with people they don't know.