ALTHOUGH the National Aeronautics and Space Administration may not have planned it that way, its manifest of nine space shuttle missions for 1992 reads like a salute to the Columbus 500th anniversary year.Not only does it have a strong international flavor, but it features the ascent of the first Italian Space Agency astronaut aboard Atlantis in July. Franco Malerba - who like the great 15th-century navigator was born in Genoa - will help test a novel Italian space satellite system. Astronauts from Canada and the European Space Agency will fly on several missions. And, in August, Japan's first professional astronaut - Mamoru Mohri - is to ride on board the new orbiter Endeavor, which is scheduled to debut in May as the replacement for the lost Challenger. This will be Japan's first major manned spaceflight mission. Its space agency is joining NASA in materials processing and life sciences experiments in the Spacelab that fits into the orbiter's cargo bay. As NASA's manifest now stands, the first 1992 flight of Spacelab should be on board Discovery in January. It is to carry out an international mission with 14 materials processing and 33 life sciences experiments planned by 200 scientists from 13 countries. A mission to study Earth's atmosphere from the orbiter Atlantis should follow in March. The new orbiter Endeavor's maiden flight is scheduled for May. It is an ambitious mission in which astronauts are to try to rescue an Intelsat communications satellite now stranded in a useless orbit. They hope to replace a failed booster rocket. Endeavor is more than just another orbiter. It carries modern electronics. It also has been designed for missions up to 28 days - three to four times the length of typical shuttle missions. In June, Columbia returns to service after extensive overhaul. It, too, now is equipped for longer stays in orbit, although not as long as those Endeavor can sustain. Columbia is to carry out another Spacelab materials and life sciences research mission. This is to last 13 days - the longest shuttle flight so far. Then, in July, Atlantis is to orbit Dr. Malerba and an all European payload. The Italian astronaut will help operate a Tethered Satellite System. The satellite, carrying scientific instruments, will be reeled out of the shuttle on a cable, moving up to 12 miles away. Atlantis also will deploy the European Space Agency (ESA) Eureca recoverable space carrier. This is designed to orbit on its own for about 10 months, carrying out materials and biological experiments under weightless conditions. Then a futur e shuttle mission is to bring it back to Earth. The remaining 1992 missions now planned include: * Endeavor's Japanese Spacelab flight in August. * A mission by Columbia in September to launch a geodetic satellite and conduct Canadian scientific experiments. * An October Department of Defense mission for Discovery. * Endeavor's third mission, in December, to deploy a tracking and data relay satellite. This is a very ambitious flight schedule, given NASA's strained shuttle resources. Technicians have identified several thousand defects that need correcting. These have already caused the orbiter's debut to slip from April into May. Shuttle program managers now have tight limits on funds and personnel. They have to try to cut spending by 3 percent a year for the next five years. Forrest McCartney, retiring director of the Kennedy Space Center in Florida, has said he thinks NASA will be hard pressed to maintain a launch program of eight to 10 missions a year as it now hopes to do.