THE Atlantis astronauts tested an experimental space radiator yesterday, carried out more medical research, and geared up to kindle a tiny blaze to find out how to fight fires in weightlessness.Commander John Blaha, co-pilot Michael Baker, flight engineer G. David Low, Shannon Lucid, and James Adamson spent their sixth day studying the effects of weightlessness, operating a small biological laboratory, and testing space-station computer gear. Other experiments included ongoing tests of a space-station radiator and a modest research project that utilizes the absence of gravity to obtain ultra-pure materials. Commander Blaha took advantage of the work to put in a plug for NASA's planned space station, a frequent theme in the nine-day flight. Later in the day, the astronauts planned to ignite a small fire to learn how flames propagate in the absence of gravity. "In the absence of gravity, and the absence of convection that is a result of that, you don't have ... hot gases rising that you have in one G [gravity]," explains flight director Phil Engelauf. "The mechanism of flame propagation and the result of fires is not yet very well understood in the zero-G environment." By studying film of how the test fire behaves in the safety of an experiment module, scientists hope to learn how to fight fires that might one day break out aboard space shuttles, NASA's planned space station, or other orbital vehicles. "For the design of equipment ... in orbit, the reaction of combustion and how the flames propagate is the subject of quite a bit of interest," Mr. Engelauf said. The Atlantis astronauts accomplished the primary goal of the 42nd shuttle flight six hours and 13 minutes after liftoff Friday with the successful launch of a $120 million NASA communications satellite. As the satellite eased away from Atlantis, the crew noticed an arc-shaped piece of debris floating nearby. NASA officials Tuesday said extensive photo analysis indicated the "debris" was, in all likelihood, a harmless piece of ice that broke away from the rear of the shuttle after the ship reached orbit. Since the satellite deployment, the shuttle fliers have been plowing through a battery of on-board science, engineering, and biological experiments, including six devoted to studying the effects of weightlessness. They also have been sending down spectacular shots of Earth some 180 miles below. Commander Blaha described the shuttle as "a real jewel," saying "it's a gold mine, and we've got to get the most we can out of it." "We just need more space and more electricity and longer time in orbit to even do better with our research ... and that's why we need the space station," he said.