Atlantis astronauts aimed shuttle instruments toward the sun Monday as the atmospheric research mission focused on the solar radiation that constantly bombards the Earth.
Four of seven primary instruments in the shuttle cargo bay are designed to monitor sunlight, the most powerful of all atmospheric forces.Scientists behind the experiments want to learn more about fluctuations in the sun's ultraviolet radiation and how they affect the planet. The variances are believed partly responsible for ozone depletion.
The thin, invisible layer of ozone in the stratosphere protects humans, animals and plant life from dangerous ultraviolet rays.
"The sun's energy is the driver for weather and many chemical reactions in the upper atmosphere," said astronaut Scott Parazynski. "Clarifying solar energy is critical to understanding how ozone levels change from season to season and year to year."
Another goal is to pinpoint the total solar energy reaching Earth, knowledge that could lead to a better understanding of long-term climatic changes. Researchers theorize that the ice ages were caused by only minuscule changes in solar energy.
Atlantis, in its 190-mile-high orbit, is flying most of the time with its cargo bay facing Earth so that instruments can gauge ozone and other gases. But for four periods during the flight - each lasting nine to 12 hours - astronauts point the cargo bay toward the sun. Monday was the second of those periods.
NASA said the solar-energy monitors were performing well, as were two of three atmospheric instruments. The third - a German ozone monitor - has been broken since Friday. Ground controllers tried unsuccessfully throughout the weekend to revive it by flipping its power off and on.