The United States is set to launch a satellite that the space agency, NASA, says will supply the best information yet about the health of Earth's atmosphere. The spacecraft will provide a better understanding of the atmosphere's chemistry and composition so scientists can improve forecasts of climate change.

NASA calls the mission an effort to protect the air we breathe. Agency scientist Richard Stolarski says the new satellite, named Aura, will orbit Earth measuring the gases in the atmosphere.

"Aura is a very large satellite weighing over three tons designed to attack three problems," he said. "One is to see long range transport of pollution over the globe. A second one is to observe the recovery of the ozone layer, and finally, to try to look at global climate change."

The Aura satellite will use U.S., British, Dutch, and Finnish instruments to observe the atmosphere from the top of the stratosphere at about 50 kilometers altitude down to the ground. Although this is less than half the atmosphere's height, it is the region where most of the gases exist.

The stratosphere is where the ozone layer is. This is the protective layer that filters physically harmful ultraviolet radiation from the sun. International agreements have banned the use of industrial chemicals that rise into the stratosphere as gases and destroy ozone, and earlier satellites have observed the beginnings of the layer's restoration. NASA scientist Paul Newman says Aura will continue to monitor the situation.

"These gases have lifetimes of hundreds of years, so it's going to take a long time for the ozone layer to recover," he said. "There is a worry that climate change may actually make the ozone problem a little bit worse, so we have to understand the physics and chemistry interaction between climate change and stratospheric ozone loss."

NASA says the Aura satellite will also help unravel some of the mysteries of climate change at lower atmospheric levels, where automobile and industrial exhaust and other gases trap solar heat, a process that many scientists believe causes global warming. Mr. Newman says Aura will monitor the most important of these compounds, carbon monoxide, nitrogen and chlorine compounds, water vapor, and ozone concentrations that act as pollutants in the lower atmosphere.

"All of these gases can influence climate change to some degree, so having global daily pictures over the five-year life of the mission will just be a huge improvement in our ability to actually decide of our models are doing the right thing for long term prediction," said Mr. Stolarski.

Aura completes the first series of three U.S. Earth observation satellites. These include one to monitor the planet's land and another to observe the water cycle.