... The Concern ...

ecent and substantial changes in atmospheric composition, caused in part by human activity. The people of the world are conducting continuing, inadvertent experiments with the Earth's atmosphere that alter its properties on a global scale. The outcome can not yet be predicted.

... The Response ...

ASA's Upper Atmospheric Research Satellite (UARS) - which will initiate the long-term, national program of space research into the global atmospheric change.

... UARS SOLSTICE ...

ltraviolet solar radiation in the spectral interval 120 to 300 nm is almost completely absorbed in the Earth's middle atmosphere. Small changes in the amount of radiation incident at these wavelengths will result in corresponding changes in the photochemistry and energy balance of the stratosphere, mesosphere and lower thermosphere. Recent measurements indicated that solar cycle variability is far smaller than previous estimates. Although the Sun varies by nearly a factor of two at Lyman- (121.6 nm), at wavelengths from 120 to 170 nm the Sun varies by less than 20%. From 180 to 300 nm the variability quickly decreases to less then 1%. The challenge for future observations is to make spectral measurements with a long term accuracy far better then 1%. One approach is to directly compare the solar irradiance to the UV output of a number of bright early-type stars. The SOLar-STellar Irradiance Comparison Experiment (SOLSTICE) is flown on the Upper Atmosphere Research Satellite (UARS) and will make the first comparison of this type.