The Shuttle Columbia (STS 107) carried
the Shuttle Ozone Limb Sounding Experiment-2 (SOLSE-2) as a
demonstration of new technology that will be used on the next generation
of meteorological satellites to monitor ozone change. Current
instruments flying on NOAA and NASA satellites look directly downward
toward the Earth, which limits their ability to accurately measure ozone
in the lower layers of the stratosphere, the layer of atmosphere 10 to
50 kilometers (6.2 to 31 miles) in altitude. New technology called
“limb viewing” (depicted below) allows observation of the
atmosphere from the side rather than straight down. When viewed from the
side, the Earth looks like a flat circle, and the atmosphere appears
like a halo around it. This halo is known as Earth’s
“limb.” From that side view, the layers of the atmosphere
appear like layers in a cake, allowing instruments like SOLSE-2 to see
the lower layers of the stratosphere. This is important because most of
the recently observed ozone change, like the “ozone hole,”
occurs in the lower stratosphere.
To understand how SOLSE-2 is able to measure ozone in the stratosphere, remember that sunlight is composed of numerous different wavelengths of electromagnetic energy, and that the individual molecules in the atmosphere can either scatter that incoming light or absorb it. The way each component of the Earth’s atmosphere interacts with sunlight is called the “absorption spectrum.” Like fingerprints reveal the identities of people, the absorption spectrum of the atmosphere can be used to identify the various gases and particles in it. The ozone layer near the tangent point (the point in space at which the instrument’s line of sight appears to touch the edge of the curve of the Earth) absorbs radiation at several wavelengths of light. SOLSE-2 infers the presence of ozone by using a spectrometer to separate the light it detects into individual wavelengths, and then measuring the relative absence of radiation at wavelengths that are strongly absorbed relative to wavelengths that are weakly absorbed.
The atmosphere is a blue haze that thins as it rises above the curve of the Earth’s surface. Called the “limb,” it provides a view of the structure of the atmosphere. Orbiting scientific instruments look at the limb to measure how the concentrations of trace gases vary with altitude. One of these instruments, the Shuttle Ozone Limb Sounding Experiment-2, flew in the cargo bay of the Space Shuttle Columbia on its final flight. (Image STS073-E-5113 courtesy NASA-JSC Gateway to Astronaut Photography of Earth.)
The graphs above show preliminary results from the SOLSE-2 experiment. The top panel illustrates the structure of the light—all the different wavelengths—detected by SOLSE at an altitude of 25 kilometers (15.5 miles) above the Earth. The dips in the line at specific wavelengths (listed along the horizontal axis) indicate that less light at those wavelengths made it back to SOLSE-2’s detectors. The broad dip in the line near 600 nanometers (600 billionths of a meter) shows where ozone absorbed light most strongly, while the other wavelengths have more subtle dips. The narrow dip near 760 nanometers is absorption by oxygen.
The bottom panel is an ozone index that shows the amount of light in a triplet of wavelengths (one strongly absorbed relative to two weakly absorbed) detected by SOLSE-2 between 0 and 50 kilometers above the Earth. Where ozone is high, index values will be low because ozone absorbs the light, leaving less to be detected by the instrument. Index values (green diamonds) were lowest (farthest to left on the graph) around 20 kilometers above the Earth, which is just where years of research and model calculations indicate the “ozone layer” should be. Indeed, in both cases, SOLSE-2’s measurements nearly duplicate what model calculations predict they should be in an atmosphere containing typical amounts of ozone. In the bottom panel, the red line shows the observations that atmospheric ozone models predict we should see; the blue line shows the observations that would be expected if the atmosphere contained no ozone.
The measurements made by the SOLSE-2 mission on the Space Shuttle Columbia demonstrated that the limb-sounding technique will work very well for monitoring ozone in next-generation satellites. Although the primary data storage for SOLSE-2 was on board Columbia and was therefore destroyed, data were sent down to Earth for about 15 minutes during each orbit, yielding about 40 percent of the data collected during the mission. That amount was more than sufficient to demonstrate that the experiment was successful. The efforts of the Columbia crew (Rick D. Husband, William C. McCool, Michael P. Anderson, David M. Brown, Kalpana Chawla, Laurel Blair Salton Clark, and Ilan Ramon) resulted in the collection of valuable scientific data that will help design future Earth-observation satellites. Tragically, the Columbia and her crew were lost during re-entry into Earth’s atmosphere on February 1, 2003.