NASA - National Aeronautics and Space Administration

After a very successful 14-year plus mission, Langley's HALOE instrument on the Upper Atmosphere Research Satellite (UARS), took its last measurements at the end of November and the satellite was turned off on December 14. While the end of a mission is never welcome, the more than 222 journal articles and countless presentations that used HALOE data prove that this mission has left its mark on the scientific community.

"This experiment is ground breaking and has really had a tremendous and long-lasting impact," said the instrument's Principal Investigator James M. Russell III from Hampton University. "The clearest statement on this is the mission's overview paper published in 1993 which was the most cited paper in the field of geosciences from 1991 to 2001."

The HALOE (Halogen Occultation Experiment) instrument uses solar occultations to collect profiles of atmospheric chemical composition, temperature and aerosol loading. The mission's science team, which includes a Nobel Laureate, Paul Crutzen from the Max Planck Institute for Chemistry, is able to analyze the data to tell not only what chemicals are in the atmosphere, but also how they affect each other and how they are transported.

Image right: The HALOE instrument uses the solar occultation technique. Occultation is a form of passive sensing in which the instrument views the sun both above and through the atmosphere to measure how much the sun's light is reduced by atmospheric gases and aerosols. The quantity of light as well as other characteristics can tell scientists about the chemical composition, temperature and transport of gases in the atmosphere.

"HALOE is unique because it made measurements of methane, an atmospheric tracer as it follows the net circulation," said Ellis Remsberg, HALOE project scientist. "Referencing the hydrochloric acid in relation to the methane distribution can allow us to answer questions about the effects of chemistry versus the effects of transport within our atmosphere."

While HALOE was able to observe chemical transport in the atmosphere, it was also able to study atmospheric ozone, specifically the effect of chlorine on ozone. By 1991, when HALOE was launched, there were no instruments in space to study the effects of chlorine gases called chlorofluorocarbons (CFCs) on ozone destruction, and scientists had to rely on ground measurements. However, the significant chemical reactions in which chlorine breaks down ozone leaving our planet unprotected from ultra-violet radiation, happen in the stratosphere and cannot be observed well from the ground.

Image left: HALOE made the first global measurements of stratospheric hydrogen fluoride (HF). HALOE observations of HF confirm the breakdown of CFCs. HF values are high where CFC values are low. Yellow indicates large amounts and purple indicates small amounts of HF.

HALOE was the first scientific mission to observe trends in atmospheric chlorine that matched model predictions following the signing of the Montreal Protocol an international agreement to protect our stratospheric ozone layer by reducing CFC emissions signed in 1987, and then substantially amended in 1990, 1992, 1997 and 1999. A discrepancy between HALOE's CFC observations and the model predictions of CFC quantities following the signing of the Montreal accord could possibly have led scientists to question the efficacy of the agreement and request stricter emissions standards. The HALOE Principal Investigator and others published a paper in Nature magazine in 1996 that received worldwide media attention when it showed conclusively that the main cause for stratospheric ozone destruction was manmade chlorine. Later HALOE papers published by the science team verified that the Montreal Protocol is having the desired effect of reducing atmospheric chlorine and allowing the ozone layer to begin the recovery process.

Another unique feature of the mission is its Web site. In 1995, HALOE's Web site began making use of an innovative feature -- a searchable collection of data. This feature was unique in the atmospheric science community when it was first introduced, and has inspired many other missions to adopt a similar concept for their sites.

Image right: HALOE, or the Halogen Occultation Experiment on the Upper Atmosphere Research Satellite (UARS) took its final measurements on November 21, 2005 after a very successful 14-year plus mission. Visit the HALOE data Web site: + http://haloedata.larc.nasa.gov/download/index.php .

"HALOE was developed in-house, and its near-flawless operation is really a statement of the excellent people we have at Langley," said John Wells, mission flight operations manager.

"We really want to acknowledge the dedicated software and data processing team members from SAIC and GATS and others who provided vital technical input to the mission," said Remsberg. "The mission was really a success because of the harmonious relationship among all of the team members and our partners."

The HALOE science team includes members from Hampton University; Langley Research Center; Max Planck Institute for Chemistry; University of Chicago; University of Michigan; University of California, Irvine; NOAA/Environmental Research Laboratory; and Imperial College, U. K.

For more information about HALOE on-line, visit:

+ http://haloedata.larc.nasa.gov

Katie Lorentz
NASA's Langley Research Center