Doing science in space is always a difficult business, where even the best laid plans suffer from the challenges of cold, vacuum and radiation. Engineers and scientists who design instruments for this environment are therefore conservative in their forecasts of how long their hardware will survive.
Hence, it was a jubilant gathering at the University of Toronto this past fall marking the 10th anniversary of Canada’s SciSat-1/ACE mission, or Science Satellite/Atmospheric Chemistry Experiment / Atmospheric Chemistry Experiment. The satellite, designed to make observations of the Earth’s atmosphere, was launched by NASA in 2003. It was officially given a two-year life span, although expectations were that it could make it to five.
The SciSat-1 measures global ozone processes as well as data on banned ozone-depleting substances like chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). Photo credit: NASA
Kaley Walker, a U of T physicist who has been the SciSat-1 deputy mission scientist for the past decade, says the satellite has held up even better than expected. Its batteries are still going strong, its solar panels have weathered well and the primary laser at the heart of an interferometer continues to work. This has added several extra years of research spent studying dozens of different molecules in the atmosphere, with many more years anticipated, says Walker. “If something on a satellite is going to fail, it will likely happen early in the mission.”
That is good news for anyone with an interest in the changes occurring in the Earth’s atmosphere, a field that has evolved considerably since ACE was conceived in the 1990s. At that time, ozone depletion was a top priority and the satellite’s spectrometer was originally intended to look at all of the chemical species that might be involved in this process. However, this same array now effectively measures more than 30 different molecules, such as greenhouse gases, which were not part of the mission’s original mandate. “This device was designed by spectroscopists and we wanted really good spectra,” says Walker. “We measure everything from 750 wavenumbers out to 4,400 wavenumbers. That means you have spectral signatures available.”
ACE is outfitted with two devices: a Fourier Transform Spectrometer (FTS) to measure in the infrared and MAESTRO, or Measurements of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation. As the satellite orbits at 650 kilometres above Earth, the FTS is poised to take sequences of atmospheric absorption spectra twice a day, during sunrise and sunset. The resulting data offers vertical profiles of temperature, pressure and many atmospheric trace gases at altitudes up to 150 kilometres. At the same time, MAESTRO serves as a spectrophotometer for the 285-1,030 nanometre spectral region, capable of measuring the signatures of ozone, nitrogen dioxide and aerosols to a resolution of one to two kilometres.
“We started to measure pollutants that get transported from continent to continent in the upper troposphere,” says Walker. “We measure all the main greenhouse gases and we’re able to get that vertical information. For a lot of the molecules that we measure, we are the only people who measure them. We’re very interested in putting together a series of data sets that can give us the best understanding of how things like ozone or methane are changing.”
An important example of such a data set is the global distribution of species like chlorodifluoromethane, or HCFC-22, which is being phased out under the Montreal Protocol on Substances that Deplete the Ozone Layer. ACE has recorded HCFC-22 levels in the atmosphere and compared the findings with predictive models, looking for discrepancies that might indicate whether some places are still producing this agent.