Planetary Society: AIRS: A mission to planet Earth

How would you describe what NASA has done in more than 50 years of its existence?

If you had to sum it up briefly, you might start with the Apollo program and Neil Armstrong’s first steps on the Moon. Maybe you’d describe the breathtaking photographs returned to Earth by Voyager 1 and 2 in the 1970s and 80s. You would certainly mention the space shuttle program, and perhaps the amazing tales of Mars rovers Spirit and Opportunity.

This article was also posted on the Planetary Society blog.

Often lost among the flashier events of human and robotic space exploration is the space-based observation of our own planet. Starting in the 1980s, NASA began to systematize its study of our home world, calling it their “mission to planet Earth.” That mission is now the goal of the agency’s Earth Science Division, and the space component of this program is the Earth Observing System (EOS), a collection of satellites designed to study the Earth over the long term.

Let’s take a closer look at one EOS member that’s recently been in the news. Meet Aqua, a multi-national satellite sent into orbit on May 4, 2002, with six action-packed instruments used to study Earth’s water cycle. One of these instruments is AIRS, the Atmospheric Infrared Sounder. Managed by the Jet Propulsion Laboratory (JPL), AIRS and Aqua zip around the Earth every 99 minutes at an altitude of 705 kilometers.

A rendering of the Aqua satellite orbiting Earth. Aqua carries six instruments used to study Earth's water cycle, one of which is AIRS, the Atmospheric Infrared Sounder. Credit: NASA / JPL

So what is an atmospheric infrared sounder? “Sounding” is the nautical term for finding the depth of water under a boat. Historically, this was accomplished by throwing simple weighted measuring lines overboard. Modern sounding employs more sophisticated techniques, such as transmitting pulses from a boat and measuring the time it takes them to return.

The term “sounder” has now been generalized to refer to any instrument that measures the characteristics of a medium. In the case of AIRS, the medium is Earth’s atmosphere. Another example of an atmospheric sounder is a weather balloon. As the balloon rises, it collects data such as temperature, pressure, moisture, and wind speed, all measured as functions of height.

AIRS performs many of the same functions as a weather balloon. As AIRS moves along its orbit, its mirror sweeps back and forth, collecting 1,600-kilometer-wide swaths of light from its high vantage point. The incoming light is separated into its component wavelengths, and sent to 2,378 detectors inside the instrument, each set to detect a narrow band of wavelengths of infrared light (infrared wavelengths are too long to be seen by human eyes). By analyzing the content of the infrared light in each scan swath, scientists can determine temperature, moisture content, and more. As AIRS laps around the Earth, a 3D picture of the planet’s atmosphere begins to emerge.

The data collected by AIRS are publicly available in nearly real time. Anyone with a numerical computing software package like MATLAB can download the data, process them, and start studying Earth’s climate cycles. For interested parties not inclined to crunch the numbers, JPL offers “today’s maps from space,” satellite maps showing the last three days of global carbon monoxide, ozone, air temperature and water vapor, from various altitudes.

The average global distribution of water vapor in the Earth's atmosphere from August 1-3, 2011. 50 millimeters of water vapor equals approximately 2.5 inches of potential rain, if the water vapor in the atmosphere were to fall to the ground. Credit: NASA / JPL

Because of the accessibility of AIRS data, researchers across the globe can continuously produce a steady stream of new findings about Earth’s climate. Examples include the first global maps of carbon dioxide distribution and an analysis of the major sources of the gas in the northern hemisphere. According to project manager Thomas Pagano, AIRS has already had more impact on weather forecasting than any other instrument. The National Oceanic and Atmospheric Administration (NOAA) processes AIRS data within 1-3 hours and distributes them to weather prediction centers around the world.

Recently, JPL released animations constructed from AIRS data showing the evolution of the 2011 heat wave in the United States, highlighting the deviation from average of daytime and nighttime air temperatures. Another new AIRS release is a map of surface humidity in Africa, which shows humidity levels 15 percent below average in parts of Somalia, Ethiopia, Kenya and Djibouti, highlighting the severe drought the region is facing.

A map of relative surface humidity over northeast Africa shows levels 15 percent below normal in July 2011, compared to the past seven years. Credit: NASA / JPL

The current funding crisis at NASA means that every program in the space agency – whether in the area of human spaceflight, robotic planetary exploration, or Earth observation – is under pressure to defend its very existence. While sending humans and robots into space may inspire our imagination and attract media attention, AIRS is a valuable source of information about our planet both in the short term (helping weather forecasters let us know what we can expect tomorrow) and the long term (discovering patterns and tracking changes in Earth’s climate over time). AIRS is just one example of why the importance of NASA’s “mission to planet Earth” cannot be understated.