Terra Cognita

A new generation of satellites zooms in on a familiar planet.

NASA Goddard Space Flight Center/MITI/ERSDAC/Jaros and U.S.-Japan Aster Science Team

They’re up there now, scanning the planet at all wavelengths, taking the measure of its shifting seas, winds, and landforms. Earth-viewing satellites have been around for 40 years, but none like these. A new generation of remote sensing spacecraft has brought unprecedented clarity and coverage to the study of Earth from space, and we now live on a continuously monitored planet.

In the 1980s NASA conceived of a grand “Mission to Planet Earth”—a fleet of large satellite platforms, each carrying a suite of sensors that together would provide a long-term record of environmental change. It didn’t turn out that way, mostly due to the multibillion-dollar cost. But a less expensive Earth Observing System (EOS) is reaching orbit, with the first major component launched in 1999.

Terra, as it’s called, retains the original concept’s Swiss army knife approach to Earth observation. Each of the five onboard sensors has its own specialty. A versatile spectrometer called MODIS takes regional-scale pictures in 36 wavelengths. The multi-angle MISR has nine separate cameras—four pointing forward, one straight down, and four looking backward—so that hard-to-see phenomena like atmospheric haze can be photographed in different angles of illumination. ASTER, the one Japanese instrument on board, is Terra’s zoom lens; its high resolution is suitable for a range of tasks, from studying glaciers to tracking changes in land use. MOPITT is tuned to the infrared signatures of pollutants in the lower atmosphere, and CERES measures global radiation to help answer the critical question of what role clouds play in global warming or cooling.

Documenting global change is in fact the main quest of Terra and the rest of the new satellite sensors. They watch for signs that coral reefs are dying, that snowpacks are melting, that forests are disappearing, or shorelines are shifting. More importantly, they collect fundamental data—trillions of bytes’ worth—revealing the complex interplay of land, air, ice, and water driving our planet’s weather.

Terra will be followed later this year by the second large EOS platform, Aqua, which will focus on the atmosphere and ocean. By the end of 2003, some two dozen EOS satellites of varying size and scope will be in space. Add the data from non-EOS projects, like the Shuttle Radar Topography Mission, which last year mapped 80 percent of Earth’s surface in 3-D, and Earth scientists are happily swamped with information. “These days there’s so much data around that you can’t possibly look at it all,” says Alexander Goetz, who heads the University of Colorado’s Center for the Study of Earth from Space.

More is on the way. With the launch of the EO-1 (Earth Observing 1) technology-testing satellite in November, NASA has made its first foray into space-based hyperspectral imagery, which sees in more than 200 wavelengths instead of the few bands covered by older satellites like Landsat, and lets scientists better characterize surface materials based on the way they reflect or absorb light. The first commercial space images with one-meter resolution have already hit the market, with more sharp-eyed competitors on the way.

For students of planet Earth, the view is getting better all the time.



Browse images in the Photo Gallery at right.


Terra's zoom lens, the Advanced Spaceborne Thermal Emission and Reflection (ASTER), captured this view of Kunlun fault in northern Tibet last July. The image combines visible and infrared data and shows, among other details, the shadows of passing clouds. The fault line is marked by lines of vegetation, which appear red. NASA Goddard Space Flight Center/MITI/ERSDAC/Jaros and U.S.-Japan Aster Science Team
Central Oregon's Cascade region shows the scars from widespread logging in this false-color image from the Japanese ASTER instrument, which is on Terra. This view combines red, shortwave infrared, and near-infrared light detected by the satellite. Snow-covered mountains to the east appear blue, forests are green, and clear-cut areas are orange-pink. ASTER, the only Terra sensor that can match Landsat 7's 15-meter resolution, is designed to study thermal (heat) emission and reflection from the land, yielding detailed maps of surface temperature. The maps enable scientists to investigate problems ranging from deforestation to urban growth to soil erosion. The satellite can "revisit" any target to detect change over time, with visit intervals varying from 4 to 16 days. ASTER scientists plan eventually to publish a single, cloud-free composite image showing the entire land surface of Earth. NASA GSFC/MITI/ERSDAC/Jaros and U.S.-Japan Aster Science Team
The living Earth is revealed in this image, compiled from data taken over a period of three years by the SeaWiFS (Sea-viewing Wide Field of view Sensor) instrument on the commercially owned OrbView-2 satellite. The SeaWiFS detects the spectral signature of chlorophyll-bearing plankton, tiny marine organisms that are responsible for about half of Earth's primary biological production. Red areas in the ocean are highest in chlorophyll, yellow-green are intermediate, and blue-violet are the lowest. The ocean data represents a three-year average from September 1997 to August 2000. Land vegetation is based on data taken in July 1998, with dark green showing the areas of dense growth and yellow-brown showing the absence of plants. SeaWiFS Project, NASA GSFC and ORBIMAGE
Because radar imagers can view Earth day or night, even through clouds, Canada's RADARSAT-1 needed only 18 days to produce this exquisitely detailed map of Antarctica. Compare that to how long it took to make the previous best cloud-free satellite map of the continent, assembled from pictures taken by weather satellites over the course of 13 years. The Radarsat images, gathered in October 1999, are being used by scientists to study previously unexplored features, including 500-miles-long ice streams flowing from the continent's interior. Canadian Space Agency
Information on clouds is vital to understanding global climate change, so clouds are a primary target for NASA's Earth Observing System. In this view of the Great Lakes region, taken by Terra's Moderate Resolution Imaging Spectrometer (MODIS), cloud composition and altitude are revealed by how the clouds emit or reflect radiation. Pink areas in the false-color image show colder, higher clouds containing snow and ice, while green areas are lower clouds containing liquid water. The versatile MODIS will fly on both of the first two large EOS platforms--Terra and the soon-to-be-launched Aqua--and is the workhorse for climate change research. MODIS extends and improves on measurements that have been made by two key weather satellite instruments, the Advanced Very High Resolution Radiometer and the Coastal Zone Color Scanner. The new sensor scans the entire surface of Earth every two days. Data from MODIS already had been put to wide use, from monitoring fires in the western United States to documenting the biological productivity of the world's forests. Liam Gumley, University of Wisconsin-Madison/Terra Project
National Oceanic and Atmospheric Administration (NOAA) researchers assessing the health of shallow-water coral reefs in the Caribbean and Pacific were in the market for high-resolution pictures such as this view of tiny Baker Island, located 1,600 miles southwest of Hawaii (blue and yellow areas are reefs). So they turned to Colorado-based Space Imaging, owners of IKONOS, the world's only commercial satellite currently returning one-meter-resolution photos. The company's Washington operations director, Mark Brender, says Space Imaging had identified many uses for its close-up imagery, but never guessed that scientists would be using the pictures to look 90 feet underwater. "That wasn't in our business plan," he says. spaceimaging.com
The SeaWiFS satellite, launched in 1997, is used primarily to monitor the biological productivity of the oceans. But its camera is also well suited to spotting fires from space, as in this view of a blaze in Greece during a hot, dry spell last July. SeaWiFS is perhaps the best example to date of government-industry cooperation in remote sensing. The satellite (also known as OrbView-2) is owned and operated by Virginia-based Orbimage, a subsidiary of Orbital Sciences Corporation. But it was the commitment to a long-term data purchase by NASA Earth scientists that led to the spacecraft's being built and launched in the first place. SeaWiFS Project, NASA/GSFC and Orbimage
The Landsat 7 view of Cape Canaveral, Florida, clearly shows the area's past and present launch pads, including space shuttle pad 39A, the rounded structure near the beach at top center. The first Landsat was launched in 1972, and its successors have been documenting Earth's changing surface ever since. The most recent in the series, Landsat 7, is easily the best. Not only are its pictures sold to scientists for a fraction of past prices, but the archive of captured scenes is much larger. Landsat 7 also doubles the sharpness of its predecessors, with 15-meter resolution in black and white. And for the first time, the data is precisely calibrated to other satellite and airborne data, which "makes us more objective than we've been in the past," according to geologist and remote sensing specialist Alexander Goetz of the University of Colorado. USGS/Eros Data Center
The next big thing in remote sensing, hyperspectral sensors return data across a continuous spectrum subdivided into 200 or more channels--as compared to a handful of separate, selected bands for traditional satellites like Landsat. This "datacube" shows the amount of information contained in a single hyperspectral image of Pearl Harbor taken by AVIRIS (Airborne visible Infrared Imaging Spectrometer), an airplane-mounted instrument similar to the hyperspectral imager recently sent into orbit on NASA's Earth Observing 1 satellite. Each picture element--pixel--on the cube's face has its own spectrum, yielding a wealth of information on how the surface reflects or emits light. A slice through the cube in a plane parallel to the image would show the scene as it appears in a single narrow wavelength. AVIRIS Project, JPL/CalTech
It used to be that only airplanes could return overhead images this sharp. But the view of downtown San Francisco was take by the IKONOS satellite from an altitude of 423 miles. (Note the Transamerica pyramid building at the top center.) The computer-enhanced image adds four-meter-resolution color data to a one-meter-resolution black-and-white image to achieve the sharpness without sacrificing realism. Space Imaging, which owns IKONOS, says that demand for the hi-res orbital photography is growing. Buyers have requested everything from photos of Mt. Ararat in Turkey (a team searching for signs of Noah's Ark) to pictures that woman commissioned of her New York lake house. Apparently she wasn't daunted by the $1,000 minimum for a targeted IKONOS "scene." spaceimaging.com

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