Landsat 8 Satellite Set to Rescue Global-Change Observations

The launch next week comes at a time when the future of the world's longest-running set of Earth-observations data is at stake

By Jeff Tollefson and Nature magazine

Images from the Landsat satellite series show the Aral Sea in central Asia shrinking significantly from 1977 to 2010 because of water diversion for agricultural use. Image: USGS EROS Data Center

When Landsat 5 fell silent on 6?January, scientists across the globe mourned its passing but gave thanks for its fortitude. The satellite had lasted a record-breaking 28 years, snapping images of the changing planet from melting glaciers to burning rainforests, while its successors faltered. Landsat 6 failed during launch and Landsat?7, at 13 years old, is partially blind and has limited fuel. With the passing of Landsat?5, the future of the world?s longest-running?? and perhaps most influential?? set of data on global change rests with Landsat?8, which is scheduled to launch next week from Vandenberg Air Force Base in California.

?Landsat 8 is the most important NASA Earth-observing mission in over a decade,? says Greg Asner, an ecologist at the Carnegie Institution for Science in Stanford, California, who uses Landsat data to track tropical-forest trends. After making do for years with ageing and impaired orbiters, ?we are getting our eyes back in the sky?, says Asner. ?Those eyes are going to be extremely good.?

From the first launch in 1972, Landsat was designed to provide a global record of land-cover and land-use change. The earliest satellites captured images at several visible-light and near-infrared frequencies, all with a resolution of 80 meters. Landsats?4 and 5 reached 30-meter resolution and could see farther into the infrared, to aid studies of soil moisture and vegetation. Landsat 7 added a multi-spectral sensor that can achieve 15-meter resolution. Through careful calibration, the program has built up a continuous 40-year record of observations at the original frequencies, spanning a period of huge change for the planet.

The latest mission, commissioned in 2002, had a tortuous birth. NASA originally intended to buy data from a privately owned satellite, then formulated a plan to mount Landsat sensors on a separate set of polar-orbiting satellites. Only in 2005 did the administration of then-president George W. Bush call for an independent craft, formally known as the Landsat Data Continuity Mission.

The size of a large jeep, the US$855-million spacecraft will circle Earth at an altitude of about 700?kilometers, carrying sensors of even higher precision than its predecessors. Instead of scanning the terrain below it with a mirror and sending the signal to a few sensors, it will capture instantaneous views of a 185-kilometer swathe of Earth, using some 7,000?sensors for each bandwidth. The result is more data from each location and better image quality. ?The data are much more sensitive to change across the landscape and over time,? says Jim Irons, project scientist for the mission at NASA?s Goddard Space Flight Center in Greenbelt, Maryland. ?Maybe we will be able to better differentiate corn from sorghum, for example, or maple trees from oak trees.?

The spacecraft will also be the first in the Landsat series to collect data in an ?ultra-blue? band, particularly useful for studying oceans and atmospheric aerosols. Another new data stream will be a shortwave infrared band that is sensitive to cirrus clouds, which are powerful players in Earth?s climate.

Landsat has endured major changes in ideology and organization during its lifetime, notably in the 1980s and 1990s, when Congress gave control of the satellites and all data to a private company. Images sold for several thousand dollars each, limiting analysis to an elite corps of scientists. The government regained control in 2001, however, and from 2008 the US Geological Survey, which oversees the data, opened up the archive as a free global resource. Access exploded. Whereas the agency once sold around 15,000 images a year, annual downloads now average about 3 million.

?It provided for data democracy,? says Mike Wulder, a research scientist at the Canadian Forest Service in Victoria and a member of the Landsat science advisory team. ?Landsat really is a global resource.?

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