Introduction
Launched into the night sky nearly 26 years ago, on Dec. 19, 1999, from Vandenberg Air Force Base (now Space Force Base), Terra was NASA’s first Earth Observing System (EOS) flagship mission to study Earth’s land surface from space via a coordinated series of polar-orbiting and low-inclination satellites that produce long-term global observations useful for understanding the interactions between Earth’s atmosphere, land, snow and ice, oceans, and radiant energy balance. Scheduled for a six-year tour, Terra outlasted its life expectancy by nearly two decades. Despite its longevity, Terra’s mission scientists stopped making inclination adjustments in 2020, allowing the satellite to slowly drift out of its contained orbit. The mission team have also begun the painful process of shutting down the five key instruments as the satellite is prepped for retirement.
“Terra’s impressive human legacy stems from the fact that the mission’s history is grounded in NASA icons,” said Nyssa Rayne [NASA Goddard Space Flight Center (GSFC)—Terra Outreach & Communications Coordinator]. “Even today, Terra continues to benefit from legendary figures, including the current project scientist and instrument calibration/validation experts, who have shaped this mission in monumental ways.”
An Auspicious Beginning to More Than Two Decades of Science
Terra’s mission of discovery was designed to provide a better understanding of the total Earth system. Up to this point, the research community knew very little about how the land interacted with the atmosphere on a regional and continental scale. The community also lacked a way to quantify surface properties, such as albedo, roughness, evaporation rate, and photosynthesis, from satellite data.
Terra was designed, engineered, and programmed to address these knowledge gaps. Often described as a small bus, Terra measures almost 7-m (23-ft) long and 3.5 m (11 ft) across. In the vast expanse of space, however, Terra travels in an orbit around Earth, like a gnat circling the Sphere in Las Vegas. Carried into space aboard an Atlas-Centaur IIAS expendable launch vehicle from Vandenberg Air Force Base, CA, Terra was placed in orbit 705 km (438 mi) above the planet’s surface, capturing a viewing swatch from each overpass that could be stitched together to produce whole global images. Its flight path was designed to cross the equator to coincide with the time of day when cloud cover along the equator was at a minimum (10:30 AM local time).
Five Instruments Wrapped in a Silver Package
First named EOS-AM, the concept of the Terra mission was envisioned in the 1980s and implemented in the 1990s. Terra builds on the lessons learned from past pioneering programs, including the Upper Atmosphere Research Satellite (UARS), Landsat, the Ocean Topography Experiment (TOPEX)/Poseidon, and the series of Total Ozone Mapping Spectrometer (TOMS) instruments. After many scientific conversations and arguments, it was finally decided that Terra would carry five instruments capable of gathering data that would benefit a variety of Earth scientific disciplines – see Figure 1. An international effort, Terra carries instruments from the United States, Japan, and Canada that allow scientists to document relationships between Earth’s systems and examine their connections. The five instruments include:
Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), which obtains high-resolution images of Earth at 14 different wavelengths of the electromagnetic spectrum that can be used to create detailed maps of land surface, temperature, emissivity, reflectance, and elevation;
Clouds and the Earth’s Radiant Energy System (CERES), which measures Earth’s total radiation budget as well as cloud property estimates that enable scientists to clarify the role that clouds play in the planet’s radiative flux;
Measurement of Pollution in the Troposphere (MOPITT), which measured the distribution, transport, source, and sinks of carbon monoxide (CO) in the troposphere;
Multi-angle Imaging SpectroRadiometer (MISR), which improves the field’s understanding of the fate of sunlight in Earth’s environment, distinguishing between different types of clouds, aerosol particles, and surfaces; and
Moderate Resolution Imaging Spectroradiometer (MODIS), which combines data gathered from CERES and MISR to determine the impact of clouds and aerosols on the Earth’s energy budget.
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