Terra

Terra (originally designated EOS AM-1) is NASA's flagship Earth Observing System satellite, launched December 18, 1999 aboard an Atlas IIAs rocket from Vandenberg Air Force Base [^terra-about]. The spacecraft was built by Lockheed Martin Missiles and Space and carries five instruments designed to study Earth's land, oceans, atmosphere, and energy balance simultaneously [^eoportal-terra].

Terra operates in a sun-synchronous polar orbit; the nominal design orbit of 705 km altitude with a 10:30 AM local time of descending node (LTDN) has drifted over the mission's extended life [^terra-about]. Since October 2022 the satellite has operated at approximately 693 km altitude, and the equatorial crossing had moved to approximately 9:06 AM LTDN as of December 2025. The 16-day ground-track repeat cycle remains intact [^terra-about].

The five instruments represent a division of scientific labour: MODIS provides daily global observations across 36 bands from 0.4 to 14.5 micrometres at resolutions from 250 m to 1 km, underpinning fire detection, vegetation index, snow/ice, and cloud products [^modis-specs]. ASTER delivers high-resolution multispectral imaging at 15 m (VNIR), 30 m (SWIR, non-operational since approximately 2008), and 90 m (TIR) across a 60 km swath [^eoportal-terra]. MISR uses nine cameras at angles from 0 to plus/minus 70.5 degrees in four spectral bands to characterise surface reflectance and atmospheric aerosol properties [^eoportal-terra]. The two CERES scanners measure Earth's reflected solar and emitted thermal radiation across shortwave, longwave, and total-radiation channels to constrain the planet's energy budget [^eoportal-terra]. MOPITT, provided by the Canadian Space Agency, measured tropospheric carbon monoxide and methane for 25 years before being permanently powered down on April 9, 2025 to conserve the satellite's remaining electrical power [^terra-news].

As of May 2026, Terra operates in a degraded state: four of five instruments remain active, orbit has drifted from the original design parameters, and the mission is in its mission-end phase having vastly exceeded its original six-year design lifetime [^terra-news].