The Terra satellite and Earth Observing System (EOS): Explained

The Terra satellite plays a pivotal role in NASA’s Earth Science program, formerly known as NASA Earth Science Enterprise (ESE) and Mission To Planet Earth (MTPE). Led by Michael Freilich, this research initiative aims to develop a comprehensive scientific understanding of the Earth system and its response to both natural and human-induced changes. Let’s explore the significant contributions of the Terra satellite within NASA’s Earth Science program and its mission to benefit present and future generations through enhanced Earth system predictions.

Under this program, NASA is currently developing the Earth System Observatory or the Earth Observing System, the main purpose of which are five satellite missions providing critical data on climate change, severe weather and other natural hazards, wildfires, and global food production. 

These satellites are a coordinated series of polar-orbiting and low inclination satellites for long-term global observations of the land surface, biosphere, solid Earth, atmosphere, and oceans. This Earth Observing System (EOS) involves multi-instrument orbital platforms that will measure 24 parameters needed to understand global climate change.  

Terra Satellite

Terra satellite (formerly known as EOS/AM-1) was launched in December 1999 as a joint mission within NASA’s ESE (Earth Science Enterprise) program between the US, Japan and Canada. Terra collects data about the Earth’s biogeochemical and energy systems using five sensors that observe the atmosphere, land surface, oceans, snow and ice, and energy budget. Each sensor has unique features that enable scientists to meet a wide range of science objectives. The five Terra onboard sensors are:

• ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer)
• CERES (or Clouds and Earth’s Radiant Energy System)
• MISR (Multi-angle Imaging SpectroRadiometer)
• MODIS (Moderate-resolution Imaging Spectroradiometer)
• MOPITT (Measurements of Pollution in the Troposphere)

Let’s go over each of these components to understand the functioning of Terra. 

ASTER

Advanced Spaceborne Thermal Emission and Reflection Radiometer or ASTER obtains high-resolution images of the Earth in 14 different wavelengths, which are then used to create detailed maps of land surface temperature, emissivity, reflectance and elevation.

ASTER is the only high spatial resolution instrument on the Terra platform and thus serves as a ‘zoom’ lens for the other Terra instruments. This makes it particularly useful for change detection, calibration/validation and land surface studies. Unlike the other instruments on Terra, ASTER does not collect data continuously. Instead, it collects an average of 8 minutes of data per orbit. 

All three ASTER telescopes (VNIR, SWIR, and TIR) are pointable in the crosstrack direction. This ability to change viewing angles along with the ability to click high resolution images, enables ASTER to produce stereoscopic images and detailed terrain height models.

CERES

Terra has two identical Clouds and the Earth’s Radiant Energy System (CERES) instruments on Terra that measure the Earth’s total radiation budget and provide cloud property estimates that enable scientists to assess clouds’ roles in radiative fluxes from the surface to the top of the atmosphere. 

instrument is a radiometer which has three channels – a Total channel to measure entire spectrum of outgoing Earth’s radiation (>0.2 µm); a channel to measure Earth-emitted thermal radiation in the 8–12 µm “window” or “WN” region; and a shortwave (SW) channel to measure reflected sunlight in 0.2–5 µm region)

A set of six CERES instruments were launched on the Earth Observing System and the Joint Polar Satellite System. This includes the 2 (Flight Module 1 (FM1) and FM2) on Terra, two (FM3 and FM4) on the Aqua satellite. A fifth instrument (FM5) was launched on the Suomi NPP satellite in October 2011 and a sixth (FM6) on NOAA-20 in November 2017.

MISR

The Multi-angle Imaging SpectroRadiometer or MISR is a novel type of instrument designed to view the Earth with cameras pointed at nine different angles. This helps us understand the amount of sunlight that is scattered in different directions under natural conditions, thus enabling research on Earth’s climate, and how it may be changing.

MOPITT

Measurement of Pollution in the Troposphere (MOPITT) is an instrument flying on NASA’s Earth Observing System Terra satellite, designed to enhance our knowledge of the lower atmosphere and to observe how it interacts with the land and ocean biospheres. This module on the Terra satellite specifically focuses on the distribution, transport, sources, and sinks of carbon monoxide in the troposphere. Carbon monoxide (CO) hinders the atmosphere’s natural ability to rid itself of harmful pollutants.

It is one of the earliest satellite sensors to use gas correlation spectroscopy. MOPITT is a nadir-sounding (vertically downward pointing) instrument that measures emitted and reflected radiance from the Earth in three spectral bands. 

As this light enters the sensor, it passes along two different paths through onboard containers of carbon monoxide. The different paths absorb different amounts of energy, leading to small differences in the resulting signals that correlate with the presence of these gasses in the atmosphere.

MODIS

MODIS or Moderate Resolution Imaging Spectroradiometer is the fifth sensor on the Terra satellite. It is a 36-channel visible to thermal-infrared sensor that provides global coverage on polar-orbiting satellites like the Terra satellite and its sister satellite, Aqua.

The MODIS sensor measures the percent of the planet’s surface that is covered by clouds almost on a daily basis. Scientists use this wide spatial coverage provided by this sensor, along with the information from MISR and CERES, to determine the impact of clouds and aerosols on the Earth’s energy budget.

MODIS can: 

• Help measure really intricate climatic details like the properties of clouds such as the distribution and size of cloud droplets in both liquid water and ice clouds.

• Also measure the properties of aerosols which can be introduced in the atmosphere from manmade sources like pollution and biomass burning and natural sources like dust storms, volcanic eruptions, and forest fires.

• Help scientists determine the amount of water vapor in a column of the atmosphere and the vertical distribution of temperature and water vapor.

The ability of this sensor to study and observe the areal extent of snow and ice brought by winter storms and frigid temperatures, the “green wave” that sweeps across continents as winter gives way to spring and vegetation blooms in response, the disasters like volcanic eruptions, floods, severe storms, droughts, and wildfires can help us understand not only climate change better, but also the contributing agents to it. This includes pollutants and foreign agents like aerosols etc. 

An interesting fact is that MODIS sees changes in the Pacific phytoplankton populations that may signal the onset of the famous El Niño/La Niña climatic siblings well ahead of their arrival. It also has a unique channel for measuring chlorophyll fluorescence. All plants bombarded with light begin to fluoresce, but in wavelengths that the naked human eye cannot detect. The more plants fluoresce, the less energy they are using for photosynthesis. Thus, MODIS not only maps the distribution of phytoplankton, it also helps us gauge its health.

Some other interesting EOS and Terra Satellite Facts

The key focus areas for the Earth Observing System (EOS) are as follows:

• Aerosols as they are a key source of uncertainty in predicting climate change;

• Cloud, Convection, and Precipitation to better understand and predict climate change, air quality forecasting, and prediction of severe weather;

• Mass Change as it provides drought assessment and forecasting;

• Surface Biology and Geology to understand climate changes that impact food and agriculture, habitation, and natural resources, by answering open questions about the fluxes of carbon, water, nutrients, and energy within and between ecosystems and the atmosphere, the ocean, and the Earth;

• And lastly Surface Deformation and Change as it helps in quantifying models of sea-level and landscape change driven by climate change, hazard forecasts, and disaster impact assessments, including dynamics of earthquakes, volcanoes, landslides, glaciers, groundwater, and Earth’s interior.

The Terra Satellite NASA launched in 1999 could capture a lot of this information and meet EOS’s needs for the necessary studies and investigations. Terra was the first satellite to look at Earth system science, collecting multiple types of data dedicated to various areas of Earth science. 

While the original design life for the Terra satellite was 6 years, after 15 years in orbit, Terra has still been collecting valuable data about our planet – all thanks to the dedicated engineers and architects who designed, built and are constantly maintaining the sensors onboard and the satellite itself.

The Terra satellite was launched from the Vandenberg Air Force Base and was successfully placed into a near-polar, sun-synchronous orbit at an altitude of 705 km (438 mi) in Feb 2000. In 2020, Terra completed its final inclination maneuver, using some of its limited fuel supply, to maintain that crossing time.

Since that final inclination maneuver, Terra has drifted to an earlier equatorial crossing time (i.e. local solar time of a satellite’s passage over the equator) . With every passing day, the Terra satellite will be lowered to a new orbit where it will be able to collect valuable data at an even earlier crossing time. As Terra moves closer to Earth, the sensors’ views will become narrower leading to slightly narrower swath widths. 

Although the highest effect of this drift will be noticed in ASTER imagery, each of Terra’s sensors will be affected. Some of these changes could be beneficial to some areas of research – like land morphology, surface temperature, and climate research. The data collected by Terra over the last 2 decades will continue to contribute to meaningful research of Earth’s Systems science. This will help the Earth Observing System make significant progress on its key focus areas, for years to come!