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Take the 'A' Train


NASA artist concept of the A Train satellites above Earth
The Earth Observing System

Since its creation in 1958, the U.S. National Aeronautics and Space Administration (NASA) has been studying Earth and its changing environment by observing the atmosphere, oceans, land, ice, and snow, and their influence on climate and weather.

As astronaut Sally K. Ride, the first American woman in space, was leaving NASA in 1987, she suggested a Mission to Planet Earth to preserve our planet through coordinated research into Earth's global environment.

Exploring how Earth's systems of land, water, air, and life interact with each leads to better understanding of the global environment. Earth science blends research in the fields of study that deal with Earth and its parts, such as oceanography, meteorology, atmospheric science, and biology.

Following Ride's proposal, NASA planned a series of satellites known as the Earth Observing System (EOS) as the nucleus of a "Mission to Planet Earth." From high above Earth, EOS satellites would monitor land, sea and atmosphere for changes in the environment.

In 1991, NASA upgraded the project to a more comprehensive program studying Earth as an environmental system. The space agency refers to it as the Earth Science Enterprise (ESE). By using satellites to examine Earth thoroughly, ESE expands human understanding of how natural processes affect human beings, and how human beings affect natural processes.

Research scours such diverse resources as land surfaces; the waters, oceans, glaciers, and polar ice sheets; atmospheric chemistry and processes, and clouds; energy cycles; and ecosystem processes.

Such research improves many things, including weather forecasts, predictions of climate changes, agricultural and forest management, and planning by diverse users such as fishermen and local governments.

ESE has three components: The early science results from the ESE came from a variety of small satellites, as well as space shuttle missions, and aircraft flights.

Today, EOS is the centerpiece of NASA's Earth Science Enterprise. To help us understand Earth as an integrated system, EOS maintains a series of polar-orbiting and low inclination science satellites for long-term global observations of the land surface, biosphere, atmosphere, and oceans. EOS is the first observing system to integrate measurements of Earth's processes. The A-Train

The so-called A-Train is a flotilla composed of the Aqua, Aura, CloudSat, PARASOL and CALIPSO satellites flying in formation in low polar orbits 438 miles (705 km) above Earth at an inclination of 98 degrees. Together, their overlapping science instruments give a comprehensive picture of Earth weather and climate.

The satellites are referred to as the A-Train because the caravan has been said to resemble a train of satellites flying around Earth. The A-Train reference comes from a jazz tune, Take the 'A' Train, about riding a subway to New York City's Harlem, written in 1941 by Billy Strayhorn and made famous by jazz maestro Duke Ellington.

However, the railroad metaphor doesn't give an accurate picture. The satellites do not follow each other in single file. Rather, they fly independently and cross over the equator a few minutes apart starting just after 1:30 p.m. local time.

The A in A-Train also stands for "afternoon" because the satellites cross the equator shortly after noon. Aqua leads the train. It is the largest satellite in the group and the first to cross the equator each day (about 1:40 p.m. on ascending passes) and night (at 1:40 a.m. on descending passes).

The spacecraft travel around the planet at more than 15,000 mph. Ground controllers maintain their orbits within 15 minutes of the leading and trailing satellites. CloudSat and CALIPSO fly within 15 seconds of each other, so they can measure the same clouds at the same time.

NASA's Orbiting Carbon Observatory will join the A-Train in 2008. OCO will be placed 15 minutes ahead of Aqua and will measure the concentration of carbon dioxide in the atmosphere.

The EOS Project Science Office (EOSPSO) makes its scientific information and resources available to scientists and the general public.

NASA has combined its Earth science and space science programs into an integrated Science Mission Directorate. The directorate is involved in NASA's Vision for Space Exploration through its support of scientific exploration activities.

Learn more:
Aura: Aqua: Terra: TOMS and UARS: CloudSat:
  • CloudSat    Colorado State University
Calipso: Parasol: Orbiting Carbon Observatory:
  • OCO    Orbital Sciences

Terra Aqua Aura Earth Satellites Solar System