The state of Earth observation, January 2012

We're being watched by an army of Earth-observing satellites.

Some zip above the world's cities at high noon, creating global maps bathed in perpetual sunlight. Others hang in eerie suspension over the equator, blinking at regular intervals to send pictures of cloud patterns to meteorologists. A few hug the global terminator between night and day, sleeplessly gazing at the Sun to measure its output.

These observers form a motley crew of spacecraft designed to study the Earth as a planet; a living, breathing collection of processes that humans are increasingly affecting. As of November 2011, the Earth Observing Handbook counts 109 active missions, with 112 more approved and planned for the future. The Handbook -- created by the Committee on Earth Observing Satellites and the European Space Agency -- is a comprehensive guide containing exhaustive details like orbital periods, inclinations and individual scientific instruments aboard these fearless fliers.

Learning about these Earth-observing satellites entails an examination of how they orbit our pale blue home. Consider this: NPP, NASA's newest multi-purpose climate satellite, zips around the Earth every 101 minutes at an altitude of 824 kilometers. By contrast, members of NOAA's Geostationary Operational Environmental Satellite (GOES) system take a full day to orbit once, and sit much higher at 36,000 kilometers. Why the difference?

The GOES system -- along with European and Japanese counterparts METEOSAT and MTSAT, respectively -- are used for weather forecasting and general climatology. To do this effectively, they need to be able to watch the entire disc of the Earth at once, over a long period of time. Like a photographer taking a few steps backward to get the optimum angle for a shot, geostationary satellites orbit much higher than conventional Earth-observing satellites. Additionally, so they can keep their eyes locked on the same region of the planet, they take one day to make a complete orbit, geographically locking them in place over the same spot. This also means a geostationary satellite goes through the same day/night cycle as the rest of us; all come equipped to take images in the visible and infrared spectrum, allowing them to see in the dark and reveal information about weather patterns that can only be observed by wavelengths invisible to the human eye.

Geostationary orbits won't do the trick if you need to see more than one region of the Earth. Additionally, capturing complex measurements like the water vapor content of clouds, or land and sea temperatures requires a lower altitude. For these reasons, polar-orbiting satellites are popular with the Earth-observing crowd because they circle the globe over the North and South Poles as the planet spins beneath them.

Polar orbits also require an additional consideration: satellites like LANDSAT-7, a land-imaging satellite sponsored by NASA and the U.S. Geological Survey, are designed to see how things change over time. To do this effectively, the Sun needs to be in the same spot every time the spacecraft passes overhead and snaps an image. For this reason, many polar-orbiting satellites are also sun-synchronous, passing above their targets at the same time on each lap.

One of the crown jewels of NASA's Earth-observing system is the international Afternoon Constellation. Colloquially known as the A-train, it is a series of satellites that closely follow each other on the same polar-orbiting, sun-synchronous track. The A-train is named after the local time when they cross the equator: in the afternoon, around 13:30. However, the 'A' could just as well stand for accident, with a nod to the system's origins.

In 2002, NASA launched Aqua, an Earth observation satellite designed to gather data on the planet's water cycle in solid, liquid and vapor forms. Two years later, the space agency began making preparations for Aura, a similarly-designed satellite to measure the Earth's ozone layer and air quality. As engineers began to hash out plans for Aura's orbit, they realized it would be convenient to use a track identical to Aqua's: a polar, sun-synchronous orbit inclined at 98.6° to the equator.

Due to limitations imposed by the way data is sent by the satellites and received on the ground, the two spacecraft actually ended up flying very close together in space terms: a mere 15 minutes apart. By accident, scientists now had two data sets on the Earth's climate, captured in short succession, that could be combined and compared.

The possibilities were intriguing. Why not continue adding more spacecraft to the train, and increase the amount of data that could be grouped together? In 2006, CALIPSO and CloudSat were launched -- sharing the same rocket, no less -- to do just that. CALIPSO studies clouds and atmospheric aerosols, and CloudSat examines the role clouds play in regulating planetary climate.

For a while, PARASOL -- a joint NASA/French space agency CNES mission that also studies clouds and aerosols -- joined the A-train. NASA's Glory satellite, designed to collect a wealth of data on cloud properties, aerosol distributions and incoming solar radiation, was slated to join in March 2011 but failed to reach orbit. Japan's Global Change Observation Mission (GCOM) W1, expected to observe a multitude of information about water cycles and the ocean, is expected to roll into the A-train this year. In 2013, look for yet another add-on in the form of NASA's Orbiting Carbon Observatory (OCO) 2, which will perform global measurements of atmospheric carbon dioxide.

One drawback to this arrangement is that an accidental collision within the A-train could spray debris through the entire orbital path, endangering other members. To protect against this, NASA dictates a set of orbital parameters each spacecraft, called a control box, to which it must adhere. This diagram shows the control boxes for each A-train member, including yet-to-be-launched members GCOM-W1 and OCO-2, along with former member PARASOL.

As of mid-January, there are 13 Earth-observing satellite launches on the books for 2012, comprising 14 spacecraft (GCOM-W1 and KOMPSAT-3 are sharing a ride to orbit). The table below is a rough outline of what's yet to come. Sources for the table include the aforementioned Earth Observing Handbook, the Spaceflight Now worldwide launch schedule and a variety of other Internet sources. Generally speaking, specific launch dates for some countries can be tricky to obtain and change frequently, so missions were only included if they could be verified through two or more sources.