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Since the early 1960s, numerous satellites have been launched to observe Earth. Some of them are fairly specialized, such as meteorology (weather) or radar satellites (see Box 3.2). Others, such as the various spacecraft in NASA's Earth Observing System or Europe's Sentinel satellites, carry a wide range of sensors for multiple uses, such as environmental monitoring, land use mapping, altimetry, etc.

Two main types of orbit are used. For large‐scale or hemispheric observations, geostationary orbits are favored. From an altitude of 35,780 km above the equator, such satellites circle the planet once every 24 hours, so they appear to hover over the same spot, providing continuous monitoring of the same part of Earth's surface. These orbits are ideal for weather satellites such as the U.S. Geostationary Operational Environmental Satellite (GOES) series and Europe's Meteosats.

More detailed observations require much lower orbits. These are usually provided by satellites in near‐polar orbits, inclined almost 90° to the equator. However, polar‐orbiting spacecraft can only provide brief snapshots as they pass overhead. Such overpasses typically take place twice per day at any given spot on the surface. Global coverage is achieved by combining many swaths (strips) of data that are acquired as the planet rotates beneath the satellite. The other alternative is to orbit dozens or even hundreds of satellites that carry similar Earth observation instruments.

One of the most popular types of polar orbit is one in which illumination conditions on the surface remain constant. Such Sun‐synchronous orbits are typically inclined about 98° to the equator, enabling the satellite to cross the equator at the same local time on each orbit. Perhaps the most famous example is the U.S. Landsat series, which has been providing continuous, medium‐resolution imagery of Earth's land masses in different spectral wavelengths since 1972.

Multiple observations of the same area by different instruments on different satellites are also invaluable in providing information about changes over short and long periods of time (i.e. multi‐temporal data). The most enterprising example is the Afternoon Constellation, often known as the “A‐Train,” which includes up to seven satellites from the U.S. and France that follow the same orbital path around Earth, one behind the other. The A‐Train enables near‐simultaneous coordinated measurements of the same regions, giving a more complete overview than would be possible from a single satellite.

Each satellite in the A‐Train crosses the equator at around 1:30 p.m. local time, separated by intervals of seconds or minutes. Satellites such as Aqua and CloudSat observe many different surface and atmospheric phenomena, including hurricanes, clouds, and aerosols.

As the temperature dropped, condensation of water vapor caused the planet to be blanketed in cloud. The resultant global downpour led to the formation of the first oceans. Some carbon dioxide gas dissolved in the rain and oceans. At the same time, chemical reactions involving acidic rain and the first, primitive crust led to the formation of carbonate rocks and a further reduction in atmospheric carbon dioxide.

According to this scenario, nitrogen – a gas which is not very chemically active – continued to accumulate in the atmosphere as a result of outgassing and numerous impacts. Free oxygen was scarce, since it was soon removed through chemical reactions with rocks and other gases. The first single‐celled organisms were probably also important in regulating the climate by generating substantial amounts of methane – a greenhouse gas.

The change to a more modern atmosphere began when bacteria and algae developed the ability to split water molecules by harnessing the energy of sunlight – a key part of photosynthesis. As a result, the amount of oxygen released into the atmosphere began to increase, although for a long time, most of the gas was removed by chemical reactions with rock minerals. To this day, the majority of Earth's oxygen produced over time is locked up in ancient, oxidized rock formations.

It was not until about one billion years ago that the rate of oxidation slowed sufficiently to enable free oxygen to stay in the air and the levels of oxygen in the oceans to rise. Once oxygen gas was available, ultraviolet light began to split the molecules, producing a layer of ozone (O₃) high above the planet that acted as a shield against ultraviolet light. Only at this point did life move out of the oceans and respiration evolve.