Earth observation (EO) satellites have been orbiting our skies since the beginning of the space era, just after Sputnik was launched in 1957. Although most of the initial missions were linked to defence and meteorological applications, the first Landsat satellite launched in 1972 by the USA, demonstrated it was possible to use this technology for civilian applications such as agriculture, and land mapping.

If the demonstration of existing technology that could be flown in space was one of the drivers to build new EO satellites, the link between science “pull” (or demand) and technology “push” was then further nurtured. As a result, nowadays, most of EO science satellites must fulfil scientific requirements and not only demonstrate new technology. Moreover, in the development of new satellites, compromises are often made by relaxing strict scientific requirements to avoid lengthy and costly technology developments. This highlights the fact that, even in space exploration, the pursuit of new scientific discoveries goes hand-in-hand with the creation of cutting-edge technologies.

Many nations and agencies have developed their own EO satellites. At the beginning NASA successfully pioneered the observation of the Earth with the Landsat series; currently Landsat-8 and Landsat-9 are operating. However Europe is currently in a leading position worldwide in this field with the prominent roles taken by European Space Agency (ESA) and the European Commission (EC). This is very likely due because Europe is currently investing almost as much as NASA in EO [1] although the overall NASA budget is more than threefold when all space activities are included.

It is worth noting the numerous and flourishing national activities of the ESA member states such as: the SPOT, SWOT programme in France, TerraSAR/TanDEM-X, EnMAP, and GRACE in Germany, COSMO-Skymed, PRISMA in Italy, PROBA in Belgium, etc. These missions are all successes in their own right, and showcased the great leaps Europe has taken in recent years in the EO field. Other countries such as Japan, Canada, India, China, Korea, and Russia are also very active in using data collected by EO satellites to monitor the Earth. A good illustration of the cooperation in this field has been demonstrated since 2000 with the creation of the “International Charter Space and Major Disasters” [2] for which 17 members worldwide are contributing with more than 270 EO satellites when a natural disaster strikes the Earth.

The work performed by the European Space Agency (ESA), together with its member states, should be particularly commended since the development of ERS-1 and ERS-2 in the 1980’s (culminating in their launches respectively in 1991 and 1995), followed by the launch of ENVISAT in 2002 which operated 5 more years than its planned lifetime until 2012.

A striking example of a groundbreaking science discovery allowed by EO satellites is the “image” of an earthquake using the concept of spaceborne synthetic aperture radar (SAR) interferometry, first developed at NASA-JPL, but only fully demonstrated and exploited with the tandem use of ERS-1 and ERS-2 (i.e. ERS-2 passing the same point on the ground one day later than ERS-1) as shown in the figure below.