An early warning satellite is a satellite designed to rapidly detect ballistic missile launches and thus enable defensive military action. To do this, these satellites use infrared detectors that identify the missile thanks to the heat given off by its engines during the propulsion phase. This type of satellite was developed in the 1960s in the context of the Cold War in order to activate early warning systems in the target territories of a missile attack. It later became a component of missile defense systems, as well as regulatory control systems for nuclear tests. Only the two main nuclear powers, Russia and the United States, have a constellation of early warning satellites. Description The objective of an early warning satellite is to detect the launch of a ballistic missile at the beginning of its trajectory, when its detection is possible since its propulsion system gives off heat. For a typical ICBM fired from a distance of 10,000 km, this propulsive phase lasts about 3 minutes for a total flight time of about 30 minutes. After these first 3 minutes, the flight continues by inertia and the missile becomes practically undetectable by the satellite. The early warning satellite has the advantage over a radar of being able to scan almost 50% of the Earth's surface if it is at a sufficient altitude and therefore gives the attacked country more time to react compared to a system based solely on radar. The detection of the missile is carried out by sensors that observe infrared wavelengths corresponding to the temperature of the flames of the missile engines (greater than 1000 °C). The on-board computer that processes the signal must be able to eliminate radiation sources linked to the reflection of sunlight on the ground or in the clouds. The image is magnified by a telescope whose aperture reaches one meter on the latest US satellites. Programs United States The United States was the first country to attempt to establish a space-based early warning system. The goal was to detect Soviet ballistic missile launches and give 20 to 33 minutes notice of the missile's arrival (against 10 to 25 minutes for the BMEWS ground-based radar network). The MIDAS satellites were launched between 1960 and 1966, and although they never entered a truly operational phase, they allowed the development of this type of satellite. DSP satellites in geostationary orbit took over in the early 1970s.