* Astronomy

Watching the stars set from the surface of the Earth may be a romantic pastime but when a spacecraft does it from orbit, it can reveal hidden details about a planets atmosphere.
 The technique is known as stellar occultation. Jean-Loup Bertaux, Service d'Aeronomie du CNRS, France was the first to suggest its use on an ESA mission. It works by watching stars from space, while they drop behind the atmosphere of a planet under investigation, before disappearing from view below the planets horizon.
When the stars are shining above the atmosphere, they give off radiation across a wide spread of wavelengths. As the orbit of the spacecraft carries it around the planet, the star appears to sink down, behind the atmosphere of the planet. The atmosphere acts as a filter, blocking out certain wavelengths of the stars radiation. The key to this technique is that the blocked wavelengths are representative of the molecules and atoms in the planets atmosphere.

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