All known life in the universe is contained within the thin sliver of air seen in this image, underlining the precious fragility of our Earth environment.

This image, showing the edge of our planet - known as the Earth's limb - at twilight, was taken by one of the STS-100 crew on 26 April 2001 from Space Shuttle Endeavour with a digital still camera.

Near the centre the silhouette of cloud layers can be seen in the atmosphere, above which lies an airglow layer (left).