Space Exploration: Above the Green Veil

The night sky is never completely black. Even when the moon is down, the stars and nebulae give off some light. And that small glow is equalled by a chilly, luminous layer far out in space which surrounds the earth like a diaphanous green veil.

Scientists have been studying this "airglow" layer for more than 40 years, and astronomers were cursing it long before that. Its faint green luminescence, which is probably caused by the recombination of irradiated oxygen atoms, masks dim but fascinating stars from earthbound telescopes. And not until men learned how to climb above that shimmering stratum in...

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