Only a narrow band of "visible" radiation reaches the Earth's surface unscathed. All radiation from the extreme ultraviolet to high-energy gamma rays fail to penetrate below an altitude of about 30 km. Most incident solar X-rays are absorbed through the photoelectric effect by atoms and molecules in the upper atmosphere. It is the requirement of observing above the atmosphere that makes the study of the X-ray universe a modern one, because until recently, the visible part of the spectrum was all that was accessible from the ground based telescopes.

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