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Keck I and II with comet. Composite image of comet Hale-Bopp seen above the Keck I and II telescope domes on Mauna Kea, Hawaii, USA. The optical telescopes of Keck I and II each have a 10 metre diameter mirror, which can be used together to provide more detailed images. Mauna Kea, an extinct volcano 4200 metres high, has several telescopes on its summit. Comet Hale-Bopp, seen in early 1997, was one of the brightest comets of the 20th century. Here, its two tails are seen. The gas or 'ion' tail (blue) consists of ionised glowing gas blown away by the solar wind. The dust tail (white) consists of dust grains pushed away from the comet head by the radiation of sunlight. (KEYSTONE/SCIENCE PHOTO LIBRARY/DAVID NUNUK)