caption: A huge cloud of hydrogen surrounded<PERSON> when it neared the Sun in the spring of 1997. Ultraviolet light, charted by the <PERSON> instrument on the SOHO spacecraft, revealed a cloud 100 million kilometres wide and diminishing in intensity outwards (contour lines). It far exceeded the great comet's visible tail (inset photograph). Although generated by a comet nucleus perhaps 40 kilometres in diameter, the hydrogen cloud was 70 times wider than the Sun itself (yellow circle to scale)
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