Solar rays broke up water vapour released from the comet by the warmth of the Sun. The resulting hydrogen atoms shone by ultraviolet light invisible from the Earth's surface. Even a satellite's view of the Hale- Bopp cloud would be spoiled by hydrogen around the Earth. Stationed 1.5 million kilometres out in space, SOHO had a clear view. Tuned to see hydrogen, SWAN scans the sky and studies the solar wind's effect on hydrogen atoms coming from interstellar space.
Comets reveal themselves as local sources of hydrogen.
With SWAN's maps, Michael Combi of the University of Michigan studies gas outflow from comets. He also uses the Hubble Space Telescope, but that instrument cannot safely look at comets near the Sun. The unique SWAN observations of Comet Hale-Bopp imply that the outflow of water vapour peaked at almost 50 million tonnes a day.