A Well-Studied Comet
With the LASCO coronagraph, SOHO observes many sungrazing comets as they plunge into the Sun. Some of the brighter ones are also observed in ultraviolet light, when possible, with the UVCS instrument. The above image shows the appearance of the recent comet C/2002 S2 at 3 heights (insets), in light scattered by hydrogen atoms produced by the comet.
Sunlight boils water off the comet's surface, breaking the water molecules down into hydrogen and oxygen. The brightness of the Ultraviolet emission indicates how much water boils off the surface, typically 100 kg per second for bright sungrazers. The hydrogen stops scattering light after being ionized through interaction with the surroundings; the denser and hotter the surroundings, the faster the "glow" disappears. The length and width of the tail above imply that the comet encounters relatively cool, low density coronal gas.
Taken: 17-19 September 2002
Picture credits:SOHO/UVCS & SOHO/LASCO (ESA & NASA)