Discovery of a comet by its Lyman-alpha emission

Several searches for near-Earth objects have recently been initiated, as a result of increased awareness of the hazard of impacts on the Earth. These programs mainly search for asteroids, so amateur astronomers can still cont ribute to the discovery of comets, especially out of the orbital plane of t he Solar System. An ideal way to search for comets would be to use a spaceb orne instrument capable of imaging the whole sky on a daily basis in a syst ematic and repeatable way. Such an instrument already exists on the solar o bservatory SOHO; it operates at the Lyman-alpha wavelength of neutral hydro gen, which is the main component of the emission cloud of a comet. Here we report the discovery, using archival data from this satellite, of a hithert o unnoticed comet which reached a perihelion of 1.546 a.u. on 26 June 1997. We derive the water production rate of the comet as a function of time and rnd that it increases after perihelion, like that of comet Halley.