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Discovery of a comet by its Lyman-α 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 contribute to the discovery of comets, especially out of the orbital plane of the Solar System. An ideal way to search for comets would be to use a spaceborne instrument capable of imaging the whole sky on a daily basis in a systematic and repeatable way. Such an instrument already exists on the solar observatory SOHO; it operates at the Lyman-α wavelength of neutral hydrogen, 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 hitherto 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 find that it increases after perihelion, like that of comet Halley.

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Figure 1: Bright comets visible in the southern ecliptic hemisphere from May to July 1997.
Figure 2: Water production rate Q H 2 O of comet K2 as a function of time.

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SOHO is an international cooperative mission of ESA and NASA. SWAN was financed in France by CNES with support from CNRS and in Finland by TEKES and the Finnish Meteorological Institute. The work of J.T.T.M., H.L. and T.S. was supported by the Academy of Finland.

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Correspondence to J. Teemu T. Mäkinen.

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Mäkinen, J., Bertaux, JL., Laakso, H. et al. Discovery of a comet by its Lyman-α emission. Nature 405, 321–322 (2000).

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