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The Hyakutake Comet
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Michael Brown (University of Melbourne), Chris Fluke (University of Melbourne) and Mount Stromlo and Siding Spring Observatories

DISCOVERED! X-rays from comet Hyakutake
News story originally written on May 18, 1997

A bit of background

A small portion of the solar wind is comprised of minor ions, atoms of oxygen, carbon, and neon, that have been nearly stripped of their electrons. Dr. Thomas Cravens of University of Kansas theorizes that these minor ions found in solar wind can steal electrons from neutral atoms found in comets. These stolen electrons start off in the excited state and fall into lower orbitals. For an electron to fall down to a lower orbital, it must get rid of energy, often in the form of radiation. When this occurs in a comet this excess energy can be given off in the form of x-ray radiation. So by detecting this x-ray emission, we have a new and exciting way to track comets and solar wind phenomena.

The details of the discovery

On March 27, 1996, observations were taken of the Comet Hyakutake by Germany's ROSAT satellite. X-ray emissions (the first from any comet) were detected at this time. A simulation of these emissions was conducted using an Earth sciences super computer at the NASA Goddard Space Flight Center. The computer simulation showed that the x-ray emissions depend mainly on the solar wind composition, not on the comet. Because of this independence, researchers can determine where the comet's nucleus is in relation to the comet's atmosphere.

"Cometary X-rays present a potentially powerful new tool to monitor comet activity far from Earth, as well as the composition and flux of the solar wind," said co-author Dr. Tamas Gombosi of the University of Michigan, Ann Arbor. "By capturing these X-rays' detailed energy spectrum, it might be possible to monitor the propagation and evolution of spectacular solar wind phenomena, such as the coronal mass ejections seen this January and April." This new theory of cometary x-ray emission will be tested on the Comet Hale-Bopp using Japan's ASCA x-ray satellite in September.

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