BRIDGEPORT, WV (S-M) - Astronomers announced in the October Astrophysical Letters that Comet Encke was recently observed to have its tail torn off by an ejection from the Sun's corona.


"We were speechless when we saw this," said Angelos Vourlidas of the Naval Research Lab. Vourlidas and other researchers had been using the Heliospheric Telescope aboard the STEREO/A spacecraft to observe Encke as it reached perihelion, its closest approach to the Sun, last April. As luck would have it, just as the comet was rounding the Sun, a solar outburst sent a billion tons of hot gas and magnetic fields heading toward the comet.


Even though the solar ejection was moving at more than a million miles per hour, it alone would not be enough to tear off the comet's tail. "The speed sounds impressive, but because the volume of the ejection was so large, its density was actually very low," said Senior Editor Glen Ward. "So, the overall pressure exerted by the gas was the equivalent of a slight breeze here on Earth."


Scientists believe the tail was actually ripped off by magnetic fields in the solar ejection. "In a sense, the comet experienced a geomagnetic storm," said Vourlidas. The magnetic field in the solar ejection must have been oriented opposite to that of the comet's magnetic field. The fields may have suddenly joined together in what is known as a "reconnection event," releasing a blast of energy that tore the comet's tail apart.


Studies like these can have practical applications. Solar ejections can wreak havoc on communications and power systems here on Earth. By studying interactions of the ejections with other bodies, scientists can learn more about how solar activity influences our own planet. "We're going to be looking at the ejection-comet interaction in much greater detail to understand the processes at work," said Vourlidas. - GW

Copyright 2007 Starry Mirror and Glen Ward

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Comet Encke rounds the Sun on April 20, 2007.  (NASA)