APOD: The Magnificent Tail of Comet McNaught (2009 Dec 06)

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The Magnificent Tail of Comet McNaught

Explanation: Comet McNaught, the Great Comet of 2007, was the brightest comet of the last 40 years. Its spectacular tail spread across the sky and was breathtaking to behold from dark locations for many Southern Hemisphere observers. The head of the comet remained quite bright and was easily visible to even city observers without any optical aide. Part of the spectacular tail was visible just above the horizon after sunset for many northern observers as well. Comet C/2006 P1 (McNaught), which reached an estimated peak brightness of magnitude -6 (minus six), was caught by the comet's discoverer in the above image soon after sunset in 2007 January from Siding Spring Observatory in Australia. The robotic Ulysses spacecraft fortuitously flew through Comet McNaught's tail and found, unexpectedly, that the speed of the solar wind dropped significantly.

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<<The Ulysses spacecraft made an unexpected pass through the tail of the comet on February 3, 2007. Evidence of the encounter was published on the October 1, 2007 issue of the Astrophysical Journal. Ulysses flew through McNaught's ion tail 160 million miles from the comet's core and instrument readings showed that there was "complex chemistry" in the region.

The Solar Wind Ion Composition Spectrometer (SWICS) aboard Ulysses measured the composition speed of the comet tail and solar wind, and detected unexpected ions within the comet's tail and found that it had a major impact on the surrounding solar wind. It's the first time that researchers have detected O3+ oxygen ions (atoms of oxygen with a positive charge because they have five electrons instead of eight) near a comet. This suggested that the solar wind ions, originally missing most of their electrons, picked up some of their missing electrons as they passed through McNaught's atmosphere.

Besides that, SWICS found that even at 160 million miles from the comet's nucleus, the tail had slowed the solar wind to half its normal speed. The solar wind should usually be about 435 miles per second at that distance from the Sun, but inside the comet's ion tail, it was less than 249 miles per second.

• "This was very surprising to me. Way past the orbit of Mars, the solar wind felt the disturbance of this little comet. It will be a serious challenge for us theoreticians and computer modellers to figure out the physics,"
  
  —space science professor, Michael Combi.

Prof. George Gloeckler, the principal investigator on the Solar Wind Ion Composition Spectrometer (SWICS), said the discovery was important as the composition of comets told them about conditions approximately 4.5 billion years ago when the solar system was formed.

• "Here we got a direct sample of this ancient material which gives us the best information on cometary composition. We're still in the process of figuring out what it tells us. We're contributing part of the whole puzzle".
  
  "The benefits of such an observation are important. They constrain the interactions of such comets with the Sun, including how the comets lose mass. They also examine the question of how a sudden injection of neutral and cold material interacts with hot solar-like plasmas. That occurs in other places of the universe and we were able to study it right here,"
  
  —space science professor, Thomas Zurbuchen.

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