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physical_sciences : Message: News: Supersonic snowballs in hell: How comets explode, fizzle out, or survive a flight through the Sun's atmosphere

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  • Roger L. Bagula
    http://tech.groups.yahoo.com/group/physical_sciences/message/6608 Supersonic snowballs in hell: How comets explode, fizzle out, or survive a flight through the
    Message 1 of 1 , Apr 4, 2012
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      http://tech.groups.yahoo.com/group/physical_sciences/message/6608
      Supersonic snowballs in hell: How comets explode, fizzle out, or survive
      a flight through the Sun's atmosphere
      April 2nd, 2012 in Space & Earth / Space Exploration
      Supersonic snowballs in hell: how comets explode, fizzle out, or survive
      a flight through the Sun's atmosphere(PhysOrg.com) -- Since the 1980s
      astronomers have seen thousands of comets falling towards the Sun, most
      of them too small to survive a close approach, let alone to re-emerge.
      Until recently no such objects had been seen very close to the Sun as
      the glare of sunlight made them impossible to observe. Now a team of
      scientists led by Professor Emeritus John Brown, Astronomer Royal for
      Scotland and former Regius Professor of Astronomy at Glasgow University,
      have worked out which comets make it through this fiery journey, which
      fizzle out high up and which explode just above the surface. Prof. Brown
      will present this new work in a paper at the National Astronomy Meeting
      in Manchester on Friday 30 March.

      Comets are giant dusty snowballs believed to date from the epoch of the
      formation of the Sun and planets, so carry important information about
      the early history and composition of the Solar system. The comets we see
      spend most of their time very far from the Sun, orbiting in the so
      called Oort Cloud, before being disrupted into orbits that carry them
      towards our nearest star over tens of thousands of years.

      When comets reach the inner Solar System, their dusty ices melt and
      vaporise to form huge tails blown back by the solar wind and by
      sunlight. The largest, like the famous Comet Hale Bopp seen in the late
      1990s, have nuclei tens of kilometres across and masses of 10 million
      million tonnes. Objects this large only lose a tiny fraction of their
      material on each passage around the Sun, so are able to survive
      thousands of journeys through the Solar System. In contrast, the
      smallest objects may only be 10 metres across with a mass of 1000
      tonnes. If these small comets make a close approach to the Sun, they are
      vaporised by sunlight and by the friction of the atmospheric gas.

      This video is not supported by your browser at this time.

      Credit: NASA
      In the culmination of work carried out over the last few years,
      Professor Brown and his colleagues are now able to predict how comets
      lose their mass and are destroyed in the solar atmosphere, their
      behaviour depending on whether or not their orbital path reaches into
      the ‘lower atmosphere’ 7000 km (roughly 1% of the solar radius) from the
      top of the brightest visible solar layer, the photosphere. The team
      worked out the different ways comets give up their mass, momentum and
      energy to the Sun’s atmosphere according to their height. Above 7000 km
      the cometary nuclei are slowly vaporised by sunlight and the gases
      streaming off into the coma and tail lose energy and momentum by
      atmospheric drag. In the low solar atmosphere material is stripped away
      not by sunlight but by the drag of the solar gas surrounding the comet
      and by exploding under the ‘ram pressure’ force of the atmosphere as the
      comet runs into denser layers.

      The group found that sunskimmer comets (those with their closest
      approach more than 7000 km from the Sun) are destroyed in a slow
      ‘fizzle’ lasting hundreds to thousands of seconds, depending on their
      mass. During their demise they should emit weak but detectable extreme
      ultraviolet (XUV) radiation. In contrast, the ‘sunplungers’ that
      approach the Sun more closely will be destroyed in a few seconds as they
      crash into the dense layers of the lower solar atmosphere. The resulting
      explosions produce effects similar to those of solar flares, such as
      sunquakes on the Sun’s surface. Finally, if the most massive comets were
      to collide with the Sun they would produce dramatic explosions just
      above the photosphere itself.

      In July and December last year the NASA Solar Dynamics Observatory (SDO)
      satellite made the first direct observations of comets making close
      approaches to the Sun. The first comet, C / 2011 N3 (SOHO) was
      completely destroyed after passing 100,000 km above the photosphere
      whilst the second and larger comet, C / 2011 W3 (Lovejoy) survived a
      close approach to a similar distance (140,000 km) although it lost a
      significant fraction of its mass in the process. Both events were in
      line with the predictions of Prof. Brown and his collaborators. They
      emphasise that, contrary to some news releases, the death or attrition
      of such comets has nothing to do with the high temperature (two million
      degrees Celsius) of the outer solar atmosphere since, though hot, it is
      so tenuous that it contains little heat.

      Prof. Brown comments: “In modelling how icy comets behave in this
      extreme environment, we really are starting to understand what happens
      to these ‘supersonic snowballs in hell’ when they make a close approach
      to the Sun.

      "The two sun skimmers seen last year have already given us a new insight
      into the Sun’s atmosphere and it’s only a matter of time before we see
      the flare from a sun plunger – with a low enough orbit to reach the
      lower atmosphere of the Sun. When that happens we will be able to
      analyse the light from the resulting ‘cometary flare’ and find out even
      more about the composition of the interiors of comets.”

      Provided by Royal Astronomical Society

      "Supersonic snowballs in hell: How comets explode, fizzle out, or
      survive a flight through the Sun's atmosphere." April 2nd, 2012.
      http://www.physorg.com/news/2012-04-supersonic-snowballs-hell-comets-fizzle.html

      Posted by
      Robert Karl Stonjek
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