From: dietz@dls.net   
      
   Henry Spencer wrote:   
      
   >>Couldn't the spacecraft have it's own magnetic field?   
   >   
   >   
   > To be a useful barrier to incoming particle radiation, the field would   
   > have to be immensely strong or would have to extend over a huge distance   
   > (which means either making it immensely strong at the source, or   
   > generating it with a physically very large structure).  It's possible   
   > in theory but impractically hard in practice, at least for now.   
      
   To put some numbers on this...   
      
   To deflect particles of a given energy, the strength of the magnetic   
   field B is inversely proportional to the linear dimensions of the   
   field (assuming identical geometry).  Since the stored magnetic   
   energy is proportional to volume * B^2, the total energy stored   
   in the magnetic field will scale in proportion to the linear   
   dimensions of the protected volume.   
      
   The Earth's magnetic field outside the atmosphere has a stored   
   energy equal to about that of a 200 megaton bomb.  To similarly protect   
   a 12 meter sphere (as opposed to a 12,000 km sphere) would require   
   a magnetic field with the energy of a 2 kiloton bomb.  (This is   
   probably overkill, though.)   
      
   	Paul   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)