Message #204425

[Jennifer Murdoch]

for interstellar communications  have been performed by Betz (1986),
       Kingsley (1992), Ross (1980), and Rather (1991).

       The first international  SETI  in   the   Optical  Spectrum  (OSETI)
       Conference was organized by Stuart Kingsley, under  the  sponsorship
       of The International   Society   for  Optical  Engineering,  at  Los
       Angeles, California, in January of 1993.

       There have also been independent suggestions by Drake and Shklovskii
       (Sagan and Shklovskii,  1966)  that  the  presence  of  a  technical
       civilization could be  announced  by  the dumping of  a  short-lived
       isotope, one which  would  not  ordinarily  be expected in the local
       stellar spectrum, into the atmosphere of a star.  Drake suggested an
       atom with a  strong, resonant absorption  line,  which  may  scatter
       about 10exp8 photons  sec  -1  in  the stellar radiation  field.   A
       photon at optical  frequencies has an energy of about 10exp(-12) erg
       or 0.6 eV, so each atom will scatter  about  10exp(-4)  erg sec-1 in
       the resonance line.  If we consider that the typical  spectral  line
       width might be  about  1  ^O,  and  if  we assume that a ten percent
       absorption will be  detectable, then  this  "artificial  smog"  will
       scatter about (1A/5000A)x10exp(-1)  =  2x10exp(-5)   of   the  total
       stellar flux.

       Sagan and Shklovskii  (1966) considered that if the central star has
       a typical solar flux of 4x10exp33  erg  sec-1, it must scatter about
       8x10exp28 erg sec-1  for  the line to be detected.   Thus,  the  ETC
       would need (8x10exp28)/10exp(-4)  =  8x10exp32 atoms.  The weight of
       the hydrogen atom (mH) is 1.66x10exp(-24)  g,  so  the  weight of an
       atom of atomic weight n is nxmH grams.