(note: We conclude our series on the history of barium related research in atmospheric science with a simple explanation in an excerpt on page 180 from the author(s) of the book, The Century of Space Science, by J. A. M. Bleeker, Johannes Geiss, M. Huber, Published 2001)
PROPOSAL FOR AN ARTIFICIAL COMETARY TAIL
After the successful launches of the first artificial satellites and space probes in the late fifties, I discussed with Biermann at the beginning of 1960 whether German scientists should also get involved actively in space research by using sounding rockets, satellites or space probes and what role the Max Planck Institute for Physics and Astrophysics in Munich could play. In the astrophysical part of the Institute, only theoretical work was then being performed.
During the discussion, the idea of creating an artificial cometary tail in order to understand much better Biermann's theoretical concept of the interaction between the solar wind and the ionized cometary tail was launched.
Of course, using the same molecules observed in a natural cometary tail for such an artificial one would have been most attractive. But the calculation showed that several tons of carbon monoxide (CO) would be needed to create a visible artificial cometary tail (Biermann et al 1961).
Therefore other elements or molecules had to be found. In order to keep the cost down and the payload of the sounding rocket as light as possible, it was clear from the outset of the programme that the best energy source for ionizing and exciting the atoms in an artificial cloud was solar radiation. Furthermore, the cloud had to be observable from the surface of the Earth.
These conditions led to a number of requirements for suitable elements or molecules:
(1) The resonance lines of the ions had to be within the "optical window" of the Earth's atmosphere.
(2) The time scales involved in exciting the expected lines of the ions, and photoionizing the neutral atoms had to be sufficiently short,
(3) Since a chemical technique for the release was to be used, a low evaporation temperature was highly desirable.
The most promising elements to meet these requirements were some alkaline-earth metals, particularly barium, and probably some of the rare-earth elements, namely europium and ytterbium.
We tried strontium and barium, and discovered that visible clouds could be created by using barium. The required quantities were very low, of the order of some 10 to 100 grammes of Ba[rium] ions.