ter is liquid above 15.7 degrees C. Yet none of these elements has been identified among the samples we have reviewed.

     A different approach has been proposed by J. Roser in correspondence with the author. Noting the composition of the Bogota specimen as mentioned in our earlier publication16 he hypothesized a nuclear design for the object's power plant, utilizing direct energy conversion rather than a heat driven mechanical prime mover. "A closed cycle MHD generator using a liquid metal working fluid with no vapor staging pumping could be configured in a torus or circular shape and would make very little noise due to the lack of moving parts," he wrote, adding that the nuclear process known as beta decay might allow the design to extract a surplus of power in the form of free electrons.

     Assuming a working fluid of Aluminum 27 plus some percentage of Phosphorus 31 (solitary stable isotopes of their respective elements) Roser speculates that depleted fluid might need to be occasionally ejected: "This discarded material would contain Al.27, P.31, iron from original melt or housing erosion, plus isotopes of nuclei close to aluminum and phosphorus such as Mg, Na, Si and S" Accordingly he suggests isotopic analysis of the Bogota sample to determine if it reveals anomalous isotopes such as Si.32 (half-life 280 years) which would indicate a nuclear-based power source.

Conclusion

     Reports of unusual metallic residue following the observation of an unexplained aerial phenomenon are detailed enough for a comparative study to be undertaken. This research is hampered, however, by several problems of methodology where lack of money or analytical resources is only a secondary obstacle. The primary concerns have to do with inaccuracies in data gathering, lack of information about exact dates and times, lack of detailed, critical field investigation, and failure to provide an irrefutable chain of evidence in the collection, transportation and examination of the samples.

     In spite of these shortcomings (which could be addressed through the setting up of better standards and through collaborative agreements among investigators), this paper has shown that significant progress had been made toward the analysis of a number of relevant incidents. In one case at least (Council Bluffs), the conditions of witness availability and reliability, on-site testimony by law enforcement officers, chain of custody, and timely analysis were met. Other cases, such as Ubatuba and Bogota, are sufficiently intriguing to encourage investigators to expand their work in the field.

     Over the years discussions of the UFO issue have remained narrowly polarized between advocates and adversaries of a single theory, namely the extraterrestrial hypothesis (ETH), defined as contact with an alien civilization originating in another solar system in our universe. This fixation on the ETH has narrowed and impoverished the debate, precluding examination of other

     ¹⁶Vallee, J. F.: Confrontations, 1990, pp. 44-45.