We say that life replicates. Life uses energy. Some forms of life have developed large central processing networks. In at least one instance, life has become profoundly self-aware. There may be no scientific mystery so tantalizing at the brink of the new millennium and yet so resistant to an answer.
Extraterrestrial life represents an enormous gap in our knowledge of nature.
With instruments such as the Hubble Space Telescope , scientists have discovered a bewildering amount of cosmic turf, and yet they still know of only a single inhabited world. We all have our suppositions, our scenarios. The late astronomer Carl Sagan estimated that there are a million technological civilizations in our galaxy alone. His more conservative colleague Frank Drake offers the number 10,000. John Oro, a prominent comet researcher, calculates that the Milky Way is sprinkled with a hundred civilizations.