EXTRATERRESTRIAL LIFE.

The search for life away from planet Earth has been called a science without a subject matter. Despite the countless hours that dedicated scientists and amateurs alike have spent searching the skies, there is no evidence that life exists anywhere in the universe except on Earth. Exobiology is a branch of biology that deals with the search for extraterrestrial life, especially intelligent life, outside our solar system. Exobiology is sometimes called xenobiology or astrobiology.

Urey and Miller experiment.

One approach to investigating the existence of extraterrestrial life is to discover the conditions necessary to replicate the earliest forms of life on Earth. The first experimental simulation of primitive conditions believed to duplicate Earth's atmosphere before life existed was carried out in 1953 by an American graduate student S.L. Miller under the guidance of chemist Harold C. Urey. In the experiment a mixture of methane, ammonia, water vapour, and hydrogen was circulated through a liquid water solution and continuously subjected to an electric spark to simulate lightning. After several days amino acids had been produced. These amino acids are building blocks in contemporary life forms. Most, if not all, of the essential building blocks of proteins, carbohydrates, and nucleic acids can be readily produced under similar primitive conditions.

Green Bank equation. The Green Bank equation, devised by United States astrophysicist F.D. Drake, represents an attempt to estimate the number of technically advanced civilisations in the Milky Way galaxy. It expresses mathematically the relationship of the number of stars, the number of stars with planetary systems, the number of planets in each system having conditions suitable for the origin of life, the number of planets on which life could actually develop, the number of those planets on which intelligent life could evolve, the number of intelligent populations that could develop civilisations capable of interstellar communications, and, finally, the average lifespan of technical civilisations. Depending on how estimates for various terms in the equation are made, the number of advanced civilisations in the Milky Way is estimated at from 1 to 1,000,000.

Interplanetary exploration. The most elaborate use of an unmanned space probe to investigate a neighbouring planet involved the United States Viking 1 and 2 probes that were launched in 1975. These twin spacecraft each consisted of an orbiter and lander. The landers were equipped with cameras, seismometers, soil-sampling scoops, and biological experiments specially designed to search for traces of bacterial life on Mars. Both probes continued to relay detailed close-up pictures and data from Mars. The results of the biological tests proved inconclusive.