The ideas that life may exist somewhere out in space have been with us for a long time. The first and most natural place for life to exist (1) earth was on the moon, and for centuries man has speculated that life of some kind might exist there. When there was the (2) that the moon was both too hot and too cold, there then came the suggestion that life might exist below the surface, protected from the extreme (3) . However, with the series of unmanned and then manned landings on the moon in the 1960s and 1970s, the idea that life of any kind existed on the moon had to be abandoned. Similarly, Mars was for a long time rumoured to be the home of (4) creatures, and there were many theories about the so-called "canals" on Mars. For a long time, many people, including scientists, believed that they might have been constructed by non-human intelligences. The Viking spacecrafts landing on Mars during the second part of the last decade have effectively (5) all such speculations. It seems that no life has (6) existed on Mars.