2042 A.D., powerful orbiting telescopes scour the heavens for signs of life. They focus on a dim yellow globe. Spectrum analyser results, negative, no oxygen, no life. The autotracker zooms into another world, a pale blue dot, oxygen signal affirmative, and there appears to be water, this could be it. The scenario belongs to the future, but the future is almost here. Life on earth may have begun with a bang when microorganisms hitching a ride on a comet or asteroid, crashlanded on our planet.

Finding themselves in a mild and watery world, the single-celled organisms could have slowly evolved into the rich and varied life forms that exist today. There are clues that this may have been what happened, but what do these early microorganisms look like and from which planets or moons might they have come? Single-celled organisms are just one example of life that might exist in the depths of space, there could also be intelligent life out there, way beyond our own solar system.

Some people go as far as to say that aliens have already appeared on earth, that governments have hidden them away in secret military establishments. Today, serious efforts are being made to search for life on other worlds. Radiotelescopes listen for signals transmitted from civilizations at least as advanced as ours. Seth Shostak works for SETI, the Search for Extraterrestrial Intelligence, the offshoot of an earlier NASA program.

"People in the last century thought about possibly searching for aliens using lights and stuff like that, but in fact, radio searches began in this century, and really, after the war. It was 1960 that the first really scientific radio search was conducted."

(..invaders from Mars, weird and fascinating beings of a super intelligence.)

Popular fantasies about the appearance and intentions of alien beings are as varied as human imagination. Paul Davies is a physics professor and writer of popular science. He has studied the cultural presence of aliens in human history.

"When most people think about intelligent alien beings, they have something very much like ourselves in mind, that they would look like us, think like us and behave like us, but they would be sort of super beings. And when you look back in history, stories of angels and giants and Greek gods and so on, they're always presented in this human or semi-human form. I think that's just some sort of deep cultural needs to believe in something like ourselves but better." ...

scour:to search a place or thing very carefully in order to try to find something 巡视；搜索

scenario:a description of possible actions or events in the future 想定

asteroid:one of many rocky objects, varying in width from over 900 kilometres to less than one kilometre, which circle the sun [天文]小游星, 小行星,

offshoot: something which has developed from something larger which already existed 分支, 支流