The spindle-legged creature flew over the bleak forbidding landscape, examining the terrain very carefully. The ground was covered with treacherous pits and dangerous looking rocks, was not what had been expected. Great care was needed before it could select a place to land.

Slowly, and very cautiously, the creature lowered itself onto the rock and dust covered plain, following which it emitted a high-pitched squeak of success. It was 4:13 local time.

The air was alien and very cold, but the creature was well adapted to these conditions and quickly set about its predestined tasks.

First it used is multipurpose optical sensors to picture the area nearby, and, when satisfied that there was no danger, it began to "feed".

A tubelike arm extended itself from the creature's main body. A scoop at one end scratched up a sample of the dust and deposited the material into its oral apparatus. Within the creature's "body", fluids were mixed with the dust and digestion began.

Not long afterward, the creature "burped" sadly, its long journey of expectation ending in disappointment. The dust it had just analyzed contained no nourishing organic of living material. The creature' search had been, at least in this respect, a failure.

Once more it sent out a signal. On a different planet, 164 million miles away, at the National Aeronautics and Space Administration (NASA) Center in Pasadena, the billion-dollar, eight-year search for life on the planet Mars came to a sad end.

The Viking I mission to the red planet put down its unmanned landing vehicle on Joy 20th, 1976, exactly seven years after the famous Eagle of Apollo II had made a similar landing on the moon. Among the many important scientific experiments the Viking Lander was programmed to carry out, the one that caught the public's attention most was the search for possible life on another world.

Few things trigger the human imagination more vividly than the speculation that "we are not alone". Other living organisms may inhabit distant planets and may one day try to contact us. This idea has been fuel for a prodigious amounts of science fiction and was strong enough to motivate the United States into devoting millions of human hours, and over a billion dollars, in the search for life on Mars.

Unfortunately, the Lander vehicle, a product of human technological ingenuity, turned out to be the closest thing to a living organism on the Chryse Planitia (the Golden Plain) of that distant planet.

What exactly were the NASA scientists looking for when they set out to design a life-seeking space probe? How can "life" be recognized even in alien form?

What is life?

Unfortunately there is no one sentence answer to this question and no one simple experiment to prove conclusively that an alien was "alive". Instead, the biologists who were advising the NASA team began by asking slightly different questions: What are the characteristics of life?"

What are the properties we would expect to find associated with a living organism?

The Viking Lander, therefore, contained not one, but a variety of experiments, each designed to look for a telltale signe that life was present on the red planet. None of these signs individually would be conclusive proof, but taken together they might just add up to evidence proving that we are not alone.