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Searching for little green men on Mars
THE universe is huge and, as far as we know, we are the only ones in it. But surely we cannot be the only ones here polluting space with our satellite debris and space shuttle exhaust fumes?
Tonight BBC2's Horizon will attempt to answer the question that many stargazers from the smallest child to the greatest astronomers have asked themselves: "Are we alone in the universe?"
The programme makers say they will present dramatic new evidence to suggest not only that alien life forms might exist, but that those aliens may be on the planet next door to Earth Mars.
For Horizon the implications are extraordinary. If life can have formed on Earth's nearest neighbour, then it could be anywhere in the universe.
The search for aliens on Mars began in the 19th century when an astronomer was convinced there were canals and cities on the red planet. But the first probes to take pictures of Mars showed it to be savagely cold. If a human was to walk on the planet, its blood would boil and it would quickly drown in its own bodily fluids.
Scientists believe recent discoveries show Mars was not always a dead planet, as the building blocks of life oxygen, carbon and sunlight have all been found there.
Chris McKay, of NASA, says: "It's as if the lights are on, but nobody's home."
This year NASA believes it may have found evidence of the final building block water, detecting what they think is a vast underground ocean on the planet. If true, this raises the possibility of organisms deep below the surface, either alive or frozen.
With new probes due to venture to Mars next year, the quest to find alien life forms may finally be over.
Horizon BBC2, 9pm,