At 1.12pm on Tuesday, if the sky stays clear, thousands of Hong Kong people will watch in awe as the face of a deadly neighbour slides slowly across the dial of the sun in a spectacle no living person has ever seen. Venus - the closest planet to ours and one described as its 'evil twin', thanks to dense clouds of sulphuric acid, an atmosphere of carbon dioxide and surface temperatures of almost 500 degrees Celsius - will cross the sun as seen from Earth for the first time in 122 years. It is a place of romance and horror. Venus is the last celestial body to disappear from the night sky and the brightest planet in the firmament - it is often misreported as a UFO during the peak of its powers - and the globe Emily Bronte described as 'a silent silvery star' that lingered in the dawn sky after morning had broken. But its literary image as a lush and homely sister planet was shattered when early Russian space probes melted as they tried to land on its surface; it was later unmasked as a sun-blistered inferno of bubbling lava and poisonous gases. Nothing could survive there, scientists concurred, and they began to look elsewhere for life on other planets. Now, the conclusion that there is no life on Venus is being challenged by a group of international scientists who believe there may be primitive life forms floating in the clouds 50 kilometres above its surface. It is a notion that would have been dismissed as outlandish little more than a decade ago, but recent discoveries of bacteria in clouds above Earth and proof that organisms can survive in extreme acidity have given credence to the theory. The other factor lending weight to the hypothesis is that Venus is now considered likely to have started out similar to Earth - a planet with oceans and a cool surface after its birth 4.6 billion years ago - before cataclysmic changes turned it into the fireball it is today. If organisms adapted to the changes, runs the theory, they may have found a new home in the relatively hospitable cloud layer, where despite the high level of ultraviolet rays from the sun and the acid content, temperatures range from 40 to 60 degrees Celsius. 'Life on Venus?' was the incredulous response of Hong Kong Space Museum curator Sam Chow Kim-fung, who expects hundreds of visitors to watch the six-hour transit projected onto a museum screen. 'It's very unlikely. The clouds of Venus are full of sulphuric acid. The planet is also very close to the sun and has high surface pressure.' Dr Dirk Shulze-Makuch of the Department of Geological Sciences at the University of Texas in El Paso begs to differ. 'I would say the chances of finding life on Venus are one in three [at] a conservative estimate,' he declares. A hydrologist whose fascination with the idea of life on Venus was fired by his discovery of life forms in extreme conditions in the hot springs of Mexico, Shulze-Makuch is boyish in his enthusiasm for the quest to find life beyond Earth. Too much research work in universities focuses on the minutiae of our existence, he believes. The search for life is something that carries with it a sense of mission. 'It is one of the ultimate questions: whether we are alone or whether there is life somewhere else,' he says. 'Even if it is only microbial life, at least we know it is life.' Shulze-Makuch first looked at Titan - a moon of Saturn cloaked in a thick orange haze, which some scientists believe may conceal oceans and carbon-rich life - as a possible place for life elsewhere in our solar system. Then his attention gradually shifted towards Venus. 'For some time life was excluded as a possibility on Venus,' he says. 'Then in the late 1980s and 1990s we learned about extreme survival on Earth. This puts a really different perspective on the possibility of life on Venus.' In a three-year study partly funded by NASA, Shulze-Makuch, along with two other University of Texas scientists, and David Grinspoon and Mark Bullock of the Department of Space Studies at the Southwest Research Institute in Boulder, Colorado, drew up plans for a space probe to collect cloud matter from Venus to try to discover if there are any life forms floating above Earth's sinister neighbour. Grinspoon agrees that the possibility of Venusian life had once been written off, but added recently that discoveries had changed people's thinking, commenting: 'One thing we've learned about life on Earth is how adaptable it is at finding ways to survive in seemingly unlikely, seemingly deadly niches. If you think about what life needs in a broad sense, then the clouds of Venus might actually be a habitat in which something could live.' Even if life forms were able to survive the acidic environment they would face the challenge of intense ultraviolet rays. Grinspoon, however, says it is conceivable organisms might have adapted to make use of ultraviolet radiation: 'One life form's deadly radiation might be another's lunch.' Shulze-Makuch believes the key factor that may determine whether there is life on Venus is the speed at which it evolved from a cool planet into a hot planet. 'That is really the crucial point - the big question mark. Even if we knew the speed at which it happened we would not be sure how quickly life could adapt. Our hunch though is that microbes can adapt quickly, so if the transformation wasn't a totally catastrophic event they would have had the skill to evolve and survive,' he says. If there does turn out to be life on Venus, scientists say, it does not mean it originated from a different source to life on Earth. Meteorites blazing across the cosmos connecting Earth, Mars and Venus may have carried microbes to the triumvirate of planets in our solar system that share the most common ingredients for life, they argue. 'What we have found with different space probes is that microbes can survive interplanetary travel to an extent, so it is possible life could travel from Earth to Venus or from Venus to Earth,' says Shulze-Makuch. 'If we did find life on Venus it would make our expectation of life on other planets and moons much greater. 'The really important thing though is that we would, for the first time, establish that there is life on another planet. It might be very primitive life, but it would tell us a lot about the possibilities of life and how life can deal with stresses. We would know too if it was from the same origin as life on Earth. 'If it were from the same origin we could probably say there is life on Mars too, but on the subsurface of the planet - that is the only place life could exist on Mars. Then we would have a succession: on Mars we would have life below the surface, on Earth we would have life on the surface and on Venus we would have life above the surface.' Space agencies have already listened with interest to the scientists' findings but none has yet come forward with funding for a probe. There is a sense, however, that the possibility of life on Venus, while remote, is not being ruled out by fellow scientists. 'I think we are making progress,' says Shulze-Makuch, who admits he was surprised the hypothesis was not initially 'shot at from every side' by academics. 'We didn't get much of that,' he says. 'We got some people saying life couldn't originate in that climate but that is not what we are claiming. We think there was life first in the oceans and it then adapted to the atmosphere as Venus changed. 'I have a feeling this is a hypothesis gaining in acceptance in the scientific community. We hope the probe is taken up by some space agency - it doesn't matter whether it is the European space agency, NASA or any other. We decided not to put a cost estimate on it because there are so many variables but it would be relatively cheap - much cheaper than a mission to Mars because Venus is quite a bit closer and the probe wouldn't have to touch down on the surface. We would be taking samples from the cloud levels, which is not as technologically demanding as putting a robot on Mars.' Back in Hong Kong, as the countdown to Tuesday's transit continues, Dr Chau Hoi-fung of the University of Hong Kong is among the scientists not dismissing the possibility of life on Venus out of hand. At the same time, however, he is not exactly rushing to help raise the necessary millions of dollars to send a probe more than 38 million kilometres to the planet. 'Is it possible? Yes and no,' he says. 'The thing that worries me is how all these bacteria, or microbes, would just float in the middle of the sky.' Despite the discovery of some bacteria that could come from other sources, there has been no proof yet that life can exist in cloud layers. Significantly, Shulze-Makuch and his colleagues are conducting extensive cloud-level tests for microbes, sensing that a breakthrough in establishing the existence of organisms in the severe climate of Earth's cloud belt could give the Venus quest its most compelling case for blast-off. Until that evidence is forthcoming, sceptics will abound. Chau points out that the whole theory rests on the idea of a dynamic ecosystem in the sky in which bacteria multiply, replacing those that fall out of the cloud layer. 'Constant amounts of bacteria would have to stay in one particular region of the sky and I find that a bit hard to believe,' he says. 'We are not talking about a really calm planet here - Venus is not a place you would want to go. However, if they did find life in its clouds, according to the same logic I wouldn't be surprised to find life in the interior of Mars.' However the argument turns out, the debate about the possibility of Venusian life reflects a growing appetite for space exploration in the past decade and a common desire to find out whether we are alone in the universe. The breakthrough, Chau believes, came with the discovery in recent years of 40 or so planets orbiting other suns outside our solar system. 'So the next question is what is on those planets. Is it possible they have life? Is life common in the universe? Are the conditions for generating life common in the universe? Astrobiology is a growing field. My belief is that we have to go beyond our solar system to find life. I am optimistic we will find it, but who knows? The cosmos is full of exciting things.'