Seasons & episodes

The Earth is under constant bombardment. Each year, many fragments of debris hit our planet. Fortunately for us, most are so small that they burn up harmlessly in the atmosphere. However, there are hundreds of larger asteroids orbiting near the Earth. Many scientists now believe that one of these hit the Earth 65 million years ago, killing the dinosaurs, along with 90% of all life on the planet. What is more, it is only a matter of time before the Earth is hit again. Experts warn that nuclear weapons might not destroy an approaching asteroid. But Jay Meloch thinks he can use the power of the Sun to nudge an asteroid away from the Earth. Until recently, no one took the asteroid threat very seriously. Yet the evidence that we are in danger is on our own doorstep. We need only look at the cratered surface of the Moon to realise that it has been pounded by impacts throughout its history.

A dirty bomb is a radiological weapon but unlike a nuclear bomb, its purpose is to contaminate rather than destroy. It uses normal explosives to disperse radioactive materials in the local environment, creating a hazard to health that could last for years unless cleaned up. The relative ease of making such a bomb means it is a potent terrorist weapon but Horizon's investigation shows that the risk to health from most such devices need not be great. It also underlines the need for governments to act to secure radioactive sources from falling into criminal hands. Horizon deliberately avoids outlining the production process in any detail. Horizon publishes the results of specially commissioned research, modelling two possible dirty bomb scenarios: attacks on either London or Washington DC. The main conclusion is that the health risks from a dirty bomb explosion are localised to people who are close to the incident or are in contact with the contamination. Although the modelled attack scenarios could have wide-ranging economic repercussions, the majority of the population of either capital city would have only a negligible increase in their risk of developing cancer.

The drug Viagra revolutionised the treatment of sexual dysfunction in men on its launch five years ago. An accidental discovery, the tablet that gave impotent men the chance once more to have natural erections became the fastest selling pill in history and has earned its manufacturer, Pfizer, over $6bn. The search is now on for a similar drug that could help women. Research is revealing that female sexuality is more complex than expected. For women suffering from a loss of desire many scientists believe that drugs acting on the brain may be the way forward. A pioneering Scottish study may have identified just such a drug and begun testing it scientifically. An erection is achieved by filling the erectile tissue of the penis with blood. Blood vessels widen to allow blood in and then constrict to maintain the pressure. Male impotence was long thought to be a psychiatric effect, a result of stress, anxiety or depression. Medical advice was that there was not much to be done. Some patients refused to take this message on board.

Understanding of humans' earliest past often comes from studying fossils. They tell us much of what we know about the people who lived before us. There is one thing fossils cannot tell us; at what point did we stop living day-to-day and start to think symbolically, to represent ideas about our environment and how we could change it? At a dig in South Africa the discovery of a small piece of ochre pigment, 70,000 years old, has raised some very interesting questions. Anatomically modern humans (Homo sapiens) emerged in Africa roughly 100,000 years ago. We know from fossil evidence that Homo sapiens replaced other hominids around them and moved out of Africa into Asia and the Middle East, reaching Europe 40,000 years ago. Prof Richard Klein believes art is a landmark in human evolution. Unquestionable art that's widespread and common suggests you're dealing with people just like us. No other animals, after all, are able to define a painting as anything other than a collection of colours and shapes. This ability is unique to humans.

It was the simplest idea but one with enormous potential. If a gene is defective in the human body, just replace it with one that works properly. Gene therapy would mean that genetic disorders would become a thing of the past. Cancer would be cured, as would cystic fibrosis and hundreds of other genetic illnesses. Scientists were justifiably excited about the idea but, this enthusiasm that would end up costing one young man his life. Jesse Gelsinger was born with a liver disorder, a rare condition called ornithine transcarbamylase (OTC) deficiency that stops the liver metabolising ammonia. People with the disease can suffer from brain damage or coma. At its most extreme the illness is fatal. Jesse was lucky, able to lead a fairly normal life although he had a daily cocktail of drugs to control his condition. Jesse wanted to help others. When he was offered a chance to take part in a medical trial to test the safety of using gene therapy for OTC deficiency, he was keen to participate. He knew this was not a cure for his condition but that, by volunteering he might be able to help others in the future. Although the concept of gene therapy is simple, the practice of administering the treatment is much more difficult. In order to replace defective genes, doctors must get working ones into the body and to the place where they are needed.

Earthquakes are among the most devastating natural disasters on the planet. In the last hundred years they have claimed the lives of over one million people. Earthquakes are destructive mainly because of their unpredictable nature. It is impossible to say accurately when a quake will strike but a new theory could help save lives by preparing cities long in advance for an earthquake. The surface of the Earth is made up of large 'tectonic' plates. These plates are in slow but constant motion. When two plates push against each other friction generates a great deal of energy. For this reason earthquakes occur most frequently on tectonic fault lines, where two plates meet. However these fault lines run for thousands of kilometres; predicting exactly where a quake will occur is nearly impossible. In 1992, Dr Ross Stein was monitoring a large earthquake in a town in California called Landers. Three hours later, there was another quake 67km away at Great Bear. Stein believed that this was not simply an aftershock, instead he theorised the event at Landers had set off the earthquake at Big Bear. Stein believes that when an earthquake occurs the stress that has built up along the fault, is in part, transferred along the fault line. It is this energy transfer that causes other quakes to occur hours, days or months after the original.

Are we alone in the Universe? Or are there aliens somewhere in space? New evidence suggests not only might other life-forms be out there, they may even be living on the planet right next door to us - Mars. Recent discoveries have shown that Mars has all the ingredients for life, including water. Now the Mars Odyssey probe, launched in April 2001, has detected huge frozen areas of permafrost, just like that found in the Antarctic on Earth. According to astronomers, the position of this frozen slush could hold the key to Mars' mysterious water cycle. And the surface ice may hide something even more exciting below.

Set against the back drop of awe inspiring geological beauty, a strange scientific adventure sets out to discover how a mineral clad cave network - the height of a 30 storey building and the length of six football fields - came to exist deep below the Guadalupe Mountains in North America. But this journey soon unravels a multitude of inexplicable phenomena and obscure geological formations, leading to the discovery of extreme rock-eating microbes - a testimony from primordial Earth and a glimpse of life elsewhere in the Solar System. Geologists believed that all limestone caves were formed by rain and underground water percolating through cracks in the rocks. Absorbing carbon dioxide from the soil, this water becomes weak carbonic acid, nibbling away at limestone, etching out networks of subterranean caves.

Rudi Affolter and Gwen Tighe have both experienced strong religious visions. He is an atheist; she a Christian. He thought he had died; she thought she had given birth to Jesus. Both have temporal lobe epilepsy. Like other forms of epilepsy, the condition causes fitting but it is also associated with religious hallucinations. Research into why people like Rudi and Gwen saw what they did has opened up a whole field of brain science: neurotheology. The connection between the temporal lobes of the brain and religious feeling has led one Canadian scientist to try stimulating them. (They are near your ears.) 80% of Dr Michael Persinger's experimental subjects report that an artificial magnetic field focused on those brain areas gives them a feeling of 'not being alone'. Some of them describe it as a religious sensation. His work raises the prospect that we are programmed to believe in god, that faith is a mental ability humans have developed or been given. And temporal lobe epilepsy (TLE) could help unlock the mystery.

265 people died when an Airbus operated by American Airlines crashed into the New York suburb of Queens in November 2001. The twin-engined jet took off from John F Kennedy Airport in fine conditions but hit trouble after just 67 seconds. In the following 38 seconds the plane started to disintegrate before nose-diving into the residential Rockaway area of the city. Everyone aboard was killed (along with five people on the ground) so the crash investigators had to rely on eyewitnesses, recovered parts of the plane and information from both air traffic control and the flight data recorders. The discovery of the Airbus' vertical tailfin hundreds of metres from the fuselage immediately focussed attention on whether the pilots lost the ability to control the plane. Why the tailfin detached was at the heart of the investigation by the National Transportation Safety Board. The airline and the manufacturer blame each other for creating a situation in which the stress on the rudder and tailfin exceeded the so-called ultimate load, the worst-case scenario set by the Federal Aviation Administration (FAA). A number of American Airlines pilots have taken matters into their own hands though: requesting transfers to other aircraft because of their safety concerns.

Severe Acute Respiratory Syndrome didn't even have its name in February 2003, when it struck its first known victim, Johnny Cheng, in Hanoi, Vietnam. Within days, an international effort led by the World Health Organization (WHO) had massed scientific expertise to fight the mystery illness and avert the nightmare scenario of an uncontrollable pandemic sweeping the globe. Amid attempts to quarantine high risk groups of people, it seemed only fear could spread more rapidly than the disease itself. Nothing was known about the condition - where it had come from, how it was passed on, how to spot it, contain it or treat it. The infection was described merely as 'flu-like'. But if this was a type of influenza, it was one that killed up to 15% of its sufferers. The doctor treating Mr Cheng, who first contacted the WHO about the unusual symptoms, was one of six medics to die of SARS at the hospital. But the alarm had been raised and the Organization began to pull together a response. Colossal effort by scientists around the world - and unprecedented co-operation - followed. Meanwhile, the media made much of the risk posed by and to international travel, and watched financial markets respond in gloomy fashion.

Remember that long, hot summer? You might never see its like again. And all that talk of global warming? Forget it. This season's first Horizon reveals that a growing number of experts fear Britain could be heading for a climate like Alaska. Our ports could be frozen over. Ice storms could ravage the country, and London could see snow lying for weeks on end. It would be the biggest change in the British way of life since the last Ice Age. The first signs that such a disaster could happen came from deep within the ice sheet of Greenland. Scientists discovered that the Earth's past was littered with sudden, drastic drops in temperature. The big question was: could it ever happen again? Clues came from tiny shells at the bottom of the Atlantic; a huge glacier on the move in Arctic and some alarming discoveries in the far north of Russia. In the end there came the terrifying revelation: the Gulf Stream, that vast current of water that keeps us warm, could be cut off. According to one scientist, there is a one in two chance it will happen in the next century. Others say a climatic catastrophe could be heading our way in just twenty years time.

This week Horizon investigates the science behind the Bible Code. What he can see is truly horrific; according to Drosnin, the world could end in an atomic holocaust - in 2006. It sounds preposterous yet Drosnin claims to have serious scientific backing. Behind his findings lies the work of one of the world's most brilliant theoretical mathematicians, an Israeli professor called Eliyahu Rips. In 1994, using exactly the same ancient code, Michael Drosnin accurately predicted the assassination of the Israeli Prime Minister Yitzak Rabin - twelve months before it occurred. Drosnin's books on the Bible Code have been translated into most of the world's major languages and are read by millions of people. If he's right, he's stumbled on one of the most important discoveries ever made.

Mixing powerful and deeply moving footage with telling forensic analysis, Horizon reveals what really went wrong on the Space Shuttle Columbia. The film's final revelation is telling. If NASA had acted differently, all seven astronauts could have been brought back to Earth alive. The film begins with the astronauts' final moments and shows the haunting scenes at Mission Control at the moment the disaster struck. What then follows is a disturbing detective story as the investigators gradually realise that the tragedy was caused by the failure of a small panel on the shuttle's left wing that had been built to be indestructible. No one had ever thought such an accident was possible. It has led to the shuttle being grounded for the foreseeable future. But that wasn't all. The film also shows that NASA had a number of options to bring the crew back safely - if only it had commandeered a spy telescope to visually inspect the damage. It could even have launched a rescue mission. But instead, NASA chose to rely on a computer programme for damage assessment. The programme got it wrong; as a result, there was no hope for those seven crew members.

Horizon tells the microbiological detective story in which some of the best brains in science have been pitted against the most extraordinary bug the world has ever seen. In 1984 it was discovered that HIV was the cause of AIDS. Straight away, there were confident predictions that there would be a vaccine ready for testing in just two years. Back then, just 1,292 deaths from AIDS had been reported. Now the figure is 25 million dead. By 2010 it is predicted there will be 85 million infections and 70 million deaths. And after 20 years there is still no sign of a vaccine. Despite work of dazzling complexity, the ambition of so many brilliant scientists has been constantly thwarted. Just as a vaccine seems to be working, the AIDS virus alters itself, and ten to fifteen years of work, and millions of pounds, go down the drain. These bitter disappointments are only compounded by the desperate human urgency of the work. This is a story where the clock doesn't stop ticking.

Could an unknown Englishman have been the first person ever to fly? To mark the hundredth anniversary of the Wright brothers inaugural flight, Horizon tells the remarkable story of Percy Pilcher. He could have been the most famous aviator of them all. Four years before the Wright brothers, he had constructed his own aeroplane. But on the day it was due to take off for the very first time, something so terrible happened that he was denied the chance of ever flying it. So Horizon has rebuilt his long lost flying machine to see if Percy Pilcher, the British amateur, could have claimed the glory and been the first person ever to fly. This film mixes dramatic reconstruction with fabulous contemporary scenes and gripping science. With a specially assembled team of historians, aviation experts and our own test pilot, Horizon painstakingly rebuilds Pilcher's flying machine and puts it to the test. The results will leave you cheering.

Horizon's Time Trip is a thrilling journey deep into the strangeness of cutting-edge physics - a place where beautiful, baffling ideas are sometimes indistinguishable from the utterly crazy. On this journey, we meet a time-travelling pizza, a brilliant mathematician in a ski mask and even God. The journey ends with a strange and dark conclusion - one which calls into question our very existence. Ever since Einstein showed it was theoretically possible, the quest to travel through time has drawn eccentric amateurs and brilliant scientists in almost equal numbers. The amateurs include Aage Nost, who demonstrates his time machine in front of the cameras. The professionals include the likes of Professor Frank Tipler of Tulane University. His time machine sounds good - but it would weigh half the mass of the galaxy. There is, however, one way that time travel to the past could be possible. And it would be much more convenient. Future civilisations could use computers to create exact replicas of the past. Unfortunately that idea has physics trembling in its socks. Because if you can generate a perfect virtual reality version of the past, who's to say we are not one of the replicas?

In a film that is in turns charming, disturbing and poignant, Horizon explores the relationship between science and the chimpanzee. It began with a magical story. A young girl ventured alone into the jungle and befriended a group of chimpanzees. What she saw became the stuff of scientific legend. But then, last year came a terrible tragedy. Frodo, one of the chimpanzees she had helped make famous, killed a human baby. That shocking act brought into focus a huge debate about the relationship between humans and chimps, and what these primates have taught us about the origins of our own behaviour. The saga of how Jane Goodall went into the jungle to study the chimps of Gombe in Tanzania has inspired novels and movies. Her observations revealed that chimpanzees were in many ways like humans. They used tools, had culture and even language. And what's more they had empathy. They were also capable of savage brutality against their own kind. Just like us. In fact many began to think that the origins of aggressive human male behaviour could be traced back to our shared evolutionary ancestry with chimps. In other words, men are genetically programmed to be violent. But then came some disturbing questions.

With the help of doctors and scientists in America, Germany and Britain, Horizon unpicks the mystery of the Moscow theatre siege. In October 2002, Chechen terrorists took a thousand people hostage in a Moscow theatre and threatened to kill them. The problem was how to get them out alive. A bloodbath seemed inevitable. Three days later Russian special forces stormed the theatre using a secret gas to knock everybody out. 129 hostages died - apparently killed by the very gas that was meant to save them. Horizon investigates the mystery substance, and why so many died. The Russian authorities insisted their secret weapon was not lethal. The claim provoked contempt from the victims families, and incredulity among doctors and scientists around the world. But were the Russians actually right? The Russians offered just one clue. And in Germany there was a scientist who had the means to test it: a urine sample taken from one of the survivors shortly after he was freed. Horizon follows as extremely sensitive tests are performed to find out if the Russians were telling the truth, and uncovers a deeper secret.

This is the truth about the world's most famous, most glamorous and most controversial diet. The Atkins diet says that eating fat can make you thin. It says you don't need to bother watching the calories. Rene Zellweger, Geri Halliwell and a host of other celebrities swear by it. But many scientists think it is scientific nonsense. Some even believe it is dangerous. Horizon cuts through the confusion and provide the answers. When Dr Atkins first launched his diet, he was accused of breaking one of the most fundamental laws of nature. Scientists said that if you eat more, you'll get fatter. They also said it could kill. Fat increases your cholesterol levels. You'd get a heart attack. The only problem was that people who followed the Atkins diet got thinner. Much of the rest of us got fatter. Then came studies showing that cholesterol levels can actually improve on the Atkins diet. So what was going on? Horizon's investigation seems to show that the diet may really work - but for a reason and in a way that no scientists or even Atkins himself had seriously considered.

This is the extraordinary story of how a small metal disc is rewriting the epic saga of how civilisation first came to Europe, 3600 years ago. When grave robbers ransacked a Bronze Age tomb in Germany, they had no idea that they had unearthed the find of a lifetime. But they knew that it was worth selling. It was a small bronze disc of exquisite design. So they contacted the archaeologist Harald Meller, offering to sell it to him for €300,000. Meller went deep into the criminal underworld and, after a police sting, he got his disc. It depicted the sun, the moon and the stars. This suggested an understanding of the heavens greater than that of the first great civilisations, like Egypt. Could it possibly be real? After exhaustive tests, the disc was declared genuine. Then a team of crack scientists pieced together what it meant. What emerged is a true marvel. This disc, it seems, is a Bronze Age Bible, combining an advanced understanding of the stars with some of the most sophisticated religious imagery of the age. In intellectual achievement and also age, it surpasses anything yet found in Egypt or Greece. It seems that civilisation had already dawned in Europe.

Imagine a world where disease could be eradicated by an injection of tiny robots the size of molecules. That is the hope offered by nanotechnology - the science of microscopically small machines. But others fear nanotechnology could lead to a non-biological cancer - where swarms of tiny nanobots come together and literally devour human flesh. Sounds like science fiction? It certainly did until a brilliant young scientist called Hendrik Schön seemed to bring it a step closer. Schön's great breakthrough was to make a computer transistor out of a single organic molecule. It was an achievement of almost incalculable brilliance. Some speculated this technology could spell the end of the entire silicon chip industry. Crucially, Schön's transistor was organic. Suddenly, this seemed to be the first step towards true nanotechnology, where minute computers could grow as living cells. Scientists speculated about how these tiny machines could be used to target diseases with astonishing precision. Others wondered - could the military use them as a new weapon? Others, including Prince Charles, were terrified. If these machines can grow by themselves, how do we stop them from growing? What happened next would destroy reputations and shatter lives - because there was more to Hendrik Schön's discovery than anyone knew.

Thalidomide was one of the biggest medical tragedies of modern times. The images of children born with shrunken limbs still haunt anyone who sees them. And the tragedy is not over. Those children are adults today, still coping with their disability. For many, thalidomide is a drug that should be consigned to the dustbin of history - an awful cautionary tale of the errors that science can make. But now it is making a comeback - as a radical treatment for incurable blood cancers. But can it possibly be safe to use such a dangerous drug again? In a powerful and deeply moving film, Horizon tells the tale of thalidomide and how this drug that has become so infamous may now be giving hope to people who otherwise face death. It also explores the mystery at the heart of thalidomide. It seems that the reason why it works for cancer may at least partly explain something that has long baffled scientists - why thalidomide caused such terrible damage to babies in the womb all those years ago.

Top quality diamonds at knock down prices? The only catch is: these rocks don't come out of the ground, but are made in a lab. This is the promise offered by a series of recent scientific breakthroughs. For most of us, it seems we may soon be able to bejewel ourselves like movie stars. But for De Beers, the world's largest diamond trader, could this, one day, be a serious threat? Following a dodgy meeting in Moscow, retired US Army General Carter Clarke acquired some experimental diamond growing machines, originally destined for the Russian military. He created the world's first gem diamond production line, to mass produce highly prized coloured diamonds. In a secret location south of Boston, a father and son team developed a different technique. Robert Linares and son Bryant have made colourless diamonds, allegedly higher quality than those found in nature. De Beers, at vast cost, set up a new scientific division called the Gem Defensive Programme. Its goal: to find ways to tell apart their natural diamonds from these new synthetic gems. But will the new synthetics slip through De Beers detection net? And could anyone really tell the difference? Horizon tells the story of the Diamond Labs.