Killer asteroids, solar flares and potentially lethal alien viruses are some of the dangers our planet faces. A small number of individuals and organizations have started to recognize this threat and are working to defend the Earth.

It is now widely believed that a giant asteroid was at least partly responsible for the extinction of the dinosaurs 65 million years ago and many people believe it is only a matter of time before we are faced with another killer asteroid. Worldwide, organizations such as NASA are working to find what objects are out there. We travel to New Mexico to visit Grant Stokes, head of the LINEAR search program. In the last three years Stokes has helped find over 10,000 asteroids, many of which have been classified as \'Potentially Hazardous Objects\'. We also speak to the people who are developing ways of deflecting or destroying asteroids that might pose a threat to our planet.

As NASA plans to send man to Mars and to bring back samples of Martian rock to Earth, it has become a concern that these samples could contain alien bacteria. John Rummel is head of the Planetary Protection office at NASA, responsible for making sure any samples of Mars rocks are not allowed to contaminate the Earth. Many people believe that Mars houses life and that alien viruses could be transported back to our planet. We speak to the head scientist from NASA\'s Viking mission, Dr Gilbert Levin; he claims there is bacterial life on Mars and that we should not bring samples back without knowing more.

The Sun may give life to the Earth, but as human technology advances it is starting to pose real threats to our planet. Solar weather and solar storms can cause havoc with satellite systems and wipe out power stations - leaving towns and cities at a standstill. Astronauts in space are also at risk from life-threatening radiation from solar storms. We visit the Space Environment Center in Colorado and speak to Larry Combs, who is responsible for monitoring the surface of the Sun and warning the world when solar weather erupts.