Panspermia Today, it is generally accepted that the universe is a huge reactor in which not only elemental chemistry is performed but complex molecules are also produced, similar to those used by existing living systems. Moreover, these molecules travel freely between the different cosmic regions. It is obvious that our solar system will encounter some of them during its journey through the Milky Way galaxy. But if many different components that are related to life are reaching our planet at this very moment - by different means of "piggyback" transport, for instance, inside comets or meteorites - what is then the possibility that life originally came to Earth from elsewhere, all ready? Once it landed here, it developed further into the enormous variety of organisms we now find on Earth. This hypothesis is known as "panspermia". It is an old idea, and until recently many scientists considered it to be ridiculous, although important scientific names have been associated with it (Svante Arrhenius, Francis Crick, Fred Hoyle). However, it is now catching on because new experimental and observational data challenge some of the main objections against it. Bacteria survive on the Moon An important observation that has influenced this change of attitude concerns the survival of a very common bacterium, Streptococcus mitus (a normal inhabitant of the throat). Some bacteria of this type were unintentionally taken to the Moon, on one of the cameras mounted on the Surveyor 3 lander in the late 1960's. Amazingly, they remained alive ("viable") after more than one year of full exposure to the very inhospitable conditions on the surface of the Earth's natural satellite. The rescue, in sterile conditions, of the camera by the crew of the Apollo 12 mission, and the bacterial growth observed after incubation demonstrated that normal bacteria, like S. mitus, clearly have the means to survive under those extreme conditions to which they were exposed. This unplanned experiment opened by chance the way to a new area of research that is focused on finding out what the limits of life really are. Radiation in space One of the biggest problems that life has to face during an interplanetary excursion is radiation. In this sense, the discovery of another bacteria, Deinococcus radiodurans, that is able to resist enormous doses of radiation, using efficient but otherwise common repair systems, underlines the fact that life has created mechanisms to deal with this important universal challenge. These results, together with the study of other organisms capable of surviving or even to develop under other extreme conditions (the extremophiles) have opened an important area of astrobiological research into the mechanisms of protection that have been developed by life and the additional protection that is provided by external elements (rocks, ice, new materials, etc.). Life forms on Mars But all these observations pale when compared with the enormous noise introduced into the scientific world in the summer of 1996 when D. McKey and other colleagues from NASA reported possible signs of (fossil) life inside the Martian meteorite ALH 840001. Obviously the scientists did not claim that their results were solid proof that life could resist an interplanetary trip. Their main interest was to discuss the possibility of life on our neighbouring planet. But the combined possibilities that life existed on a neighbouring planet in our Solar system, together with the notion that its signature resisted all of these destructive pressures is highly interesting. If there are really signs of fossil life inside ALH 840001, then they have "survived" the violent impact on Mars that ejected the meteorite into space, the subsequent long interplanetary wandering, the fiery entrance into the Earth's atmosphere and hard fall onto the surface, and finally its cold lodging for many years in a frozen Antartic valley. All of this has obvious implications for the discussion about the panspermia hypothesis. Are we the aliens? In spite of the raging, bitter debate that continues around that Martian meteorite, a growing number of scientists, led by K. Zahule from the AMES Research Center, have begun to consider the possibility that life could have originated on Mars. "If you want to see a Martian look in the mirror", is the graphic way to explain this idea. And obviously the transfer of matter via ballistic orbits, as happened for ALH 840001 is not just confined to Mars and Earth. This method must be recurrent in our universe and might well form one basis for panspermia. In spite of this, the concept of Cosmic Ancestry that has been put forward by Fred Hoyle and Chandra Wickramasinghe is still considered a very exotic proposal. According to this idea, not only foreign life systems can arrive on our planet, but biological evolution on Earth itself is considered to be a direct consequence of panspermia under a universal Gaian process. Although it is unlikely that conventional science will accept this point of view, astrobiology can not ignore it because it has to consider all possible options. The search is on!