Abstract Experiments are reviewed which were carried out on a number of balloon, rocket, and spacecraft missions and which have shown that inactivation of bare bacterial spores by solar ultraviolet radiation (UV) is, at most, a matter of a few minutes only. In addition, laboratory experiments have indicated that a temperature close to that of Space together with a protective coating by ice of simple molecules reduces sensitivity considerably. Extrapolation of these data to Space conditions allows the speculation that under these circumstances spores may survive for hundreds of thousands to millions of years; this time is sufficient for spores to travel from one solar system to another. Together with the enormous number of “protected” spores deposited from spacecrafts into Space these numbers allow for “modern panspermia” to work and point to a serious problem in “planetary quarantine”. In addition, they reveal constraints for panspermia in its classical sense. Here, however, UV irradiation may not be the limiting factor; but, HZE-particles of the cosmic galactic radiation may well be so.