The inactivation of Bacillus subtilis spores during long-term exposure (up to several months) to extreme dryness (especially vacuum) is strain-dependent, through only to a small degree. During a first phase (lasting about four days) monolayers of spores lose about 20% of their viability, regardless of the strain studied. During this phase loss in viability can be equally attributed both to damages of hydrophobic structures (membranes and proteins) and DNA. During a second phase lasting for the remaining time of experimental observation (weeks, months and years) the loss in viability is slowed. A viability of 55% to 75% (depending on the strain) is attained after a total exposure of 36 days. The loss in viability during the second phase can be correlated with the occurrence of DNA double strand breaks. Also covalent DNA-protein cross-links are formed by vacuum exposure. If the protein moiety of these cross-links is degraded by proteinase K-treatment in vitro additional DNA double strand breaks result. The data are also discussed with respect to survival on Mars and in near Earth orbits.