Research in microgravity is indispensable to disclose the impact of gravity on biological processes and organisms. However, research in the near-Earth orbit is severely constrained by the limited number of flight opportunities. Ground-based simulators of microgravity are valuable tools for preparing spaceflight experiments, but they also facilitate stand-alone studies and thus provide additional and cost-efficient platforms for gravitational research. The various microgravity simulators that are frequently used by gravitational biologists are based on different physical principles. This comparative study gives an overview of the most frequently used microgravity simulators and demonstrates their individual capacities and limitations. The range of applicability of the various ground-based microgravity simulators for biological specimens was carefully evaluated by using organisms that have been studied extensively under the conditions of real microgravity in space. In addition, current heterogeneous terminology is discussed critically, and recommendations are given for appropriate selection of adequate simulators and consistent use of nomenclature. Key Words: 2-D clinostat—3-D clinostat—Gravity—Magnetic levitation—Random positioning machine—Simulated microgravity—Space biology. Astrobiology 13, 1–17.

A Random Positioning Machine (RPM) is a laboratory instrument to provide continuous random change in orientation relative to the gravity vector of an accommodated (biological) experiment. The use of the RPM can generate effects comparable to the effects of true microgravity when the changes in direction are faster than the object’s response time to gravity. Thus, relatively responsive living objects, like plants but also other systems, are excellent candidates to be studied on RPMs. In this paper the working principle, technology and control modes will be explained and an overview of the previously used and available experiment systems will be presented. Current and future developments like a microscope facility or fluid handling systems on the RPM and the option to provide partial gravity control modes simulating for instance Mars or Moon gravity will be discussed.