# Advantages and Limitations of Current Microgravity Platforms for Space Biology Research
## 1 Introduction  
Space exploration has deep scientific implications that are not solely astrophysical, but also biological. Space biology is concerned with discovering the origin of life on Earth and finding extra-terrestrial forms of life, but also understanding how the conditions on Earth shape life and how organisms are affected by the Space environment (microgravity, radiation, etc.). Fruit flies were the first organisms sent to Space in 1947. Many experiments followed before the first human Space flight by [PERSON] in 1961. Since 2000, technological progress has allowed humans to constantly inhabit Space on board of the International Space Station (ISS). As missions have progressed, so has the knowledge of how living organisms respond to the conditions of Space. The Space environment is not compatible with life as known on Earth; a near vacuum, with temperatures ranging from hundreds of degrees to near absolute zero, and a conspicuous dose of solar and cosmic radiations. Bacteria, trabridgates, fungi, and other forms of life have survived in dormant form to outer Space exposure [1; 2; 3; 4; 5], but life needs protection and insulation from these conditions to thrive away from Earth. The variations in the gravitational force have also been shown to have profound effects on living organisms. Life on Earth has evolved under the influence of constant gravity; it provides a directional stimulus once thought to be perceived only by gravitropic organisms, like plants [7]. It is now known that gravity influences life at physiological and cellular level, affecting development, membrane exchange, cell metabolism, and growth. Alterations of the gravitational force are perceived as stress. In humans, for example, microgravity induces important physiological alterations, such as muscle atrophy [8], bone loss [9],cardiovascular deconditioning [10; 11], immunological [12; 13], cerebrovascular [14] and cognitive alterations [15; 16], and metabolic problems [17]. In some cases, these physiological alterations can adversely affect performance and health in astronauts, both during and after the mission. Indeed, these adaptive responses seem to be partially reversible after return to Earth, but can take up to years, if ever, to fully recover [8; 18]. Moreover, future missions beyond Low Earth Orbit (LEO) will engage astronauts for longer and these effects could become even more critical. The importance of an adequate understanding of the physiological responses of living organisms to microgravity has grown since the beginning of human Space exploration. Scientists have developed ground-based microgravity simulators to study the physiological and molecular responses to altered gravity, and to develop effective countermeasures for spaceflight. Depending on the scientific question, different experimental platforms are available and experimental applications can range in size from molecules to humans. There are different ways to reproduce the effects of microgravity on Earth. Research in this field was given a head start by the pioneers of plant gravitropism, starting from [PERSON]. [PERSON] at the beginning of the XIX century. The so-called microgravity simulators do not reproduce microgravity conditions, but rather achieve omnilateral stimulation by randomizing the direction of gravity over time (clinostat and random positioning machine) or compensate the gravitational force with a counteracting force (magnetic levitation). Microgravity analogues just reproduce the effects of microgravity on physiological responses. Analogues are bed rest and bed rest associated with head-down tilt (HDT), water immersion (neutral buoyancy), and total or partial body suspensions. Even though simulated microgravity conditions only reproduced some effects observed in a true microgravity environment, they represent effective, affordable, and readily available test beds.