# Advantages and Limitations of Current Microgravity Platforms for Space Biology Research
## 5 Orbiting Microgravity Facilities
### The International Space Station  
The International Space Station (ISS) is a unique laboratory for scientific research and technology demonstration. In continuous free-fall, orbiting at 400 km high with a speed of 28.000 km/h, the ISS is the only platform where constant microgravity can be achieved. This has allowed scientists to investigate, among other factors, the effects of long-term exposure to microgravity on biological systems, especially humans, and test technologies needed for astronauts to live and work in such an environment. Since 2011, the International Space Exploration Coordination Group, which gathers fourteen Space agencies, has been defining the Global Exploration Roadmap (GER), a shared 20-year strategic plan for Space exploration. In the third edition of the GER, published in 2018, the International Space Station (ISS) was recognized as the steppingstone towards the expansion of human presence beyond LEO [72].  
Research carried out on the ISS spans from biology to physical sciences, from Earth and Space science to educational activities and technology development. The results from ISS biological research have provided insight into the complexity of microgravity responses in whole organisms, complex cell cultures, down to protein crystals growth [73].  
Figure 4: Schematic representation of **(a)** a CubeSat, a modular miniaturized satellite for Space research, composed of two units, with yellow antennas; **(b)** altitude in km of orbiting (CubeSats, ISS) and non-orbiting (sounding rockets, parabolic flights) facilities and atmospheric zones: the majority of CubeSats orbit at an altitude between 350 and 700 km, but they can be at lower or higher orbits; the International Space Station (ISS) orbits at an altitude of about 400 km; sounding rockets typically reach an altitude of 250–350 km; parabolic flights reach an altitude of 7 km.  
Recently, new technologies have allowed for the first onboard analysis of microbiological and genetic samples in spaceflight history. A large range of astrobiology experiments have been performed under Space conditions by pairing long-duration external exposure onboard the ISS with on-ground sample analysis after return to Earth [3; 74; 75; 76; 77]. Plant biology experiments have been conducted for the development of bioregenerative life support systems in preparation for future long-duration exploration missions, but have also highlighted some of the basic processes of plant growth on Earth. Biological and human physiological studies driven by the need to support astronaut health have yielded important results that can have a positive impact on human health on Earth. These results include new ways to mitigate bone loss; innovative wound-healing techniques; insights into bacterial behaviour; and advances in psychological stress response, disease models, and nutrition [73]. In the first twenty years of ISS activity, research on the ISS has produced almost 2500 among publications in peer-reviewed journals, conferences, and grey literature, reporting the research of more than 5000 scientists worldwide [78].  
The ability to conduct research in real microgravity provides a unique chance to treat gravity, which is ubiquitous on Earth, as a variable. Nevertheless, performing experiments in Space comes with its specific challenges. In particular, the limiting factors are mass and volume, environmental requirements, time constraints, and controls. Experimental design is also constrained by operational limitations associated with launch, execution, and data collection while in Space, as well as sample return.