# Advantages and Limitations of Current Microgravity Platforms for Space Biology Research
## 4 Non-Orbiting Microgravity Facilities
### Parabolic Flight  
Parabolic flight allows for short-term (15-20 s maximum) exposure of humans, animals, tissues, or cells to microgravity (\(\sim\)10\({}^{-2}\) g) (Table 3). Such short-term simulations of microgravity are suitable for studies of acute phenomena, such as cardio-vascular and neuro-sensory response [44]. Parabolic flights are also used to test technical equipment, train astronauts for Space missions, and validate operational and experimental procedures.  
A typical NASA parabolic flight lasts 3 h. First, the plane carries experiments and crewmembers to a starting altitude of approximately 7 km above sea level. Then, it performs a rapid ascent at 45\({}^{\circ}\) (pull up), traces a parabola at altitudes ranging from 7.3 to 10.4 km (pushover), and then descends at 45\({}^{\circ}\) (pull out) (Figure 3b). Crew and experiments experience hypergravity (between 2 g and 2.5 g) during the pull-up and pull-out segments. During the parabola, net acceleration drops as low as 1.5 \(\times\) 10\({}^{-2}\) g for a brief period of time (>15 s). ESA (European Space Agency) parabolic flights slightly differ from NASA parabolic flights in the level of hypergravity (1.5-1.8 g) and altitude (7.5-8.6 km) during pull-up and pull-out, and microgravity experienced (10\({}^{-2}\) g, for 20 s) [60]. Experiment area available in the cabin is up to 200 m\({}^{2}\) and electrical power is supplied during the flight. Payload can be several square meters, up to 2.9 m in height [59], and up to several tons in weight. A standard parabolic flight campaign consists of three flight days, each with a 3-4 h long flight with 31 parabolas in a row. After five parabolas, a 4-8 min pause allows scientists to quickly analyse the experimental run and to change parameters or experiment setup. A crew of up to 40 people is onboard, performing 12 to 15 different experiments.  
Figure 3: Non-orbiting microgravity facilities. Schematic representation of (**a**) drop tower; (**b**) parabolic flight; and (**c**) sounding rocket. As microgravity can only be achieved transiently, these facilities are widely used for short-term experiments.  
\begin{table}
\begin{tabular}{l c c c c c} \hline \hline  & **Drop Tower** & **Parabolic Flight** & **Sounding Rocket** & **CubeSat** & **ISS** \\ \hline
**Microgravity Duration** & 2.2 s up to 9.5 s & 20 s repetitive & 5–20 min & Weeks to months & Months to years \\
**Microgravity Quality** & 10\({}^{-6}\) g & 10\({}^{-2}\) g & \(\leq\)10\({}^{-4}\) g & 10\({}^{-6}\) g & 10\({}^{-6}\) g \\
**Hypogravity** & Yes & Yes & No & No & Yes \\
**Biological System** & Cells, microbes, plants & Cells, microbes, humans & Cells, plants, microbes, animals & Microbes, plants & Cells, microbes, plants, animals, humans \\
**Cost** & Medium & Medium & Medium & Medium & High \\
**Accessibility** & Medium & Medium & Medium & Medium & Hard \\
**Late access before** & 2 h & 10 min & \textless{}3 h & 24 h & 24 h \\
**experiment** & 45 min & 1 min & 1–2 h & Not Applicable & \(\geq\)48 h \\ \hline \hline \end{tabular}
\end{table}
Table 3: Specifications of non-orbiting and orbiting microgravity facilities. ISS, International Space Station.  
The main advantages of parabolic flights are the late access and early retrieval of the sample [61], the short time between proposal and experiment implementation (in term of months), and the medium costs (Table 3). In addition, there is the possibility of performing a series of experimental runs within three days; of using typical laboratory-type instrumentations; and of directly controlling, like for drop towers, the experiments during flight. Moreover, by changing the flight manoeuvres, it is possible to reproduce partial gravity conditions, with gravity levels corresponding to those of the Moon and Mars. Contrary, the greatest disadvantage is that microgravity is intermixed with hypergravity phases, and this must be considered when interpreting results.