Microgravity Sci. Technol (2009) 21:203–207 DOI 10.1007/s12217-008-9062-z The Human Centrifuge Jack J. W. A. van Loon Received: 15 October 2007 / Accepted: 15 April 2008 / Published online: 21 August 2008 © The Author(s) 2008. This article is published with open access at Springerlink.com Abstract Life on Earth has developed at unit gravity, working in the field of gravitational physiology, but also 9.81 m/s2, which was a major factor especially when other disciplines, will be discussed. vertebrates emerged from water onto land in the late Devonian, some 375 million years ago. But how would Keywords Hypergravity · Artificial gravity · nature have evolved on a larger planet? We are able to Microgravity · Weightlessness · Centrifuge · address this question simply in experiments using cen- Gravity continuum · Mars · Moon · trifuges. Based on these studies we have gained valu- Human exploration · Human hypergravity habitat able insights in the physiological process in plants and animals. They adapt to a new steady state suitable for the high-g environments applied. Information on mam- Introduction malian adaptations to hyper-g is interesting or may be even vital for human space exploration programs. It has In the past decades numerous studies have been per- been shown in long duration animal hypergravity stud- formed on cellular, plant and animal development and ies, ranging from snails, rats to primates, that various adaptation for longer durations, even multi genera- structures like muscles, bones, neuro-vestibular, or the tions, under hypergravity conditions (see e.g. Beams cardio-vascular system are affected. However, humans and Kessel 1987; Hemmersbach et al. 1999; van Loon have never been exposed to a hyper-g environment et al. 2005; Clément and Slenzka 2006). However, vir- for long durations. Centrifuge studies involving humans tually no data are available on humans in long duration are mostly in the order of hours. The current work on hypergravity. human centrifuges are all focused on short arm systems When we divide gravity related research in hu- to apply short periods of artificial gravity in support man physiology, we can distinguish two main areas of of long duration space missions in ISS or to Mars. In research or application; the aero-medical and the space- this paper we will address the possible usefulness of a related research. In the aero-medical field gravity large human centrifuge on Earth. In such a centrifuge fluctuations are relevant in flying modern, especially a group of humans can be exposed to hypergravity for, combat aircraft where steep maneuvers at high speeds in principle, an unlimited period of time like living on a are important or even crucial matters. It seems that the larger planet. The input from a survey under scientists human being is one of the limiting factors in such sys- tems (e.g. Pancratz et al. 1994; Convertino 1998;Scott et al. 2007). Both the cardiovascular and vestibulo- J. J. W. A. van Loon (B) ocular systems are challenged to their limits. While Dutch Experiment Support Center (DESC), going through the various combat maneuvers the pilots Dept. Oral Cell Biology, ACTA-Vrije Universiteit, have to be trained to rely on on-board instrumentation van der Boechorststraat 7, 1081 BT Amsterdam, with regard to orientation in stead of their own visual, The Netherlands e-mail: [email protected] proproseptive and vestibular sensors. They also have URL: www.desc.med.nu.nl to be trained in special breathing techniques and wear 204 Microgravity Sci. Technol (2009) 21:203–207 anti-g suits to withstand the high g forces involved in Earth Bound Large Radius Centrifuge support of their cardiovascular system. As the first field for Space Exploration is mainly dominated by the military, the second field has emerged since mankind has decided to go into If the sole purpose of the application of a human space. Related to spaceflight numerous experiments of centrifuge is to get humans on Mars, we could only some significant duration have been performed, first on focus on the recent developed short radius centrifuges board the Soviet Russian Salyut (Grigoriev et al. 1994) (e.g. Cardus and McTaggart 1994; Iwasaki et al. 1998; and later on the American Skylab crews (Johnston Warren et al. 2006; Edmonds et al. 2007;Wuyts2007). and Dietlein 1977) in the early 1970s. These cosmo- The main rationale for these relatively small centrifuges nauts/astronauts have been under near weightlessness is that they would be applied on board a Mars mission. conditions for several months and longer. The radius has to be small in order to fit into a space system of some sort. The crew members are exposed to the artificial gravity in a supine position, with the sub- A Large Radius Human Centrifuge ject’s head near the center of rotation. This should be beneficial for the musculoskeletal and cardiovascular Why would we need to explore the use of a ground system while effects like Coriolis are still present. based large radius human centrifuge? However, if we elaborate a bit on the possible ap- A first and, in my view, very important reason to plication of a large radius Earth based centrifuge in contemplate on a large centrifuge system is that the preparation of Moon/Mars missions, what could we use research emerging from this would be completely new, it for? providing basic knowledge and understanding on how the human physiology is regulated with respect to sus- 1. If we consider the paradigm of a gravity contin- tained hypergravity. When we consider physical para- uum, e.g. that physiological processes scale with meters acting upon the human body the factor weight the magnitude of applied acceleration (van Loon is basically not any different from e.g. temperature or et al. 2005; Tou et al. 2002;Wade2005) one may pressure, and in order to understand how the body learn about the long term adaptation of the body deals with environmental variables we need to modu- to different g-loads. How fast does the human body late them. Numerous studies have been performed ex- adapt to the hyper-g load, but also how fast does posing man to hypobaric and hyperbaric environments the body respond when returning to Earth gravity (see e.g. Miles 1957; Pedoto et al. 2003) or hypothermal after centrifugation? We might even combine such or hyperthermal atmospheres (see e.g. Sawka et al. hypergravity studies with bed rest experiments be- 2001; Solonin and Katsyuba 2003). So, what about grav- fore or after hyper-g exposure. Such studies pro- ity and humans? Most studies have been performed vide valuable insights in adaptive processes also using relatively short-arm centrifuges and short expo- occurring in hypo-gravity adaptations. sure times. The average arm length of current cen- 2. We can apply a period of hypergravity to a Mars trifuges world wide is 7.6 m (Singh 2005). However, crew before their voyage to the planet. They might if one is interested in the long term effects of gravity be pre-conditioned by building up enough “re- on human physiology we need to expose humans to serves” in terms of muscle and bone or any other both hypo- and hyper-gravity conditions for periods system to overcome the microgravity and hypo- of days or months. In his flying career the Russian gravity periods of a mission to Mars. cosmonaut Sergei Krikalev has been exposed to hypo- 3. It has been shown that humans can withstand a gravity conditions on board orbital space stations for period in microgravity, while making use of various more than 800 days. However, there has never been a countermeasure devices, of much longer than a person exposed to more than 1.0 g for such a period of year as was shown in the flight by Valeriy Polyakov time. The longest period to which humans have been in 1994–1995 when he stayed on board the Mir exposed to hypergravity was few weeks during study station for 438 consecutive days. Based on this one performed in Dowley (California, USA) in the 1960s. In might argue that we could go to Mars, a one way this facility 3–4 subjects lived in a camper size and like voyage of about 9–10 months without any new system attached to a large arm centrifuge. No publicly countermeasure protocols. If the adaptive effects accessible publications have been released from this for e.g. muscle, bone or the cardiovascular system study. Recently NASA looked into the operation of a have a higher threshold level than Mars gravity nearly 16 m diameter human centrifuge for studies in (3.73×g) one could apply centrifugation on the the order of weeks (NASA 2005). Mars surface. This argument holds even more for Microgravity Sci. Technol (2009) 21:203–207 205 Moon missions where the weight is reduced to less tational physiology spread over four continents some than 17% from that on Earth, although the travel general conclusions might be drawn. time is less relevant compared to a Mars mission. As mentioned before we can have insight in the While on the planet the crew can be exposed to any adaptation of the human body to long duration hy- suitable g-level, building up sufficient ‘reserves’ for pergravity. Questions like: Does the paradigm of a their return trip to Earth. Such a centrifuge could gravity continuum also apply for humans? What is have any suitable dimension not dictated by the size the adaptation time for the various organ systems, or of the spacecraft transporting the crew to Mars. their re-adaptation from hyper-g to 1 g or bed-rest? 4. If we consider a really large centrifuge, see later, Psychological and social studies while running weeks there is an additional associated field of research or months hypergravity studies are advantageous in namely psychology.