Artificial gravity as a countermeasure for mitigating physiological deconditioning during long-duration space missions

Abstract

In spite of the experience gained in human space flight since Yuri Gagarin's historical flight in 1961, there has yet to be identified a completely effective countermeasure for mitigating the effects of weightlessness on humans. Were astronauts to embark upon a journey to Mars today, the 6-month exposure to weightlessness en route would leave them considerably debilitated, even with the implementation of the suite of piece-meal countermeasures currently employed. Continuous or intermittent exposure to simulated gravitational states on board the spacecraft while traveling to and from Mars, also known as artificial gravity, has the potential for enhancing adaptation to Mars gravity and re-adaptation to Earth gravity. Many physiological functions are adversely affected by the weightless environment of spaceflight because they are calibrated for normal, Earth's gravity. Hence, the concept of artificial gravity is to provide a broad-spectrum replacement for the gravitational forces that naturally occur on the Earth's surface, thereby avoiding the physiological deconditioning that takes place in weightlessness. Because researchers have long been concerned by the adverse sensorimotor effects that occur in weightlessness as well as in rotating environments, additional study of the complex interactions among sensorimotor and other physiological systems in rotating environments must be undertaken both on Earth and in space before artificial gravity can be implemented.

Keywords: adaptation; centrifuge; countermeasure; gravity; international space station; microgravity.

Figure 1

Artificial gravity. Continuous rotation of a large spacecraft that creates a centrifugal force of 1 G in the habitat would give the static crewmembers the sensation of standing upright as on Earth. The magnitude of the centrifugal force is function of the square of the rotation rate (ω) times the distance (r) from the axis of rotation. In the example of the spacecraft shown in the insert, a 4-rpm rotation rate would generate 1 G in the crew habitat located at 56 m from the axis of rotation.

Figure 2

Hypothetical comfort zone bounded by values of artificial gravity level and rotation rate based on theoretical studies in the 1960s (see Hall, , for details). The “comfort zone” is the area in blue delimited by a maximum rotation rate of 6 rpm. According to the model of Stone and Letko (1965) the Coriolis and cross-coupled angular accelerations generated at these rotation rates during walking, climbing and handling materials should be the most comfortable for the crewmembers. However, very little experimental data were actually collected to validate this model. Recent data indicate that the limit of 6 rpm is overly conservative.

Figure 3

Constraints for short-radius centrifugation. On Earth, the actual forces exerted on the body during centrifugation are the resultant of the gravitational force (in blue) and the centrifugal (inertial) forces (in red). These gravito-inertial forces (in green) are larger than 1 G and tilted relative to vertical. In space, the centrifugal forces are the only forces generated by centrifugation and aligned with the longitudinal body axis. Note also the gravity gradient, i.e., the different magnitude of centrifugal force along the longitudinal body axis.

Figure 4

Rationale for evaluating the effects of intermittent short-radius centrifugation during bed rest.

Figure 5

Rationale for evaluating the effects of Martian gravity during head-up tilt.

Figure 6

Partial-gravity simulators. (A) A harness connected to a rolling-trolley mechanism ensures that only a vertical force is applied to the subject. (B) Subject walking on a treadmill with lower body positive pressure (LBPP) support that reduces weight bearing. (C) The reduced-gravity walking simulator at NASA Langley Research Center used long cables to support a subject walking on a tilted surface Photo credit: NASA.