Artificial gravity of short radius as a countermeasure to bone, muscle, and cardiovascular atrophy during long-term space missions
Artificial gravity created by rotation — one of the oldest ideas for combating muscle atrophy in microgravity, dating back to von Braun and concepts of orbital stations with diameters in hundreds of meters. But such a giant radius is unrealistic for a real spacecraft — so NASA, ESA, and DLR researchers have been studying much more compact variants: short-radius centrifuges (Short-Radius Centrifuge, SRC) with diameters of only 2–6 meters, which can be integrated into the living module of a station or ship.
A short-radius centrifuge is a rotating platform or gondola at the end of a radial arm 1.5–3 meters long, driven by an electric motor near the central axis. The astronaut lies horizontally (radially) so that their feet are as far from the rotation axis as possible (where the centripetal acceleration is greatest), and their head is as close to the axis as possible (where the acceleration is minimal).
Main components: • Drive unit on the axis: electric motor, reducer, speed control system with smooth acceleration • Radial beam or frame: rigid structure that withstands loads during rotation • Gondola or crew seat: fixes the body in radial position with safety belts • Counterweight: another radial element for balancing moments and vibrations • Emergency stop system: rapid deceleration in case of malfunction
Unlike the large centrifuge on the station (with a diameter of hundreds of meters that rotates very slowly and provides uniform gravity), short-radius centrifuges are compact devices for periodic 'sessions' of gravitational loading, not permanent living environments.
The main advantage of a short radius is compactness: SRC can fit inside an existing station module or even a transport ship, whereas a rotating ring with an acceptable gravity gradient would require a radius in hundreds of meters and its own massive apparatus.
Centripetal acceleration increases linearly with radius: a = ω²·r, where ω is angular velocity and r is the distance from the axis. This means that gravity varies significantly along the length of a lying person's body at short radii: it is maximal on their feet (r ≈ centrifuge radius) and much lower on their head (r ≈ radius minus height), sometimes approaching zero.
On a large rotating ring (hundreds of meters), this gradient is negligible because human height is an insignificant fraction of the radius. But at a radius of 2-3 meters, the gradient becomes physiologically significant: the difference between g on degrees and g on head can exceed 2-3 times.
This creates an unusual loading: blood and bodily fluids feel different 'gravitational' pressure depending on their position along the body, unlike uniform fields on Earth or in large stations.
The second physiological challenge of short-radius systems is coriolis acceleration, which occurs each time the body or end of motion moves relative to the rotating reference frame: a_cor = −2·(ω × v). The smaller the radius and the higher the accelerations, the stronger this effect during any head movement.
This is most noticeable during head turns: a movement that on Earth is completely normal creates a conflict between the vestibular apparatus (inner ear) and the visual system — known as cross-coupled illusion, which subjectively feels like an unexpected tilt or rotation of space and often accompanies nausea.
Historically, the comfortable limit was considered to be about 6 rotations per minute, but adaptation studies show that after several training sessions, people can tolerate 20-30 rotations per minute without significant boredom, as the brain progressively recalibrates the vestibulo-ocular connection.
Unlike continuous habitation in a rotating environment, short-radius centrifuges are used as intermittent countermeasure procedures: daily or every-other-day sessions lasting 20-60 minutes during which the crew receives a dose of centripetal loading on bones, muscles, and cardiovascular systems.
A typical session protocol includes a gradual warm-up (a few minutes), a plateau at target rotations, and a smooth cooldown — sudden changes in speed can intensify Coriolis discomfort and the risk of disorientation.
The AGBRESA (DLR, Cologne, 2019) study — a 60-day bedrest experiment with head-down tilt involving 24 volunteers — became one of the most comprehensive tests of artificial gravity through short-radius centrifuges as a countermeasure, comparing continuous and intermittent rotation modes.
On a large rotating ring of the station, gravity is practically uniform along the body if the radius is large. However, in short-radius centrifuge, the gradient is a fundamental physical property that cannot be eliminated but can only be mitigated by choosing the radius, rotations, and position of the body.
Acceleration at a point r from the axis: a(r) = ω²·r, where ω = 2π·RPM/60. For a lying person with feet at radius R and head at radius (R − 1.7 m), the acceleration ratio is:
a(feet)/a(head) = R / (R - 1.7)
At R = 2.5 m, this ratio is ≈3.1×; at R = 4 m — only ≈1.7×; at R = 10 m — approximately 1.2×. Thus, the gradient quickly softens with increasing radius, but it's precisely a large radius that makes the structure impractical for in-cabin installation.
This is a fundamental engineering compromise for SRC: compactness requires a short radius, which in turn means a strong gradient.
The gradient primarily affects the distribution of fluids within the body. On Earth, hydrostatic pressure of blood and lymph along the body is determined by uniform gravity. In a short-radius centrifuge, 'effective weight' of the fluid column between the heart and feet significantly exceeds that between the heart and head — this intensifies the load on the cardiovascular system compared to an equivalent uniform field.
Subjects in studies also subjectively report an unusual sensation of 'stretching' their body — feeling as if their limbs are heavier, and their head almost weightless, which does not correspond to any natural experience on Earth.
The conclusion from NASA and DLR researchers is that for tolerable physiological loading, the radius should be approximately greater than 2 meters, and the target g-force on the steps should not be too high (typically 0.5-1 g) to keep the gradient within what the body can easily tolerate during short sessions.
Engineers and physiologists propose several approaches:
• Maximizing the available radius within the module — even a difference of 1 meter significantly reduces the gradient • Body orientation: placing legs closer to the maximum radius and bending knees — to shorten the effective body length along the radius • Limiting the target g on degrees to a physiologically sufficient minimum (instead of full 1g) — fewer revolutions at the same radius reduce both the absolute gradient and Coriolis effects • Gradual adaptation: organisms and vestibular systems partially adapt to the gradient during a series of training sessions
Coriolis acceleration is not just discomfort but a fundamental consequence of motion in a non-inertial reference frame. In a short-radius centrifuge, it is the main cause of disorientation, boredom, and confusion—much more so than the gravitational gradient itself.
When a person is in a rotating centrifuge and moves their head (for example, by nodding or turning), the semicircular canals of the inner ear record a combination of two rotational movements simultaneously: the actual rotation of the centrifuge and an additional rotation of the head. The brain perceives this combination as an unexpected, 'impossible' third movement — an illusory rotation or tilt that does not physically occur.
This effect, known as the cross-coupled (Coriolis) illusion, is much stronger source of discomfort in SRC than the actual centripetal force or gravity gradient. Historically, it was coriolis illusions, not g-gradients, that limited the maximum rotations acceptable for the crew.
Research shows that sensitivity to coriolis illusions is significantly influenced by individual vestibular systems, but systematic repeated exposure (repeated exposure) allows most people to adapt and tolerate much higher accelerations without boredom over 2-4 weeks of regular sessions.
Researchers and engineers have tested several approaches to reducing the impact of Coriolis forces:
• Limiting head movements during sessions — instructions and fixation of the head in a neutral position reduce the number of illusion episodes • Gradual acceleration and deceleration — avoiding sudden changes in angular velocity, which themselves cause transitional Coriolis and Euler accelerations • Visual reference points — a fixed visual orientation (e.g., screen synchronized with rotation) helps the brain reconcile visual and vestibular information • Progressive training — starting with low rotations (e.g., 10 rpm) and gradually increasing over several weeks forms vestibular adaptation • Pharmacological support — in some protocols, anti-motion sickness medications are used during early stages of adaptation
Unlike continuous habitation in a rotating environment, short-radius centrifuges are applied as periodic medical procedures — a dose of gravitational loading, similar to physical exercises. The design of the protocol balances physiological benefits, coriolis discomfort, and practical time constraints for the crew.
A typical short-radius centrifuge session protocol usually consists of three phases:
1. Ramp-up: gradual increase in rotations over 1-4 minutes to the target value to minimize Eulerian transient accelerations (related to changes in angular velocity) and allow the vestibular system time to adapt
2. Plateau (steady-state): stable target rotations during the main part of the session (typically 20-50 minutes), during which the body experiences a centered loading close to the target g-level at hip level
3. Ramp-down (deceleration): symmetric smooth deceleration to a stop with limited rate of change
The AGBRESA study (DLR, 2019) compared a continuous 30-minute daily protocol with an intermittent one (multiple short sessions) to determine the optimal balance between combating atrophy effectiveness and crew tolerance.
Protocol design is an optimization problem with multiple variables that affect each other:
• Higher revolutions at the same radius → higher g on feet → stronger physiological stimulus but also a stronger gradient and Coriolis discomfort • Longer session → greater cumulative 'dose' of load, but more mission time spent on other tasks • More frequent sessions (daily instead of every other day) → faster adaptation and potentially better effect, but higher scheduling demands
Without a final consensus on an 'ideal' protocol, most NASA, ESA, and DLR research programs converge on approximate parameters: 0.5–1 g on degrees, 20–60 minutes per day or every other day, with a mandatory phase of gradual acceleration and deceleration.
Microgravity over months leads to progressive bone density loss, muscle atrophy, and cardiovascular deconditioning. Artificial gravity through short-radius centrifugation is considered one of the few countermeasures capable of simultaneously affecting all these systems, unlike physical exercises which only partially compensate for certain aspects.
Since orbital tests are expensive and rare, most artificial gravity studies on Earth use a 6° head-down tilt bed rest (head-down tilt bed rest, HDBR) model — volunteers lie in a tilted bed at −6° for weeks or months, mimicking the redistribution of fluids and loading of bones and muscles similar to microgravity.
During this bed rest period, participants spend some time in a short-radius centrifuge (as a countermeasure), and results are compared with a control group without centrifugation. The AGBRESA study (DLR :envihab, Cologne, 2019) is one of the most cited: 60 days of head-down bed rest (HDBR), 24 participants, comparison between continuous and intermittent rotation protocols.
Previous studies using short-radius centrifugation as a countermeasure indicate partial alleviation:
• Bone tissue: reduced loss of mineral density in lower extremity bones compared to control groups without rotation • Muscles: better preservation of strength and volume of fast-twitch and slow-twitch muscles with regular sessions at sufficient g on the feet • Cardiovascular system: improved orthostatic tolerance (the ability of the organism to maintain arterial pressure upon standing) after returning to normal position • Vestibular system: despite initial discomfort, repeated sessions improve resistance to motion and reduce symptoms of space adaptation sickness after actual flight
No single study has conclusively shown that a short-radius centrifuge fully replaces physical exercises or full artificial gravity, but accumulated data point to it as a promising additional countermeasure, especially for future multi-year missions to Mars, where cumulative bone and muscle mass loss poses a serious risk.
The compactness of short-radius centrifuges makes them one of the few realistic options for implementing artificial gravity on next-generation ships and stations, unlike giant ring habitats which remain mostly conceptual due to their mass and orbital deployment complexity.
Current research directions include: optimizing protocols (duration, frequency, target g), engineering solutions for reducing drive vibrations and noise, individualized programs for adapting to Coriolis effects, as well as combined protocols that combine centrifugation with traditional physical exercises for maximum effect with minimal crew time investment.