Artificial gravity for long-duration human spaceflight: physiological rationale, evidence gaps, risks, and research priorities

1 Introduction

Human physiology evolved under a nearly constant 1-g gravitational field. Removing that field alters mechanical loading, hydrostatic pressure gradients, vestibular input, movement strategies, and the relationship between the body and its environment. These changes are tolerable for short missions, but their cumulative and interacting effects become increasingly important as agencies plan extended stays in cislunar space and on the lunar surface, and eventual Mars missions. Transit phases may involve months of microgravity, whereas surface operations will occur at approximately 0.16 g on the Moon or 0.38 g on Mars. Whether either partial-gravity environment provides a sufficient physiological stimulus—and for which systems—remains unresolved (Richter et al., 2017; Clément et al., 2016).

The term artificial gravity is used here for an apparent gravity produced by rotation. At radial distance r and angular velocity ω, centripetal acceleration is a = ω²r. The same target acceleration can therefore be achieved with a large radius and low rotation rate or a small radius and high rotation rate, but these configurations are not physiologically equivalent. In a short-radius system, acceleration rises with distance from the axis; consequently, the feet may experience a much larger stimulus than the heart or head. This gradient can strengthen lower-limb loading while providing a weaker vestibular and cephalad-fluid stimulus. During voluntary movement, Coriolis acceleration (2ω × v) alters limb trajectories and produces cross-coupled vestibular cues; its magnitude increases with rotation rate and movement velocity. Thus, radius, rpm, acceleration at anatomically relevant locations, body orientation, head and limb movements, spin-up profile, and exposure duration jointly determine the physiological dose. Large rotating habitats reduce gradients and Coriolis effects at a given target g-level but introduce substantial structural, guidance, docking, balancing, and energy challenges (Clément and Bukley, 2007; Isasi et al., 2022).

AG is appealing because it can simultaneously restore body loading, fluid gradients, and vestibular and proprioceptive stimulation. Nevertheless, a plausible multisystem rationale is not equivalent to demonstrated clinical or operational effectiveness. This review therefore distinguishes acute physiological responses from durable protection, intermittent centrifugation from continuous rotation, and AG from complementary countermeasures such as lower-body negative pressure (LBNP). It asks three questions: which spaceflight-associated changes could AG reasonably mitigate; what has human evidence demonstrated; and what biological, human-factors, and engineering questions must be resolved before deployment?

2 Review approach and scope

This article is a structured narrative review, guided by SANRA principles (Baethge et al., 2019), rather than a systematic review or meta-analysis. The review questions were defined a priori around physiological rationale, direct human evidence, exposure geometry, comparison with established countermeasures, safety, ethics, and translational readiness. Literature was identified through targeted combinations of terms relating to “artificial gravity”, “short-arm centrifugation”, “rotating habitat”, “head-down tilt bed rest”, “dry immersion”, “partial gravity”, and the physiological domains discussed in the article. Reference lists of relevant reviews and key protocol papers were also examined. Priority was given to peer-reviewed human spaceflight, centrifugation, bed-rest, and dry-immersion studies; authoritative reviews and engineering analyses were used for context. Animal, cellular, conceptual, and commercial sources were not used as direct evidence of human efficacy.

Studies were included when they described a defined gravity-related exposure and reported physiological, performance, tolerability, human-factors, or engineering outcomes relevant to long-duration missions. Studies without a sufficiently described exposure, purely promotional concepts, and claims that could not be traced to a scientific source were excluded from evidentiary conclusions. Because the search was designed for a broad narrative synthesis and the included evidence spans reviews, protocols, overlapping secondary analyses, and engineering sources, a PRISMA flow diagram and pooled participant total would imply a completeness and independence that the source set does not support. The article instead reports the design and sample size of pivotal human protocols and explicitly identifies overlapping analyses from the 24-participant AGBRESA cohort. Table 1 sets out the principal human and analogue protocols considered in this review, together with their main findings and limitations.

TABLE 1. Key human and analog protocols emphasized in this review. Participant counts should not be summed across rows because several publications derive from the same AGBRESA cohort.

Evidence was weighted qualitatively according to directness (spaceflight > validated human analog > acute laboratory centrifugation > mechanistic or engineering inference), study design, sample size, comparator quality, exposure characterization, outcome relevance, consistency, and replication. No formal risk-of-bias instrument or meta-analysis was applied because of marked heterogeneity and the small number of comparable trials. Conclusions are therefore graded in the text as demonstrated acute effects, suggestive but heterogeneous protection, insufficient evidence, or hypothesis-generating only. The reproducibility of the review may be limited by the absence of fully standardized reporting of search dates, databases, and quality-assessment procedures.

3 Why an integrative gravity countermeasure is considered

3.1 Musculoskeletal unloading

Microgravity removes habitual weight bearing and markedly reduces loading of antigravity muscles and the axial and lower-limb skeleton. Bone remodeling shifts toward net resorption, particularly in weight-bearing sites, while reduced mechanical signaling impairs osteoblast function and alters osteocyte-mediated regulation. Loss is heterogeneous among astronauts and anatomical sites, and recovery may be prolonged. Spaceflight and analog evidence also show reductions in muscle volume, strength, power, and endurance, with preferential effects on postural and lower-limb muscles (Grimm et al., 2016; Juhl et al., 2021; Man et al., 2022). The commonly quoted 1%–2% monthly bone loss and very large muscle-loss estimates should not be treated as universal rates because mission duration, anatomical site, exercise adherence, nutrition, and individual susceptibility materially influence outcomes.

Resistance and aerobic exercise are indispensable but impose substantial hardware, time, maintenance, and energy costs. They also deliver episodic, task-specific loading rather than a persistent head-to-foot gravitational vector. Nutrition and antiresorptive approaches may support skeletal protection, but do not replace mechanical stimulation and may introduce mission-specific monitoring or adverse-effect considerations (Genah et al., 2021; Baran et al., 2022). AG could add axial loading and permit exercise under a gravity gradient, but evidence that available centrifugation prescriptions prevent clinically meaningful bone loss remains limited.

Evidence must also be separated by tissue. Bone endpoints change slowly and vary by skeletal site; the available centrifugation studies are generally too short or too small to establish fracture-relevant preservation. Tendon, ligament, and cartilage responses to AG have not been adequately characterized in humans, and it is not justified to infer protection from muscle loading alone. Mechanical loading could plausibly support connective-tissue homeostasis, but magnitude, strain rate, joint kinematics, recovery, and exercise mode are likely to matter. These outcomes should therefore be treated as explicit evidence gaps rather than presumed extensions of skeletal benefit.

3.2 Cardiovascular and fluid-regulatory adaptation

Entry into microgravity causes a cephalad fluid shift and loss of the upright hydrostatic pressure gradient. Subsequent responses include plasma-volume reduction, altered cardiac filling, vascular and baroreflex adaptation, and reduced tolerance to orthostatic stress after landing. These changes interact with physical inactivity, mission duration, sex, hydration, heat stress, and individual autonomic responsiveness. Cephalad venous congestion is also relevant to venous thrombosis risk and to spaceflight-associated neuro-ocular syndrome (SANS), although SANS pathophysiology is multifactorial and should not be reduced to a simple increase in intracranial pressure (Patel et al., 2025).

Rotation can re-establish a footward pressure gradient and challenge cardiovascular control. This provides a strong mechanistic rationale for AG, especially when combined with exercise. However, the cardiovascular response depends on body orientation, acceleration at the heart and feet, exposure duration, intermittent versus continuous dosing, and whether exercise is performed during rotation. A countermeasure that produces a robust acute orthostatic stimulus may not necessarily preserve plasma volume or long-term vascular function.

3.3 Sensorimotor adaptation and operational performance

Microgravity changes the relationship among vestibular, visual, proprioceptive, and somatosensory cues. Early in flight, sensory conflict contributes to space motion sickness, disorientation, and altered eye–head and postural control. Adaptation supports performance in orbit, but creates a second challenge when crews return to a gravitational environment or transition to a planetary surface. Impaired balance, locomotion, manual control, and spatial orientation can be operationally consequential during landing, emergency egress, or immediate surface tasks.

AG offers repeated vestibular and proprioceptive stimulation, but rotation itself creates unfamiliar cues. Cross-coupled angular accelerations generated by head movements can provoke illusory motion, nausea, and performance decrements, particularly at high rotation rates. Accordingly, AG is both a potential countermeasure and a sensorimotor stressor; its success depends on adaptation protocols and on whether training transfers to the target gravitational environment.

3.4 Hematological, immune, endocrine, circadian, neurocognitive, and behavioural outcomes

Spaceflight is associated with persistent hemolysis, altered immune-cell function, viral reactivation, and changes in inflammatory signaling (Trudel et al., 2022; Dhar et al., 2021). Endocrine, circadian, sleep, cognitive, and behavioural findings are heterogeneous and are strongly confounded by light–dark schedules, workload, isolation, radiation, altered nutrition, and mission phase. Some studies report changes in spatial cognition, prospective memory, neural connectivity, sleep architecture, or performance under high workload, whereas other domains remain stable (Stahn and Kühn, 2021; Hao et al., 2025). The reviewed human AG studies do not isolate durable effects on endocrine regulation, circadian phase, sleep, immune competence, or behavioural health.

Accordingly, AG cannot presently be claimed to normalize immunity, endocrine regulation, circadian rhythms, cognition, sleep, collective mood, or cooperation. A stable gravity vector and more Earth-like routines could improve environmental legibility in a rotating habitat, while centrifugation sessions could also disrupt schedules, provoke discomfort, or transiently impair performance. Both benefit and harm remain plausible; these domains should be prospectively measured with validated biomarkers, sleep and circadian assessments, cognitive tasks, adverse-event reporting, and behavioural-health instruments.

4 What current countermeasures achieve—and what they do not

Exploration countermeasures should be evaluated as a system rather than as isolated interventions. Exercise targets aerobic capacity, muscle, and bone; nutrition supports energy balance and substrate availability; pharmacological measures may address selected risks; sensorimotor training accelerates adaptation; compression garments, fluid loading, cooling, thigh cuffs, and LBNP target aspects of fluid distribution or orthostatic tolerance. None reproduces all consequences of an ambient gravity field. Conversely, AG is unlikely to eliminate the need for exercise, nutrition, behavioural health support, or mission-specific medical countermeasures.

LBNP is particularly important to distinguish from AG. By applying subatmospheric pressure around the lower body, LBNP shifts blood and interstitial fluid toward the legs and can provide cardiovascular stress without rotating the crew member. It may therefore complement or, for selected objectives, compete with centrifugation. It does not create whole-body mechanical loading or a vestibular gravity reference and should not be described as an AG technology (Harris et al., 2020; Petersen et al., 2019).

A defensible comparison should use a common mission-value framework rather than rank countermeasures by a single physiological endpoint (Clément, 2017). Candidate metrics include effect size and responder proportion for prespecified outcomes; durability after exposure; adverse-event and intolerance rates; crew time; mass, volume, power, cooling, and maintenance; training burden; compatibility with other tasks; and the extent to which one intervention replaces or complements another. Exercise should be evaluated for aerobic, muscle, bone, and functional outcomes; LBNP for fluid distribution and orthostatic tolerance; nutrition and pharmacology for their target-specific benefits and adverse effects; and AG for multisystem effects across a defined geometry and schedule. Head-to-head or factorial trials, with harmonized endpoints and cost or resource accounting, are needed before quantitative net-benefit comparisons are possible.

5 Evidence for artificial gravity

5.1 Human evidence base

Most human evidence comes from short-arm centrifugation during head-down tilt bed rest or dry immersion. These analogs reproduce unloading and selected fluid-shift features but not radiation, confinement, operational workload, or all vestibular characteristics of spaceflight, and they cannot establish feasibility under spacecraft constraints. Across studies, AG prescriptions differ in radius, rotation rate, acceleration at the feet and center of mass, duration, frequency, body position, exercise, and outcome timing. Samples are generally small, and multiple publications may analyze different outcomes from the same cohort; treating such reports as independent trials would overstate the evidence (Clément et al., 2016; Clément et al., 2022).

5.2 Musculoskeletal and connective-tissue outcomes

Centrifugation increases lower-limb loading and can be combined with squats, cycling, or other exercise. Bed-rest studies suggest that AG-containing protocols can attenuate selected indices of muscle deconditioning, particularly when active exercise is included. However, passive centrifugation alone has not consistently preserved muscle volume or strength. Evidence for bone is less mature because remodeling outcomes require longer observation and are site specific; direct human evidence for tendon, ligament, and cartilage protection is essentially absent. An effective prescription must specify acceleration at anatomically relevant locations, cumulative exposure, session frequency, joint loading, and concurrent muscle activation rather than reporting only ‘1 g at the feet’ (Davis-Street and Paloski, 2005; Isasi et al., 2022).

5.3 Cardiovascular and vascular outcomes

Daily centrifugation can provide a reproducible orthostatic challenge, but protection is outcome specific. In the 24-participant AGBRESA campaign (eight participants per group), time to presyncope declined by 323 ± 235 s after continuous AG, 296 ± 508 s after intermittent AG, and 801 ± 354 s in controls (between-intervention p = 0.0249), indicating attenuation rather than prevention of orthostatic intolerance. The same exposure did not prevent several changes in autonomic cardiovascular control, and changes in plasma volume did not correlate with changes in orthostatic tolerance. Earlier studies likewise found that brief daily exposure could support selected postural-muscle outcomes without preserving plasma volume (Hoenemann et al., 2023b; Clément et al., 2016). These findings argue against a universal prescription and support phenotype-aware dosing and combination with exercise or fluid-volume countermeasures.

5.4 Neurovestibular, sensorimotor, and cognitive outcomes

Within the AGBRESA cohort, 30 min/day of AG was associated with better performance on some tasks administered during or immediately after centrifugation, while intermittent exposure produced less post-rotation illusory motion than continuous exposure. The intervention did not clearly prevent all declines in balance or mobility (Tays et al., 2022). Neuroimaging analyses from the same cohort suggest altered sensory reweighting and attenuation of selected vestibular-network changes, but these overlapping mechanistic analyses should not be translated into independent evidence or broad claims of cognitive protection (McGregor et al., 2023; Tays et al., 2024). Bed-rest studies have also not demonstrated consistent protection against ocular structural changes; daily AG delivered approximately 1 g at the centre of mass but only approximately 0.3 g at eye level in the AGBRESA ocular analysis, emphasizing the importance of anatomical acceleration gradients (Laurie et al., 2021). Table 2 assesses the potential effects of artificial gravity across physiological domains, distinguishing mechanistic plausibility from demonstrated protection.

TABLE 2. Evidence-based interpretation of potential artificial-gravity effects. The table distinguishes mechanistic plausibility from demonstrated protection.

6 Design options and constraints

6.1 Intermittent short-radius centrifugation

A short-radius centrifuge can be integrated into a spacecraft more plausibly than a continuously rotating habitat and can deliver a controlled daily dose. Its disadvantages include steep acceleration gradients, high rotation rates, motion sickness, constrained posture, crew-time demand, moving-part hazards, vibration, and integration with exercise and life-support systems. The biologically relevant dose is not captured by a single g-value: radius, rpm, acceleration at the feet, heart and head, exposure duration, movement, and exercise must all be reported.

6.2 Continuous or intermittent rotating spacecraft and habitats

Large-radius rotation can provide a more uniform field at lower rpm and may support normal locomotion and habitability. Whole-vehicle rotation, tethered configurations, and internal rotating modules each impose different requirements for structural loads, mass balance, attitude control, docking, non-rotating utilities, emergency response, maintenance, and transition zones. Historical concepts and recent commercial proposals illustrate design possibilities but do not constitute health validation or flight-ready engineering evidence. Claims concerning announced stations or hotels should therefore be clearly labelled as concepts unless supported by detailed, current technical documentation.

6.3 Partial gravity is not necessarily a sufficient countermeasure

Lunar and Martian gravity may reduce some deconditioning relative to microgravity, but the threshold and exposure time needed to maintain each physiological system are unknown. A single threshold is unlikely because bone, muscle, cardiovascular regulation, vestibular adaptation, and development may have different dose–response relationships. Surface activity, suit mass, locomotor strategy, habitat design, and exercise will modify the effective stimulus. AG requirements must therefore be mission-phase and outcome specific.

7 Risks, safety analysis, and ethics

7.1 Vestibular and behavioural risks

Rotation can produce sensory conflict, motion sickness, altered perceived vertical, cognitive distraction, and performance errors. Risk increases with rotation rate and head movement but is modified by prior exposure and individual susceptibility. Mitigation should include progressive preflight and inflight adaptation, conservative spin-up and spin-down profiles, low rpm where architecture permits, visual orientation cues, task restrictions during early adaptation, and explicit evaluation of head-movement strategies. Habitability studies must also assess noise, vibration, privacy, sleep, lighting, and transitions between rotating and non-rotating zones.

7.2 Cardiovascular and mechanical risks

Centrifugation can provoke presyncope or excessive strain in susceptible individuals, particularly with rapid transitions, dehydration, heat stress, or high footward acceleration. Continuous physiological monitoring, automated stop criteria, restraint and egress provisions, individualized dose escalation, and medical exclusion criteria are therefore required. Mechanical systems need fault-tolerant braking, containment of rotating components, imbalance detection, vibration control, fire and evacuation procedures, maintainability, and safe modes that do not create new single-point failures.

7.3 Prospective safety-analysis protocol

Safety should be analyzed prospectively across acute, adaptation, and chronic phases. A core protocol should record motion-sickness severity, vomiting, headache, presyncope, arrhythmia, injury, sleep disruption, anxiety, spatial disorientation, cognitive or motor errors, and withdrawal or dose-reduction events using predefined severity grades and exposure denominators. Testing should include nominal sessions, repeated exposure, head and limb movements, transitions between rotating and non-rotating zones, communication and navigation tasks, and representative emergency scenarios. Continuous physiological monitoring, independent stop authority, automated thresholds, fault injection, evacuation drills, and post-event medical review should be specified before deployment. Chronic tolerability cannot be inferred from brief laboratory exposures and requires repeated-measures follow-up.

Emergency procedures must address sudden spin-down, power loss, imbalance, fire, medical incapacitation, rescue from restraints, loss of communication, and spatially disorienting transitions. During early adaptation or after configuration changes, restrictions should apply to mission-critical piloting, navigation, maintenance, and emergency tasks until predefined performance and symptom criteria are met. Safety evidence should report both group averages and susceptible individuals because a low mean symptom score can conceal mission-limiting intolerance in a minority of crew members.

7.4 Ethical and crew-selection considerations

AG studies and operational use require informed consent that distinguishes uncertain benefit from established protection and explains foreseeable vestibular, cardiovascular, psychological, and mechanical risks. Medical screening should be relevant and proportionate, with particular attention to syncope, vestibular disease, cardiovascular disorders, migraine, musculoskeletal limitations, and medications that may alter tolerance. Screening thresholds and crew-selection decisions should be transparent, periodically reviewed, and designed to avoid unjustified exclusion when training or individualized dosing can mitigate risk.

Research participation should remain voluntary wherever operationally possible, with confidential adverse-event reporting and no penalty for withdrawal. When AG exposure becomes a mandatory mission procedure, the ethical basis changes from research consent to occupational risk governance: the requirement should be justified by demonstrated mission benefit, minimized through adaptation and individualized dosing, and overseen independently. Hierarchical pressure, career incentives, and the small astronaut population create a real risk of perceived coercion; consent processes, data governance, and return-of-results plans should explicitly address that context.

8 Research and development priorities

The central translational problem is not whether rotation can generate centripetal acceleration, but which exposure profile produces a meaningful net operational benefit. A common reporting framework should define radius, rpm, g at the feet/heart/head or center of mass, body orientation, session duration and frequency, cumulative exposure, exercise mode, adaptation procedures, adverse events, and timing of outcome assessment. Table 3 translates the main evidence gaps into specific research requirements and the operational decisions they are intended to support.

TABLE 3. Priority evidence needed to progress from physiological rationale to mission-level decisions.

9 Discussion

AG has a unique conceptual advantage: it targets the absence of gravity itself rather than treating each downstream consequence separately. Yet this advantage can encourage overgeneralization. Current evidence supports acute gravitational loading and shows potential for selected cardiovascular, muscular, and sensorimotor outcomes, but does not demonstrate that a brief daily centrifugation session prevents the full spectrum of spaceflight deconditioning. Evidence for bone, tendon, ligament, cartilage, SANS, immunity, endocrine and circadian regulation, cognition, sleep, and behavioural health is especially incomplete. Where hypotheses are biologically plausible, they are labelled as such rather than presented as demonstrated benefits.

The relevant comparator is not no countermeasure, but the evolving package of exercise, nutrition, medical monitoring, sensorimotor training, fluid-management tools, and targeted pharmacology. AG must demonstrate additive benefit, superior resource efficiency, or the ability to reduce other hardware or crew-time costs. Comparative decisions should therefore integrate standardized physiological effect sizes and responder proportions with adverse events, durability, mass, volume, power, maintenance, training, and operational compatibility. The answer may differ by mission phase: intermittent centrifugation may be attractive during transit, whereas surface operations may combine partial gravity, exercise, and task-related loading. A rotating habitat could deliver continuous exposure and improve daily function, but its engineering and adaptation costs are fundamentally different from those of a short-arm centrifuge.

The literature also needs greater precision in terminology. Centrifugation, whole-habitat rotation, partial gravity, hypergravity, and LBNP are related but non-interchangeable exposures. Grouping them obscures mechanisms and impedes comparison. Future studies should report full exposure geometry and use outcomes linked to mission capability, such as landing and egress performance, injury risk, maintenance burden, and recovery time, alongside physiological biomarkers.

10 Conclusion

Artificial gravity remains one of the most compelling candidate countermeasures for long-duration human spaceflight because it can restore several inputs lost in microgravity. It should not, however, be presented as a proven comprehensive solution. The evidence currently supports a strong mechanistic rationale and selected benefits under specific analog conditions, alongside material uncertainty about effective dose, long-term protection, tolerability, and systems-level value. A staged programme -standardized ground studies, comparative AG–exercise protocols, orbital demonstrations, and mission-architecture trade studies -is needed. AG will become mission enabling only if biological effectiveness, human acceptability, and engineering feasibility can be demonstrated together.

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Footnotes

Edited by: Volker Hessel, University of Adelaide, Australia

Reviewed by: Amit Agrawal, All India Institute of Medical Sciences, India