How common is motion sickness on short commercial spaceflights?

Two gravity transitions, two distinct syndromes

The paper separates spaceflight-associated motion sickness into two phases. Space motion sickness (SMS) appears after the transition from Earth gravity into microgravity; terrestrial readaptation motion sickness (TRMS) appears after the reverse transition, when a neurovestibular system adapted to microgravity meets Earth gravity again. The authors treat both as forms of ordinary motion sickness provoked by unusual environments, comparable to carsickness or seasickness, rather than as separate disease entities.

That distinction matters because the two phases answer different operational questions. SMS affects the first days on orbit, when crews are still learning to work in weightlessness. TRMS affects the hours after splashdown, when crews may need to exit a capsule, assist one another, or begin recovery. The paper reports incidence, severity, and persistence separately for each phase rather than pooling them into one motion-sickness number.

Head tilts before every symptom report

Prior spaceflight studies rarely controlled what crewmembers were doing before they rated their symptoms, even though head movements are thought to provoke them. This study inserted a fixed provocative stimulus: a sequence of head-on-body tilts (left ear to left shoulder, right ear to right shoulder, back upright, nose toward chest, nose up), repeated four times, performed immediately before each symptom query.

Crewmembers then rated intensity on a 0-20 scale where 0 meant no motion sickness whatsoever, 10 meant halfway to vomiting, and 20 meant vomiting or about to vomit. Reports were entered on a tablet during flight rather than reconstructed in postflight debriefs, which addresses a limitation the authors attribute to most earlier datasets. When a crewmember was too sick to complete the survey, or reported over space-to-ground communications outside the scheduled timepoints, those entries were integrated and marked separately in the figures.

The cohort was balanced by sex (4 female, 4 male), aged 28-62, and included seven first-time flyers and one person with roughly three prior days in space. Preflight MSSQ-Short scores placed the group at an average of the 50.1st percentile for terrestrial motion sickness susceptibility, so this was not a selected highly susceptible group.

87.5% inflight versus 50% after return

Incidence here means reporting a rating above zero at any timepoint in that phase. Inflight, 7 of 8 crewmembers did so, an incidence of 87.5% with a 95% confidence interval of 47.35% to 99.68%. Postflight, 4 of 8 did, an incidence of 50.0% with a confidence interval of 15.7% to 84.3%. The within-participant comparison showed a trend toward lower postflight incidence that was not statistically significant (McNemar's exact test, p = 0.25).

The overlap pattern is more informative than either rate alone. Four crewmembers reported both SMS and TRMS; three reported SMS without TRMS; one reported neither; and nobody reported TRMS without having reported SMS. Vomiting is a separate and stricter measure: 5 of 8 (62.5%, CI 24.5% to 91.5%) vomited inflight, including all four Fram2 crewmembers and one Polaris Dawn crewmember, while no one vomited postflight (0%, CI 0% to 36.94%).

The wide intervals are the point, not a footnote. With eight participants, an 87.5% estimate is compatible with a true rate anywhere from roughly 47% to nearly 100%, so the headline percentage should be read as a first systematic baseline for commercial flyers rather than a precise population parameter.

Postflight sickness was milder, not absent

Severity was defined as each crewmember's most severe report in a phase. Mean peak inflight severity was 14.9 of 20 versus 4.8 postflight, a mean difference of 10.1 points (95% CI 3.8 to 16.5) that was statistically significant in a paired t-test (t(7) = 3.76, p = 0.007). Five crewmembers reached 20 inflight, meaning vomiting.

The individual-level picture is uneven. Two crewmembers had severe postflight sickness at 15 of 20, two were mild at 4 of 20, and all four Polaris Dawn crewmembers reported no TRMS at all. No crewmember reported more severe sickness postflight than inflight, but the correlation between an individual's inflight and postflight peak severity was positive and not significant (Pearson r = 0.5, p = 0.2; Spearman r = 0.71, p = 0.14). A severe inflight response therefore did not reliably predict a severe response after return in this small group.

Symptoms lasted days, and not always on schedule

Persistence was measured as the time until ratings dropped below 3 and stayed there. Inflight motion sickness continued for an average of 1.85 days (44.4 hours, 95% CI 0.81 to 2.9 days), and postflight sickness for an average of 0.78 days (18.7 hours, 95% CI 0.065 to 1.50 days). The postflight phase was shorter on average but the difference was not significant (paired t-test, t(7) = 1.89, p = 0.1003). Three crewmembers were still above 3 at their last inflight report, so their true persistence may have been longer than the value used.

The abstract's statement that SMS persisted at least three days on orbit in 25% of crewmembers corresponds to two of the eight. The individual trajectories also departed from the textbook pattern. Four crewmembers showed the expected early onset, plateau, and decay; two had low symptoms early followed by a delayed rise after two to three days; one relapsed on flight day 3 after an early onset and decay; and one reported no inflight sickness at any of four reports. The authors suggest anti-motion-sickness medication may have temporarily masked symptoms and shaped these atypical courses, but they could not control for that.

Medication use was common and confounds the picture

Six crewmembers reported inflight use of anti-motion-sickness medication and the other two used it preflight; six took medication just before return. Promethazine, typically 25 mg, was most common, with meclizine, typically 25 mg, used by two crewmembers. Because medication timing and dose varied by person, the reported intensities are symptoms experienced under whatever pharmacological protection each crewmember chose, not untreated motion sickness.

The two missions also differed in ways the authors flag as plausible contributors. All four Polaris Dawn crewmembers reported no TRMS, while all four Fram2 crewmembers did. Capsule motion after splashdown, measured from inertial sensors over the 2-to-20-minute window, was roughly twice as large on Fram2 for body-rate RMS (8.0 versus 3.7 deg/s), body-rate peak (21.6 versus 10.2 deg/s), detrended specific-force RMS (1.55 versus 0.76 m/s2), and peak-to-peak hang angle (30.6 versus 17.7 degrees), while hang-angle mean and dominant rocking period were similar. Fram2 crew also performed a self-egress from the capsule on the recovery ship, which could have further provoked symptoms.

That sea-state explanation cannot cover the inflight difference. Fram2 crew showed a trend toward more severe SMS than Polaris Dawn (p = 0.08 inflight), and postflight the difference was significant (p = 0.024), even though both crews were in the same spacecraft in microgravity. The authors note the inflight gap is presumably not due to environmental stimuli, leaving open what else differed between the crews.

Sex, age, and prior susceptibility did not predict severity

In this cohort, severity did not differ significantly between males and females for inflight SMS (p = 0.18) or postflight TRMS (p = 1). Age showed no correlation with severity inflight (p = 0.82) or postflight (p = 0.49). Preflight MSSQ score, which records a history of terrestrial motion sickness, also failed to correlate with inflight (p = 0.88) or postflight (p = 0.46) severity, and the conclusion held when using MSSQ percentile instead of raw score.

The authors are explicit that predicting individual differences was outside the study's scope, so these null results should not be read as evidence that susceptibility, sex, or age are irrelevant in general. With eight participants, the study is not powered to detect modest associations. What the nulls do establish is narrower: in this balanced commercial cohort, none of these readily available preflight or demographic variables separated the crewmembers who vomited from those who did not.

Why the commercial baseline is the contribution

The paper's central claim is methodological and comparative rather than mechanistic. Earlier agency incidence figures are inconsistent and depend heavily on which programs are pooled: the authors cite Mercury and Gemini at 0%, Apollo at 36%, Skylab at 56%, and a large Shuttle investigation at 73%, with pooled agency estimates elsewhere described as roughly 40%, 50%, 60%, or 60-80%. Those numbers came largely from male, NASA-selected astronauts, were often collected retrospectively, and generally lacked a controlled provocative stimulus before symptom queries.

This study's 87.5% SMS incidence sits at the high end of that range, but the authors caution against direct comparison because the reporting scale, the head-tilt stimulus, the real-time tablet entry, and the commercial cohort all differ from prior methods. The more defensible reading is that a standardized, stimulus-controlled protocol applied to commercial flyers produced a high measured incidence and a clear severity gap between the two gravity transitions, and that the gap between 87.5% and 50% is a within-participant comparison that earlier retrospective designs could not make cleanly.

The operational relevance the authors draw is that SMS can impair crews during the first days on orbit, which is why extravehicular activities are not scheduled before 72 hours and why crews are cross-trained so early critical tasks do not depend on one person. Their persistence data bear on that threshold: average inflight persistence was 1.85 days, but two crewmembers remained above the threshold at three days, and three were still above 3 at their final inflight report.

Short flights, eight people, one spacecraft

Both missions were short: Polaris Dawn spent 4.9 days (118 hours 13 minutes) in orbit and Fram2 spent 3.6 days (86 hours 33 minutes), both in a SpaceX Crew Dragon with about 9.3 m3 of habitable volume. The authors argue that on short flights, incomplete adaptation to microgravity may leave a partially adapted nervous system that is better positioned for the return to Earth, which would produce less severe and less persistent TRMS. They anticipate that on long-duration flights, with fuller neurovestibular adaptation, SMS and TRMS severity and persistence may correlate more clearly within individuals. That is a stated expectation, not a result of this study.

Crewmembers also conducted operational and scientific activities in parallel that may have influenced symptoms, both by adding provocative motion and by changing how far each person adapted. The authors state these could not be controlled for. The sample of eight, the two missions, the single spacecraft, and the uncontrolled medication timing all bound the conclusions to short commercial flights in this setting.

The most useful takeaway is a comparison rather than a single rate: in these eight commercial flyers, the transition into microgravity produced higher incidence, higher peak severity, and longer persistence than the transition back to Earth, and vomiting occurred only in the inflight phase. The Fram2-versus-Polaris Dawn split, where all four Fram2 crewmembers reported TRMS and none of the Polaris Dawn crew did alongside roughly double the measured capsule motion after splashdown, is the paper's clearest signal that post-splashdown environmental motion may matter for readaptation sickness. Testing that link requires more missions with instrumented capsule motion and standardized symptom reporting, not a larger retrospective survey.

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