It is impossible to burp in space, because in microgravity the human stomach cannot separate gas from the liquid and partially digested food it sits inside, and any attempt to burp expels a mixture of all three directly into the astronaut's mouth.

The mechanism behind a burp on Earth is something most humans do not stop to think about. Food and drink enter the stomach. The act of swallowing brings air down with them. The acids and enzymes in the stomach break down the food and produce additional gas as a byproduct of digestion. The gas accumulates. It needs to come back out, because if it does not, the stomach distends and the pressure becomes uncomfortable. Most people release this gas dozens of times a day without noticing.

The release is possible because gravity sorts the contents of the stomach into layers by density. Heavier solids and liquids settle at the bottom. Lighter gas rises to the top. When the gas at the top of the stomach builds up to sufficient pressure, it pushes against the lower oesophageal sphincter, the ring of muscle that separates the stomach from the oesophagus. The sphincter opens. The gas rises into the lower oesophagus. A second sphincter, at the top of the oesophagus, then opens to let the gas escape through the mouth.

Convection assists the entire process. Warm gas, being less dense than the cooler air above it, rises through the oesophagus toward the mouth. The entire burp mechanism, from sphincter opening to gas release, is a passive process that relies on gravity at every step.

In microgravity, none of it works.

What happens inside an astronaut’s stomach

Aboard the International Space Station, where the residual gravity is approximately one-millionth of the gravity at Earth’s surface, the density-based separation of stomach contents does not occur. The food, the drink, the swallowed air, the digestive juices, and the gas produced during digestion all float together in the stomach as a single mixed mass. Raffi Kuyumjian, the chief medical officer of Operational Space Medicine at the Canadian Space Agency, has described the resulting state of the astronaut’s stomach as “chunky bubbles”, in which gas and partially digested food are interspersed rather than layered.

The gas, no longer rising to the top of the stomach, does not put pressure on the lower oesophageal sphincter the way it does on Earth. The sphincter does not open. The gas remains trapped in the stomach. Bloating accumulates. Discomfort builds.

If the astronaut nevertheless attempts to burp, by swallowing additional air, contracting the abdominal muscles, or drinking a carbonated beverage, the result is not a burp. It is the release of the entire mixed-state contents of the stomach back into the oesophagus and from there into the mouth. Astronauts and flight surgeons refer to this outcome, in the operational vocabulary of the International Space Station, as a wet burp. The colloquial term in the same vocabulary is bomit, a portmanteau of burp and vomit.

The astronaut Chris Hadfield, who commanded the International Space Station in 2013, has set out the practical implication directly. When you burp in space, the air, food, and liquid in your stomach are all floating together. The gas does not separate. If you burp, you throw up into your own mouth. The gas, having come back up alongside the rest, simply returns to the stomach again afterward.

The Newman workaround

There is, on the published record, one documented technique for inducing a clean burp in microgravity. The technique was developed by Jim Newman, an American astronaut and physicist who completed four Space Shuttle missions and accumulated approximately 43 days in orbit between 1993 and 2002. The technique is called the push burp, and it relies on artificial gravity briefly generated by the astronaut’s own body.

Newman discovered that by pushing forcefully off a wall of the spacecraft, his body would accelerate in the opposite direction. The acceleration produced, in the brief seconds it lasted, a sensation indistinguishable from gravity inside the astronaut’s body. The food and liquid in the stomach, briefly experiencing the equivalent of downward force, settled toward the lower part of the stomach. The gas, briefly behaving as gas behaves on Earth, rose toward the top. The lower oesophageal sphincter responded as it does in Earth gravity. The gas escaped through the mouth, alone, as a clean burp, before the brief artificial gravity dissipated and the stomach contents returned to their mixed state.

The technique is described in the 2016 book What’s It Like in Space?: Stories from Astronauts Who’ve Been There, by Ariel Waldman, a member of NASA’s Innovative Advanced Concepts program. Newman explained to Waldman that timing was the critical variable. The acceleration phase, the few seconds during which the body is still moving outward from the wall, is the only window in which the burp will work cleanly. If the astronaut tries to burp before the acceleration has fully developed, or after the body has stopped accelerating, the wet-burp outcome returns.

Other astronauts have, in informal accounts since 2016, reported developing variations on the same technique. None have produced a fundamentally different solution. The push burp remains, by the published record, the only known way to burp in space without producing what astronauts call a bomit.

What this implies about the rest of digestion

The burp problem is, on its own, a minor inconvenience in the context of a six-month deployment on the International Space Station. The broader category of digestive challenges it points toward is not minor.

NASA’s STEMonstrations educational series, filmed aboard the International Space Station, has set out the digestive physiology of microgravity in some detail. The primary mechanism driving food through the digestive tract is peristalsis, the wave-like contractions of smooth muscle that propel food from the throat through the oesophagus, stomach, small intestine, and large intestine. Peristalsis is muscular rather than gravitational. It works in microgravity essentially as it works on Earth, which is why astronauts can in fact eat, digest, and excrete food without any catastrophic disruption to the basic process.

What microgravity does change is the secondary mechanisms that gravity normally contributes to digestion. The rate at which food moves through the stomach slows. Gas accumulates rather than rising and escaping. Fluid shifts in the body, including the well-documented cephalad fluid shift in which fluids redistribute from the lower body toward the head, alter the chemistry of the digestive environment. Multiple peer-reviewed studies, conducted using both simulated microgravity in ground-based experiments and direct observation of astronauts in orbit, have documented changes in gut motility, alterations in the gut microbiome, and reduced absorption of certain nutrients in microgravity conditions.

The food itself, on the International Space Station, has been engineered around these constraints. High-flatulence foods, such as raw onions, broccoli, and beans, have been progressively removed or restricted from the menu. Carbonated beverages are not flown to the station because the carbonation, behaving unpredictably in microgravity, would produce immediate and severe wet-burp episodes. Fresh fruit and vegetables, when they are flown at all, are flown in small quantities and consumed quickly, both because of spoilage and because of the variable gas they produce during digestion.

The air circulation systems aboard the ISS, which exist primarily to prevent astronauts from suffocating in their own exhaled carbon dioxide, also handle the other gaseous outputs of the human body. Flatulence, which in microgravity does not rise away from the astronaut the way it does on Earth, is moved through the cabin by the same fans that move the breathing air. The smell does not linger in any one place. It disperses into the general atmosphere of the spacecraft.

Why it matters

The inability to burp cleanly in microgravity is, in the broader programme of human spaceflight, a small fact among many small facts that together describe how poorly evolved the human body is for the conditions of life outside Earth’s gravitational well. The astronauts who have lived in orbit have, by the testimony in the published literature, adapted to the wet-burp problem the way they have adapted to the puffy face, the temporary loss of taste and smell, the height increase of approximately 5 centimetres from spinal decompression, the bone loss of approximately 1 per cent per month, the muscle atrophy, and the visual changes now grouped under the diagnostic category of Spaceflight Associated Neuro-ocular Syndrome.

None of these individually are mission-ending. The cumulative effect of all of them is the reason the Mars-mission planning at NASA, the European Space Agency, and the China National Space Administration includes substantial research budgets devoted to maintaining astronaut physiology over the multi-year duration that a Mars mission would require.

The push burp is one of the smaller technical solutions in that programme. It works. It is also, in the words of the astronauts who have used it, undignified.

The human body did not evolve to release gas by shoving off a wall.

The fact that doing so is now the established procedure on the most expensive scientific platform humans have ever built is one of the more unexpected facts of the present era of human spaceflight.