Apollo astronauts trying to rest on the way to the Moon kept noticing the same thing in the dark. With the cabin lights down and their eyes adjusted, they saw brief flashes and streaks of light, dots and lines that no one else could see and that were not there in any ordinary sense. The cause, worked out over the missions that followed, was cosmic rays from deep space passing through the visual system. The broad cause is now well supported, even if the exact biophysics of how a passing particle turns into a flash of light is still not fully pinned down.

The flashes are an example of a phosphene, a sensation of light produced by something other than light entering the eye.

In this case, the something was radiation.

What the astronauts saw

The reports began on Apollo 11. After a period of dark adaptation, with eyes open or closed, crew members saw flashes they described as pinpoints, thin streaks, or small clouds of light. They were almost always colourless. They came at a rate of roughly one-half to two a minute, often enough to be a distraction when someone was trying to sleep.

This was not confined to one crew or one flight. Flashes were reported across the Apollo missions, and later by astronauts on Skylab, on Mir, and on the International Space Station. The phenomenon even had a kind of forecast. As early as 1952, the biophysicist Cornelius Tobias had suggested that people exposed to cosmic radiation in space might see exactly this sort of thing, and the team that investigated the Apollo reports traced them back to that idea.

How they tied it to cosmic rays

Suspecting a cause and demonstrating it are different tasks. To test whether the flashes really were caused by particles passing through the head, the Apollo 16 and 17 crews carried a device called the Apollo Light Flash Moving Emulsion Detector, worn on the head during the coast to and from the Moon. It used stacked photographic emulsion to record the tracks of charged particles, so that the path of a particle could be matched against the moment an astronaut reported a flash.

The numbers were small but telling. The detector was worn for about an hour on each of the last two Apollo missions. On Apollo 17, seventeen flashes were reported during the session, and two of them were matched to heavy cosmic-ray nuclei passing through an eye. Combined with the geometry of the eye and statistical modelling, that was enough for the investigators to attribute the flashes to cosmic rays. Ground experiments pointed the same way: when volunteers had their dark-adapted eyes exposed to beams of heavy ions, they saw comparable flashes.

What is still uncertain

The confirmed part is that cosmic rays cause the flashes. The unconfirmed part is the step in between, the biophysics of how a single fast particle produces a perceived point of light. Several mechanisms have been proposed, and more than one may contribute.

The leading idea is that the particle ionises tissue directly in the retina, stimulating the light-sensing cells or the neurons behind them as it passes through. This fits the ground experiments, where heavy ions produced flashes at intensities too low for any light to have been generated. A second idea is Cherenkov radiation: a particle moving through the clear jelly of the eye faster than light moves through that medium emits a faint glow, real photons generated inside the eye itself. The trouble is that Cherenkov light should look bluish and diffuse, and most astronauts described sharp white dots and streaks, which do not match. A third possibility is that some flashes come from particles striking the optic nerve or the visual centres of the brain, bypassing the eye altogether.

Decades on, reviews of the phenomenon still describe the specific mechanism as undefined, and note that the effects on the different parts of the visual system have not been fully separated.

The cause is well established; the wiring is not.

Why it still matters

On Earth’s surface, we are mostly shielded from this by the atmosphere and the magnetic field. In low Earth orbit the shielding is reduced but not gone, which is why astronauts on stations such as Mir and the ISS have also reported the effect. The Apollo crews saw the flashes more strongly still because they left that protection behind, travelling beyond the magnetosphere. The flashes were, in effect, the radiation environment of deep space made briefly visible inside a human head.

That is why they are treated as more than a curiosity. They are a perceptible sign that high-energy particles are passing through the body, the brain included, and the same radiation is a serious concern for long missions beyond Earth. On the International Space Station, experiments have used the visual system as a way to study how cosmic radiation affects the central nervous system. For a crew bound for Mars, who would spend months outside the magnetosphere, the dose to the brain over that time is one of the open problems of the trip.

What to watch

The people who travel back to lunar distance under Artemis and later missions will almost certainly see the flashes again. They will also carry better instruments and a sharper reason to use them. Pinning down how the flashes are produced would be a small result. Understanding what months of that radiation does to the brain behind the eye is the one that matters.

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The Apollo astronauts who tracked Moon dust back into their cabin kept filing the same odd report. Fresh lunar dust, they said, smelled like spent gunpowder. The impression was immediate, and it was reported by multiple crews. Yet the smell never survived the trip home. By the time the samples were opened in laboratories on Earth, they no longer smelled like gunpowder, and more than fifty years later there is still no settled account of what the astronauts were smelling up there.

The puzzle is real, but it is narrower than it sounds. A good deal is known about why lunar dust is chemically strange. What is missing is the specific link between that chemistry and the smell.

What the astronauts actually reported

The descriptions are unusually consistent, and unusually specific. Harrison Schmitt, the geologist on Apollo 17 and the only trained scientist to walk on the Moon, said the instant impression was of spent gunpowder, not something metallic or acrid. He even noted the timing: about seven minutes passed between the start of repressurising the lander and his first remark about the smell.

Gene Cernan, his crewmate, reached for the same comparison. Buzz Aldrin, on Apollo 11, described it a little differently, closer to burnt charcoal, or wet ashes from a fireplace. John Young, on Apollo 16, went further than smelling it and reported that it did not taste half bad.

One detail matters for what follows. The astronauts were careful to say burnt gunpowder, the smell after firing, not the smell of the powder itself. The two are not the same.

Why it vanished before Earth

The most useful clue is the disappearance. There are hundreds of kilograms of lunar material in the curation lab at NASA’s Johnson Space Center in Houston, and it does not smell of gunpowder. Gary Lofgren, a NASA geologist who has handled the dusty rocks directly, sniffed them and reported plainly that they do not smell like gunpowder.

The astronauts were not imagining it. The accepted reading is that the dust was, in the word researchers use, pacified. Whatever produced the smell was a reaction that happened once, when the dust first met the air inside the cabin, and was finished long before the samples reached Earth. A smell that depends on a one-time reaction cannot be bottled and shipped.

The leading explanation, and where it stops

The reason fresh lunar dust reacts at all comes down to where it has been. The surface of the Moon has no air and no weather. For billions of years its dust has been broken down by micrometeorite impacts and bombarded by the solar wind, producing extremely fine grains with fresh, raw surfaces and, in many of them, tiny specks of metallic iron embedded in glassy coatings. Surfaces like that are chemically hungry. They carry what chemists call dangling bonds, unsatisfied bonds left exposed because the dust has never had air to react with.

Bring that dust into a cabin filled with oxygen and a little moisture, and those surfaces oxidise quickly. The reaction is a slow form of burning, too gradual for smoke or flame, and it is the most popular candidate for the smell. Schmitt himself guessed as much at the time, suspecting his nose was responding to unsatisfied bonds of the kind found in both fresh dust and just-fired gunpowder.

This is a hypothesis with broad support, not a proven result. No instrument sampled the cabin air, so the actual molecules responsible were never identified. The gunpowder comparison is an association in the human nose, not a statement about shared chemistry. Other ideas have been raised over the years, including sulphur-bearing minerals in the dust and volatiles implanted by the solar wind. None has been confirmed, because the evidence that would settle it evaporated, in effect, during the flight home.

Why this is more than a curiosity

The smell is a footnote.

The reactivity behind it is not.

The same properties that may have produced the odour, the abrasive shape of the grains and their reactive surfaces, make lunar dust a serious problem for people and equipment. It clings to everything, works its way through seals, and is fine enough to be inhaled deep into the lungs. Dust that got into the Apollo cabins left crews with eye, nose and throat irritation, an effect NASA came to call lunar hay fever, with Schmitt’s reaction the best documented. Reviews of lunar dust as a hazard link its reactive surfaces and iron content to the way it interacts with human tissue.

That has turned an Apollo-era anecdote into a present-day engineering requirement. For NASA’s Artemis programme, and for the commercial missions intending to operate on the surface for more than a few days, dust handling is now a standing line item: cabin filtration, suit design, airlock procedures, and medical monitoring all have to account for a material whose chemistry was barely understood when the first samples came home.

What to watch

The next people to bring Moon dust into a sealed cabin will arrive with instruments the Apollo crews did not have, and with reason to use them. Whether anyone finally captures the cabin chemistry, and identifies what the astronauts were smelling, is one of the smaller questions Artemis could answer. The larger one, how to live and work in the stuff without it harming lungs and hardware, is the reason the answer would be worth having.

The post The Apollo astronauts who carried lunar dust back into the cabin kept making the same strange report — fresh Moon dust smelled like spent gunpowder — yet the smell never survived the trip home, and more than fifty years later no one has fully explained what they were breathing in up there. appeared first on Space Daily.

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