There is no altitude at which Earth’s atmosphere simply stops. The blue sky turns black, airplanes lose the ability to fly, and orbital motion takes over, but an increasingly fine population of particles continues far beyond the conventional edge of space. The most widespread neutral component is atomic hydrogen. Sunlight shines it at an ultraviolet wavelength called Lyman-alpha, creating a vast halo known as a geocorona. Measurements published in 2019 tracked that brightness to about 630,000 kilometers from Earth, almost twice the average distance of the Moon. This allows a technically true but easily misinterpreted statement to be made: the Moon orbits within the Earth’s outer atmosphere. It does not mean that astronauts at lunar distance find air that they can breathe, feel or use. Near the Moon, the study estimated only about 0.2 hydrogen atoms per cubic centimeter, a density that laboratories on Earth would call an excellent vacuum. The Kármán line is a boundary for flight, not for molecules. The 100 kilometer Kármán line is the internationally known boundary of space. It’s useful because conventional aerodynamic flight becomes impractical and orbital mechanics take over at that altitude. Separates aeronautical records from astronautical records; It is not a wall where the atmosphere ends. That distinction is easy to miss. When Space Daily reviewed the first photographs of Earth taken from space by a V-2 in 1946, the rocket’s 105-kilometer altitude was enough to place its camera beyond current conventional limits. It was nowhere near the final atmospheric particle. NASA’s guide to atmospheric layers says that 99.99997 percent of the atmosphere lies below the Kármán line. The surviving fraction is incredibly small, but the particles become less and less frequent instead of disappearing together. The exosphere is collision-free: atoms can travel long distances without colliding with each other, some following ballistic trajectories back to Earth and others escaping permanently. Therefore, the atmosphere can be visually thin and physically huge at the same time. Space Daily’s account of the narrow blue line seen by space station crews describes the dense, optically visible part that protects life. The geocorona is a much larger structure that the human eye cannot see. The geocorona is sunlight scattered by hydrogen. Hydrogen is the lightest element and the dominant neutral constituent in Earth’s distant exosphere. Some starts out as water vapor or methane in the atmosphere. Photochemical reactions release hydrogen, which diffuses upward through the thermosphere until individual atoms enter the exosphere. The Sun illuminates those atoms at 121.6 nanometers, the Lyman-alpha line. A hydrogen atom absorbs a photon and re-emits light in another direction. Seen from afar, countless scattering events form a faint ultraviolet corona around the Earth. The name geocorona means “crown of the earth.” People on the ground can’t see it because the lower atmosphere absorbs far-ultraviolet light. Even a space detector must distinguish Earth’s hydrogen from interplanetary hydrogen that also scatters sunlight throughout the Solar System. That background can be hundreds of times brighter than the geocoronal signal at its most detectable ranges. SOHO found the Moon within a two-decade-old data set. The Solar and Heliospheric Observatory, or SOHO, occupies a halo orbit around the L1 Sun-Earth point about 1.5 million kilometers toward the Sun. This placed it outside the geocorona and gave it a view of the entire cloud. SOHO’s SWAN instrument mapped Lyman-alpha across the sky. Its decisive component was a hydrogen absorption cell that could be connected to the optical path. Comparing cell-on and cell-off measurements selectively removed much of the geocoronal signal, allowing researchers to disentangle the faint Earthglow from the Doppler-shifted interplanetary background. Igor Baliukin and his colleagues reanalyzed specific observations made in January 1996, 1997, and 1998. Their 2019 Journal of Geophysical Research: Space Physics paper detected emissions within about 100 Earth radii, or about 640,000 kilometers from the Earth’s center. ESA described the range as 630,000 kilometers, a rounded distance from the planet that is about 50 times the diameter of Earth. Honest writing is “at least” this far. The researchers found a signal of about five Rayleighs within 100 radii of Earth, near the limit they could reliably separate from the background. They did not find a hard layer beyond which hydrogen was absent. The Moon is permanently embedded, but its elliptical orbit takes it between approximately 54 and 64 Earth radii from the center of the planet. Therefore, a geocorona measured at approximately 100 Earth radii surrounds the entire lunar orbit, not just the closest approaches to the Moon. Density matters more than reach. The team estimated about 70 hydrogen atoms per cubic centimeter at 60,000 kilometers above the Earth’s surface. At lunar distance, the estimate dropped to only about 0.2 atoms per cubic centimeter. One cubic centimeter of air near sea level contains on the order of 10 quintillion molecules. Calling both regions “atmosphere” is scientifically consistent but physically misleading if density is omitted. The geocorona produces no breathable pressure, weather, sound, or significant aerodynamic drag on a lunar spacecraft. An astronaut cannot feel it and it offers no useful reserves of propellant. Each Apollo crew flew through Earth’s outermost neutral hydrogen, but their spacecraft behaved as if it were in a vacuum. The Moon is also not acquiring a dense hydrogen atmosphere of its own. The paper concluded that the interaction with the lunar exosphere is probably negligible because the two media do not collide. Apollo 16 photographed the halo from within In April 1972, Apollo 16 astronauts John Young and Charles Duke placed a gold-plated ultraviolet camera in the shadow of the Orion lunar module. Designed by Naval Research Laboratory scientist George Carruthers, it became the first astronomical observatory operated on another world. The camera recorded Earth’s atmosphere and geocorona below 160 nanometers, producing the first complete view of the planet in far-ultraviolet light. The astronauts observed the Earth’s outer atmosphere from the Moon without knowing that the telescope, the lander and themselves were embedded in its farthest reaches. The position that made the image possible also limited it. Apollo 16 observed the bright halo from inside the structure and was unable to measure its remote edge. SOHO finally provided the necessary distant perspective. In planetary science, the apparent limits of a phenomenon often depend on both where the detector is located and what the phenomenon is doing. The halo is more important to telescopes than to lunar crews. The geocorona is not a significant radiological shield on the Moon. Their hydrogen atoms scatter solar ultraviolet light, but the ESA assessment found that the associated exposure is negligible compared to radiation arriving directly from the Sun. The particles also pose no atmospheric danger to astronauts. For ultraviolet astronomy, the dim foreground light does matter. The 2019 study estimated that an observatory on or around the Moon could see approximately 10 Rayleighs of geocoronal emission in directions perpendicular to the Earth-Moon line, coupled with the stronger Lyman-alpha interplanetary background. Precise measurements of stars, galaxies, and diffuse hydrogen may be necessary to model and subtract that local brightness. The halo is also a marker of atmospheric escape. Hydrogen derived in part from water rises, moves through the exosphere, and can be ionized or lost to space. Measuring their distribution helps researchers test how Earth’s atmosphere responds to solar ultraviolet radiation and space weather. Similar extended hydrogen envelopes around other planets may provide clues to water loss and long-term habitability. The geocorona and the magnetosphere are different structures. Earth’s magnetic environment may also extend beyond the Moon, but should not be confused with the geocorona. The magnetosphere is organized by the Earth’s magnetic field and populated by charged particles. The geocorona is composed primarily of neutral hydrogen, shaped by gravity, ionization, solar radiation pressure, and atmospheric exhaust. The two regions interact because neutral hydrogen can ionize. However, their shapes, densities and response to the Sun differ. Saying that the Moon is permanently embedded in the geocorona does not mean that it is always in the same region of the magnetosphere. The Earth-Moon exchange is real in more ways than one. Space Daily’s report about terrestrial oxygen reaching the Moon and helping to form hematite refers to charged oxygen ions transported through the magnetic tail. The geocorona is a separate, neutral, hydrogen-rich connection between the worlds. Carruthers is now observing the change of Earth’s corona. The 630,000 kilometer result came from just three observation windows, all near low solar activity. It is not a universal radius fixed to the Earth. Solar illumination, radiation pressure, ionization, and the supply of escaping hydrogen should change the density and shape of the halo. NASA’s Carruthers Geocorona Observatory launched in September 2025, reached a halo orbit around L1, and began its 24-month primary science mission on March 1, 2026. From beyond the geocorona, its wide- and narrow-field imagers are designed to repeatedly map Lyman-alpha, track changes on hourly time scales, and help reconstruct the hydrogen cloud in three dimensions. Space Daily covered Carruthers’ first ultraviolet views of the Earth and Moon in December. The mission is named after George Carruthers, whose Apollo 16 instrument first photographed the corona from its interior. More than half a century later, an observatory that bears his name looks back from outside the halo to see how its extent changes. The Kármán line is still a useful place to say where space begins. It was never the place where the Earth ends. The atmosphere melts away layer by layer until the last hydrogen atoms are so scarce that only their ultraviolet glow reveals that the outermost air of home still reaches around the Moon.
