There is no altitude at which Earth\u2019s atmosphere simply stops. The blue sky becomes black, aircraft lose the ability to fly and orbital motion takes over, but a vanishingly thin population of particles continues far beyond the conventional edge of space.<\/p>\n
The most extended neutral component is atomic hydrogen. Sunlight makes it glow at an ultraviolet wavelength called Lyman-alpha, creating a vast halo known as the geocorona. Measurements published in 2019 traced that glow to roughly 630,000 kilometres from Earth, almost twice the Moon\u2019s average distance.<\/p>\n
This permits a technically true but easily misunderstood statement: the Moon orbits inside Earth\u2019s outer atmosphere. It does not mean astronauts at lunar distance encounter air that can be breathed, felt or used. Near the Moon, the study estimated only about 0.2 hydrogen atom per cubic centimetre, a density that laboratories on Earth would call an excellent vacuum.<\/p>\n
The K\u00e1rm\u00e1n line is a boundary for flight, not molecules<\/p>\n
The 100-kilometre K\u00e1rm\u00e1n line is the internationally familiar boundary of space. It is useful because conventional aerodynamic flight becomes impractical and orbital mechanics takes over around that altitude. It separates aeronautical records from astronautical ones; it is not a wall at which the atmosphere ends.<\/p>\n
That distinction is easy to lose. When Space Daily looked back at the first photographs of Earth taken from space by a V-2 in 1946<\/a>, the rocket\u2019s 105-kilometre altitude was enough to place its camera beyond today\u2019s conventional boundary. It was nowhere near the final atmospheric particle.<\/p>\n NASA\u2019s guide to the atmospheric layers<\/a> says 99.99997 per cent of the atmosphere lies below the K\u00e1rm\u00e1n line. The surviving fraction is fantastically small, yet particles become less frequent continuously rather than disappearing together. The exosphere is collisionless: atoms can travel long distances without striking one another, some on ballistic paths back towards Earth and some escaping permanently.<\/p>\n The atmosphere can therefore be visually thin and physically enormous at the same time. Space Daily\u2019s account of the narrow blue line seen by space-station crews<\/a> describes the dense, optically visible part that protects life. The geocorona is a much larger structure that human eyes cannot see.<\/p>\n The geocorona is sunlight scattered by hydrogen<\/p>\n Hydrogen is the lightest element and the dominant neutral constituent in Earth\u2019s distant exosphere. Some of it begins as water vapour or methane lower in the atmosphere. Photochemical reactions free hydrogen, which diffuses upwards through the thermosphere until individual atoms enter the exosphere.<\/p>\n The Sun illuminates those atoms at 121.6 nanometres, the Lyman-alpha line. A hydrogen atom absorbs a photon and re-emits light in another direction. Seen from far enough away, countless scattering events form a faint ultraviolet crown around Earth. The name geocorona means \u201cEarth crown.\u201d<\/p>\n People on the ground cannot see it because the lower atmosphere absorbs far-ultraviolet light. Even a space-based detector must distinguish Earth\u2019s 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 furthest detectable reaches.<\/p>\n SOHO found the Moon inside a two-decade-old dataset<\/p>\n The Solar and Heliospheric Observatory, or SOHO, occupies a halo orbit around the Sun-Earth L1 point about 1.5 million kilometres towards the Sun. This placed it outside the geocorona and gave it a view back towards the entire cloud.<\/p>\n SOHO\u2019s SWAN instrument mapped Lyman-alpha across the sky. Its decisive component was a hydrogen absorption cell that could be switched into the optical path. Comparing measurements with the cell activated and deactivated removed much of the geocoronal signal selectively, allowing researchers to disentangle Earth\u2019s faint glow from the Doppler-shifted interplanetary background.<\/p>\n Igor Baliukin and colleagues reanalysed dedicated observations made in January 1996, 1997 and 1998. Their 2019 Journal of Geophysical Research: Space Physics paper<\/a> detected emission to approximately 100 Earth radii, or about 640,000 kilometres from Earth\u2019s centre. ESA described the reach as 630,000 kilometres<\/a>, a rounded distance from the planet that is about 50 times Earth\u2019s diameter.<\/p>\n The honest wording is \u201cat least\u201d this far. The researchers found a signal of about five Rayleigh at 100 Earth radii, close to the limit they could separate reliably from the background. They did not encounter a hard shell beyond which hydrogen was absent.<\/p>\n The Moon is permanently embedded, but barely<\/p>\n The Moon\u2019s elliptical orbit carries it between roughly 54 and 64 Earth radii from the planet\u2019s centre. A geocorona measured to about 100 Earth radii therefore surrounds the entire lunar orbit, not only the Moon\u2019s closest approaches.<\/p>\n Density matters more than reach. The team estimated about 70 hydrogen atoms per cubic centimetre at 60,000 kilometres above Earth\u2019s surface. At lunar distance, the estimate fell to only about 0.2 atom per cubic centimetre. A cubic centimetre of air near sea level contains on the order of 10 quintillion molecules.<\/p>\n Calling both regions \u201catmosphere\u201d is scientifically consistent but physically deceptive if density is omitted. The geocorona produces no breathable pressure, weather, sound or meaningful aerodynamic drag on a lunar spacecraft. An astronaut cannot feel it, and it offers no useful reservoir of propellant. Every Apollo crew flew through Earth\u2019s outermost neutral hydrogen, but their spacecraft behaved as if it were in vacuum.<\/p>\n The Moon is also not acquiring a dense hydrogen atmosphere of its own. The paper concluded that interaction with the lunar exosphere is probably negligible because both media are so collisionless.<\/p>\n Apollo 16 photographed the halo from inside it<\/p>\n In April 1972, Apollo 16 astronauts John Young and Charles Duke placed a gold-plated ultraviolet camera in the shadow of the lunar module Orion. Designed by Naval Research Laboratory scientist George Carruthers, it became the first astronomical observatory operated on another world.<\/p>\n The camera recorded Earth\u2019s atmosphere and geocorona below 160 nanometres<\/a>, producing the first full view of the planet in far-ultraviolet light. The astronauts were observing Earth\u2019s outer atmosphere from the Moon without knowing that the telescope, the lander and they themselves were embedded within its furthest reaches.<\/p>\n The position that made the image possible also limited it. Apollo 16 looked at the glowing halo from inside the structure and could not measure its remote edge. SOHO eventually supplied the needed distant vantage. In planetary science, a phenomenon\u2019s apparent boundary often depends as much on where the detector sits as on what the phenomenon is doing.<\/p>\n The halo matters more to telescopes than lunar crews<\/p>\n The geocorona is not a meaningful radiation shield at the Moon. Its hydrogen atoms scatter solar ultraviolet light, but ESA\u2019s assessment found the associated exposure negligible compared with radiation arriving directly from the Sun. The particles pose no atmospheric hazard to astronauts either.<\/p>\n For ultraviolet astronomy, faint foreground light does matter. The 2019 study estimated that an observatory on or around the Moon could see roughly 10 Rayleigh of geocoronal emission in directions perpendicular to the Earth-Moon line, added to the stronger interplanetary Lyman-alpha background. Precision measurements of stars, galaxies and diffuse hydrogen may need to model and subtract that local glow.<\/p>\n The halo is also a tracer of atmospheric escape. Hydrogen derived partly from water rises, moves through the exosphere and can be ionised or lost into space. Measuring its distribution helps researchers test how Earth\u2019s 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.<\/p>\n The geocorona and magnetosphere are different structures<\/p>\n Earth\u2019s magnetic environment can also stretch beyond the Moon, but it should not be confused with the geocorona. The magnetosphere is organised by Earth\u2019s magnetic field and populated by charged particles. The geocorona consists mainly of neutral hydrogen shaped by gravity, ionisation, solar radiation pressure and atmospheric escape.<\/p>\n The two regions interact because neutral hydrogen can become ionised. Their shapes, densities and response to the Sun nevertheless differ. Saying the Moon is permanently embedded in the geocorona does not mean it is always in the same region of the magnetosphere.<\/p>\n