Picture a hospital scanner — the kind that builds a 3-D image of your brain slice by slice. Now point it at the sky, 500 kilometres above your head. That is, more or less, exactly what a team at Kyoto University has just done with the thermosphere, the ghostly outer shell of Earth's atmosphere that nobody could easily measure — until now. The thermosphere sits between roughly 100 and 1,000 kilometres above the surface.

More than 99 percent of the upper atmosphere is made of electrically neutral gas found in this layer, yet it has long resisted observation. The ionosphere, which lies within the same altitude range but accounts for less than one percent of the atmospheric mass, is relatively easy to track because its charged particles bend radio waves. Neutral gas leaves no such convenient fingerprint. Scientists trying to forecast satellite trajectories — and the increasingly crowded risk of collisions in low Earth orbit — have therefore been working somewhat blind.

The Kyoto team found a creative shortcut. By analysing publicly available orbital data from approximately 1,200 Starlink satellites flying at around 482 kilometres altitude, they measured how atmospheric drag was gradually pulling each spacecraft down. Feed enough of those tiny deceleration signals into a tomographic algorithm, and a two-dimensional latitude-longitude map of thermospheric density begins to emerge. Corresponding author Mamoru Yamamoto described the work as "a multidisciplinary study between space science and space engineering," adding that researchers in both fields needed to talk to each other far more deeply.