Earth’s low orbit is becoming a laboratory for a new kind of atmospheric observation. With thousands of active satellites and growing clouds of debris circling the planet, even the nearly empty space hundreds of kilometers above Earth has become critically important. At an altitude of about 500 kilometers, the atmosphere is so thin that it cannot be felt or seen from the ground, yet it still exerts enough drag to slow spacecraft, alter their orbits and increase uncertainty in collision predictions. Researchers at Kyoto University have now used data from SpaceX’s Starlink satellites to create what they describe as the first tomographic map of thermospheric density at this altitude.
The thermosphere extends from roughly 100 to 1,000 kilometers above Earth’s surface and contains more than 99 percent electrically neutral atmospheric gas in the upper atmosphere. Although its molecules are extraordinarily sparse compared with those near the surface, their cumulative effect on spacecraft can be substantial. Atmospheric drag removes orbital energy, causing satellites to descend unless their trajectories are regularly corrected. The intensity of this drag changes with solar activity, geomagnetic disturbances and atmospheric heating, making accurate density estimates essential for spacecraft operators and space-debris tracking systems.
The challenge is that the thermosphere is far more difficult to observe than the ionosphere, the electrically charged region embedded within it. The ionosphere interacts strongly with radio waves, allowing scientists to monitor it using radar, navigation signals and other electromagnetic techniques. Neutral thermospheric gas does not offer the same observational advantages. Instead, its density is often inferred indirectly from the way it affects satellites. When atmospheric drag causes a satellite’s orbit to decay, researchers can work backward from that change to estimate the density of the surrounding air.
Kyoto University researchers applied this principle to publicly available orbital information from approximately 1,200 Starlink satellites operating near 482 kilometers above Earth. Satellite ephemeris data describe the position and motion of spacecraft over time. By examining deviations in orbital motion associated with drag, the team estimated the thermospheric density encountered by individual satellites. Each spacecraft provided a separate measurement along its trajectory, and the combined observations formed a large, distributed data set covering many regions of the planet.
The researchers then used tomography, a mathematical technique best known from medical imaging, to reconstruct the atmosphere. In a medical CT scan, X-ray measurements collected from different angles are combined to infer the internal structure of the body. In this atmospheric application, satellite paths act as moving sampling routes through the thermosphere. The team converted thousands of drag-related measurements into a two-dimensional latitude-longitude distribution, producing a snapshot of neutral atmospheric density at an altitude of approximately 500 kilometers. Rather than viewing density only along individual orbital tracks, the method reveals how it varies across a broad horizontal region.
The result represents a significant expansion of the team’s earlier work. In previous research, the scientists used Two-Line Element, or TLE, data from Starlink spacecraft to estimate how thermospheric density changed with time and altitude. That approach provided a vertical and temporal picture of the upper atmosphere. The new study adds the missing horizontal dimension, allowing researchers to examine the geographic structure of density variations. The resulting map showed strong consistency with observations from the European Space Agency’s SWARM mission, whose satellites independently measure variations in the upper atmosphere along their own trajectories.
The agreement with SWARM is important because it provides an external check on a method based largely on commercial satellite orbital data. Starlink satellites were not designed as atmospheric probes, and their primary mission is communications. However, their large number, low-Earth orbits and frequently updated orbital information create an unusually dense network of potential measurements. As satellite constellations continue to expand, ordinary spacecraft may increasingly become useful as distributed sensors, transforming the infrastructure of near-Earth space into a resource for monitoring the environment through which it travels.
The implications extend beyond atmospheric science. More accurate thermospheric density estimates can improve predictions of satellite lifetimes, reduce uncertainty in orbital maneuver planning and help operators assess the probability of close approaches with other spacecraft or debris. In the longer term, the researchers say their approach could contribute to near-real-time monitoring of density around satellites and strengthen space-weather forecasting. Solar storms can heat and expand the upper atmosphere, increasing drag abruptly and causing satellites to lose altitude faster than expected. A timely map of those changes could help operators respond before orbital predictions become dangerously inaccurate.
The study, titled “Tomography of thermospheric density from Starlink Ephemeris: initial report,” was published on 30 July 2026 in Earth, Planets and Space. Its authors emphasize that the work brings space science and space engineering into closer conversation: one field seeks to understand the dynamic upper atmosphere, while the other must calculate how that atmosphere affects spacecraft. As low Earth orbit becomes more crowded, the ability to measure invisible atmospheric forces using the satellites already in space could become an essential tool for keeping the orbital environment safe, predictable and operational.
Subject of Research: Thermospheric density and satellite orbital dynamics
Article Title: Tomography of thermospheric density from Starlink Ephemeris: initial report
News Publication Date: 30 July 2026
Web References: https://doi.org/10.1186/s40623-026-02509-5
References: Earth, Planets and Space, “Tomography of thermospheric density from Starlink Ephemeris: initial report,” DOI: 10.1186/s40623-026-02509-5
Image Credits: Kyoto University / Mamoru Yamamoto
Keywords: thermosphere, atmospheric density, Starlink, satellite drag, space debris, low Earth orbit, atmospheric tomography, space weather, satellite tracking, Kyoto University
