Astronomers have long raised concerns about the growing number of satellites in low Earth orbit. Large constellations such as SpaceX’s Starlink can appear as bright streaks in astronomical images, making it harder to study faint objects in the night sky. But researchers in Japan have found a way to use those same satellites as passive sensors for changes in Earth’s upper atmosphere.
A study published in Earth, Planets and Space by Mamoru Yamamoto and Takuya Sori of Kyoto University describes how changes in the orbits of Starlink and other satellites can be used to track changes in the thermosphere, the extremely thin layer of the upper atmosphere where many satellites operate.
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The density of the thermosphere is constantly changing, largely in response to activity from the Sun. When a solar storm sends energy and charged particles toward Earth, they interact with the planet’s magnetic field and upper atmosphere, heating the thermosphere and causing it to expand. That expansion puts more atmospheric molecules into the altitudes where satellites orbit, increasing the drag they experience.
In examining the satellites changing orbits, Yamamoto and Sori were able to calculate changes in the density of the atmosphere around the satellites. During their 49-day study, they identified three notable increases in atmospheric density, around Oct. 8, Oct. 11 and Oct. 18-20, 2024.
The first two increases followed coronal mass ejections from the Sun. As solar storms heat and expand the upper atmosphere, satellites can experience greater atmospheric drag and lose altitude more quickly. The researchers could not identify a clear solar cause for the third. They say additional observations will be needed to determine what caused that event and to better understand how reliably satellite orbital changes can be used to measure the thermosphere.
The potential application is significant because atmospheric density affects the paths of satellites and other objects in low Earth orbit. During periods of heightened solar activity, increased drag can cause spacecraft to lose altitude faster than expected, affecting their trajectories and making orbital predictions more difficult.
Starlink and other large satellite constellations could provide a rich data source for atmospheric research and potentially for space-traffic management. Rather than equipping satellites with dedicated atmospheric instruments, researchers can study readily available orbital data updated regularly by the U.S. Space Force’s 18th Space Defense Squadron.
This same data is used by WRAL meteorologists to share the best times to spot he International Space Station as it passes over.
Yamamoto and Sori emphasize that study represents the potential rather than a complete monitoring system. Longer observations and more precise orbital data will be needed to determine how reliably satellite constellations can measure the thermosphere under different conditions.
