Astronomer and science writer Tony Phillips has proposed a new way to gauge the density of Earth’s upper atmosphere: watch how quickly thousands of Starlink satellites and other megaconstellation spacecraft lose altitude. As The Register reports, the denser the air a few hundred kilometers up, the faster the satellites sink.
Phillips relies on public orbital data known as TLEs, which the US Space Command publishes. Each set includes a parameter called B*, tied to atmospheric drag. The orbital model assumes a certain air density. When the real atmosphere is denser, a satellite slows more than expected, so the algorithm bumps up B* to make up the difference. That makes B* an indirect readout of air density at the satellite’s altitude — and anyone can access it.
That density rises and falls with the Sun. When solar activity picks up, ultraviolet radiation heats the thermosphere, the layer of the atmosphere roughly 90 to 600 km above Earth. The heated gas expands upward, drag on satellites increases, and their orbits decay faster. According to Phillips, the rate of decay tracks the Sun’s radio emission at a wavelength of 10.7 cm. The atmosphere responds about two days after a change in solar activity, since heated gas takes time to expand. Geomagnetic storms work differently. They trigger sharp spikes in density and speed up satellite descent.
To make the results more reliable, Phillips looks beyond Starlink. His calculations cover about 1,000 Starlink satellites, 651 Eutelsat OneWeb spacecraft, 391 Amazon Leo satellites (formerly Project Kuiper) and 107 SuperDove spacecraft from Planet Labs. Because they fly at different altitudes, they show how different layers of the atmosphere respond. The result is effectively a distributed weather station that required no new science spacecraft. The satellites themselves become the sensors, and their motion along their orbits becomes the data.
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Large constellations in low Earth orbit usually make headlines as a threat. Six of them are already considered too big because they could clutter orbit, and companies such as Sharp are working to cut reliance on Starlink itself. Phillips’ method, by contrast, puts these spacecraft to scientific use, though it has limits.
Satellites maneuver, change orientation and fire thrusters to adjust their orbits, and TLE data is less precise than dedicated scientific measurements. Still, the sheer number of spacecraft makes it possible to statistically separate individual quirks from atmosphere-wide trends. Such a “barometer” could help sharpen space weather forecasts, better assess risks to satellites and show how the upper atmosphere responds to solar storms.
