Humanity is quickly starting to saturate our ability to track all of the satellites we have in deep space. NASA’s Deep Space Network (DSN) has been the workhorse for tracking such things for decades, but is quickly being overwhelmed by the number of requests it receives. Which in turn makes it harder to track some of the missions that have been launched recently. A new paper, available in pre-print on arXiv, from Ian Down, James McElreath, and Manoranjan Majji, of Advanced Space, Space Exploration Engineering, and Texas A&M respectively, offer a no-cost solution to that tracking problem – use solar array data from a spacecraft’s eclipse to determine where it is.
Solar panel output is something every spacecraft already tracks, typically in the form of Maximum Power Point Tracking (MPPT), which is an algorithm designed to point a craft’s solar sails at the Sun to maximize the amount of incoming solar radiation it receives. But eclipses, where a spacecraft positions itself so that a body its orbiting is between it and the Sun, is also something every spacecraft experiences, at least for a little while.
Engineers usually spend a lot of time intentionally avoiding eclipses, because they can be really hard on the spacecraft itself. The solar arrays stop generating electricity, draining the battery, and everything on the craft gets much, much colder. However, the precise timing of how those blackouts travel across the solar panels of a spacecraft holds valuable information.
Fraser interviews astrodynamacist Dr. Aaron Rosengren, talking about the three-body problem of lunar orbits.
Astronomers know the exact positions of the Sun, Earth, and the Moon years in advance using advanced planetary maps. Given how much data we’ve collected on those three bodies, we also know the precise shape and locations of the shadows those three bodies cast into space. Each shadow comes with two nested cones – the penumbra, where the Sun is partially blocked, and the umbra, where it is completely cut off.
When a spacecraft crosses into one of those cones, the amount of solar radiation hitting its solar panels drops precipitously, but its electronics continue to operate due to the on-board battery engineers placed there precisely to deal with these eclipses. By recording the precise timestamp where the voltage starts to drop, and when it starts to pick back up again after the spacecraft leaves the shadow, a navigation algorithm can determine its three dimensional location in space without needing any cameras, star trackers, or radio signals.
That sounds simple in theory, but in practice getting that precise measurement correct is difficult. The Moon has no atmosphere, so its shadow is very jagged and tracks the mountains and craters dotting its surface. Each of those features can change the eclipses’ timing by a few seconds. Earth, on the other hand, is a whole other ball of wax – or ball of gas and rock as the case may be. It’s squished slightly at the poles, and its atmosphere can blur the Sun, as well as enlarging its shadow, making it difficult to differentiate precisely what part of the planet the spacecraft is passing over at any given time.
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To solve this problem, the researchers turned to a dataset of over 22,000 historical observations of lunar craters being covered by Earth’s shadow, which was collected by astronomers between 1842 and 2011. Using this data, the algorithm was able to map certain parts of the Earth’s shadow to specific ground features, allowing a more granular idea of where the specific spacecraft is passing over during its eclipse.
To prove the system works, the researchers used another interesting data set – the actual flight telemetry from CAPSTONE, a CubeSat mission operated by Advanced Space, where Mr. Down, the lead author, works. CAPSTONE, which was intended to test the orbit for NASA’s now discontinued Lunar Gateway, recently completed its mission in July 2026. But on January 11th 2026, it traveled through the Moon’s shadow, with its solar panel voltage dropping from 23V to near zero, then picking back up to 31V when the panels hit sunlight again.
Using that data, the researchers timed the spacecraft’s entry and exit to the Moon shadow to within 15 seconds of the CAPSTONE’s actual path. In addition to running the algorithm on real-life data, they also simulated a scenario where a spacecraft missed a check-in with ground stations and had its position uncertainty drift by several kilometers. Processing the eclipse data without any further information cut that uncertainty by more than 60 %
Perhaps the most important feature of this whole system is that it’s essentially free. It doesn’t require any additional hardware, or is “zero-Size Weight and Power” (zero-SWaP) in spacecraft jargon. To be clear, it doesn’t entirely replace ground-based radio tracking. But it does provide a safety net. And as more and more spacecraft start venturing beyond low Earth orbit, and continue to overwhelm our active tracking systems, that zero-cost safety net could prove very valuable indeed.
Learn More:
I.M. Down, J. McElreath, & M. Majji – Eclipse Observables for Navigation in Cislunar Space
UT – A Small Satellite Could See a Perfect Solar Eclipse Every Month
UT – ESA’s New Mission Can See a Solar Eclipse Every Day
UT – To Keep GPS Constellations From Drifting, Look To The Moon