Ayisha Ashruf, a scientist and engineer at the Space Physics Laboratory of the Vikram Sarabhai Space Centre in Thiruvananthapuram, led a team that followed seventeen pieces of long-dead space hardware across three full solar cycles, from 1986 to 2024. The objects, all in low Earth orbit between roughly 370 and 500 miles up, included old TIROS weather satellite fragments, Thor rocket pieces and Soviet-era Cosmos debris, hardware launched when John F. Kennedy was in the White House.
The team’s finding, published in Frontiers in Astronomy and Space Sciences, is a clean break in the curve. Below a certain level of solar activity, altitude bleeds off the debris gently. Above it, the descent steepens. The threshold sits at roughly 67 to 75 percent of the sunspot peak for that cycle, well before solar maximum itself, and it is the same line whether the team measured solar activity by sunspot number or by the 10.7 centimetre radio flux.
Why dead hardware is the cleanest record
Working satellites correct themselves. They fire thrusters to stay in their lanes, masking the steady pull of the upper atmosphere. Debris does not. Every meter of lost altitude on a piece of 1960s wreckage reflects the air at orbital heights pushing back on the object, unaltered by station-keeping.
That is why Ashruf’s team did not start with operational satellites. They began with 95 candidate objects from the Space-Track catalogue and narrowed the set to seventeen long-lived pieces of debris with orbital geometries clean enough to read decade-scale drag against. The Sun’s eleven-year cycle means short studies catch only a slice of solar behaviour. Three full cycles let the team separate the steady background of atmospheric drag from the cycle-driven acceleration, and pin down where on the activity curve the acceleration actually begins.
“For the first time, we find that, once solar activity passes a certain level, this loss of altitude happens noticeably more quickly,” Ashruf said in a statement released through EurekAlert. “This observation is expected to be key for planning sustainable space operations in the future.”
The threshold in plain terms
Sunspot number is the oldest continuous record of solar activity, dating to the seventeenth century. The 10.7 centimetre radio flux, known as F10.7, has been measured daily since 1947 and tracks the same underlying physics through a different window. Both indices climb together as a cycle approaches maximum, and the Ashruf team found that both showed the same inflection point in the debris decay records.
What the team did not find was a clean linear response. The atmosphere does not respond proportionally to incremental solar input across the whole cycle. It sits roughly flat for a while, then tips. Above the threshold, decay rates increased sharply across all three cycles in the dataset, with the strongest cycle, Cycle 22, producing the steepest drops and the weakest cycle, Cycle 24, producing the gentlest.
EUV, sunspot number and the secondary role of storms
Extreme ultraviolet emissions deposit energy directly into the thermosphere, the layer between roughly 60 and 620 miles up, where temperatures swing between about 930 and 4,530 degrees Fahrenheit depending on solar input. When the Sun is busy, that layer puffs outward. More air molecules drift into orbital paths. Drag rises. Speed drops. Altitude follows.
The mechanism has been known for decades. The novelty in Ashruf’s analysis is that sunspot number and F10.7 tracked long-term decay much more strongly than geomagnetic indices such as Ap, AE and Dst. Geomagnetic storms produce short, sharp drops in altitude, but the decade-scale decay curve is driven by the cycle’s EUV and radio output rather than by the storm count.
That distinction matters for forecasting. Storms are erratic and short-lived. The threshold in sunspot number and F10.7 is a slower, more predictable signal that operators can plan against in advance.
The polar gap
Most of the seventeen debris objects fit existing atmospheric model predictions once the team adjusted for ballistic coefficient. Two did not. Both happened to be in high-inclination, near-polar orbits, and the standard NRLMSIS empirical atmospheric model could not reproduce their behaviour without significant residuals.
The authors are cautious about what that means. It may point to limitations in atmospheric modelling at high latitudes, where geomagnetic storms dump more energy into the upper atmosphere and where the standard density models have always been less reliable. It is a more measured reading than saying polar orbits are immune to solar-driven drag. The opposite seems closer to the truth, with the polar bands behaving in ways the current generation of models cannot yet predict cleanly.
Why this lands at an awkward moment
Low Earth orbit is no longer sparse. ESA’s 2025 Space Environment Report counted about 40,000 tracked objects in orbit, including around 11,000 active payloads, with the population of smaller, untracked fragments running into the millions. Mega-constellations have changed the stakes of every percentage point of atmospheric drag.
A separate analysis of Starlink reentries during the rising phase of Solar Cycle 25 found that 523 satellites reentered faster as geomagnetic activity rose, and that reentry prediction errors grew along with storm intensity. The Ashruf threshold provides a longer-baseline framing for the same physics. The Oliveira team measured the short-cycle response in active satellites. The Ashruf team measured the decadal response in debris that has been falling since before either Starlink or geomagnetic forecasting existed in their modern form.
Both findings point in the same direction. The Sun is not a constant backdrop. It is an active forcing function on every spacecraft in low orbit.
What operators can do with a threshold
The drag effect concentrates in the lower part of low Earth orbit, the band the seventeen debris objects occupied. For constellations and crewed platforms operating between roughly 200 and 500 miles up, the practical takeaway is to build fuel reserves against the number of months sunspot number is forecast to spend above two-thirds of the cycle peak, rather than against the cycle maximum alone.
That window typically opens a year or more before solar maximum and stays open well into the declining phase. NASA and NOAA announced in October 2024 that the Sun had reached the maximum period of Solar Cycle 25, which means the threshold window for the current cycle is already wide open and will remain so into the second half of the decade.
Crewed platforms like the International Space Station, which already perform debris avoidance manoeuvres and altitude reboosts on a regular cadence, face a quantifiable elevation in conjunction risk that operators can now forecast more cleanly. The relevant input is no longer a vague instruction to monitor solar activity. It is a specific index crossing a specific line.
Caveats worth keeping in view
The threshold emerged from three solar cycles that were all relatively moderate by historical standards. Cycles 22, 23 and 24 do not include anything close to the strongest cycles on record, such as Cycle 19 in the late 1950s. A stronger cycle might shift the inflection point or steepen the response above it, and the only way to find out is to keep adding cycles to the dataset as they happen.
The seventeen-object sample is also small. The team narrowed it deliberately to objects with clean, long-baseline orbital records, but a future study with a larger debris set could reveal whether the threshold holds across a wider range of ballistic coefficients and altitudes. The polar-orbit anomaly suggests at least some of the answer will look different at high inclinations.
There is also the question of what faster reentries actually do to the atmosphere they pass through. A 2024 AGU release warned that satellites burning up on reentry leave aluminium oxide particles in the upper atmosphere, with possible implications for ozone chemistry. The old framing of atmospheric drag as a free disposal service does not survive close inspection. The atmosphere can pull dead hardware down, but it is not an inert chute.
From observation to prediction
Long-baseline solar observation is what made this kind of study possible. The SOHO mission, jointly operated by ESA and NASA since 1995, has provided continuous EUV coverage through Cycles 23, 24 and the rising and peak phases of Cycle 25. The German Research Centre for Geosciences in Potsdam contributes the sunspot and radio flux archives that Ashruf’s team paired with the debris trajectories.
Forecasting work continues elsewhere in the orbital physics community. A separate Space Daily report on the STORIE instrument heading to the International Space Station describes a NASA and US Space Force payload designed to distinguish ring-current particles of solar origin from those of terrestrial origin, a separation that affects exactly the kind of drag forecasting the Ashruf team highlights as the next operational priority.
Older debris keeps doing useful work. Hardware launched in the era of the Telstar and Vostok programmes is still circling the planet, still losing altitude, still being read decades later as a record of how the upper atmosphere behaves when the Sun gets loud.
