The sun does not just threaten satellites with radiation and electromagnetic storms. It also clears some of the junk out of orbit, and a new study has pinned down exactly when that clearing process kicks into high gear.
Research published in Frontiers in Astronomy and Space Sciences tracked 17 pieces of space debris in low Earth orbit over 36 years, covering three full solar cycles. The findings show that debris starts falling significantly faster once solar activity reaches roughly 67% of its peak, a threshold that had not been identified before.
The mechanism is well understood in principle. The sun runs on an 11-year cycle tied to sunspot counts. Near the peak of each cycle, the sun intensifies its output of ultraviolet radiation and charged particles. That extra energy heats Earth's thermosphere, the atmospheric layer between about 100 and 1,000 kilometers altitude, causing it to expand upward. As the thermosphere puffs out, the atmospheric density at orbital altitudes rises, and that increased density creates more drag on anything in orbit. Drag slows objects down, which lowers their orbit, which eventually sends them into a reentry trajectory where they burn up.
What the new study adds is a specific threshold. Lead author Ayisha M. Ashruf, a scientist and engineer at the Space Physics Laboratory of the Vikram Sarabhai Space Centre in Thiruvananthapuram, India, said the team found a nonlinear relationship between solar activity and orbital decay rate. "Once solar activity passes a certain level, this loss of altitude happens noticeably more quickly," Ashruf said.
The debris objects followed in the study orbit between 600 and 800 kilometers altitude, completing a full orbit every 90 to 120 minutes. Because they are inert and perform no station-keeping maneuvers, every change in their descent rate reflects the surrounding atmosphere rather than any onboard system. That passivity makes them unusually clean data sources. "This makes space debris an excellent tool for tracing" thermospheric conditions, the researchers noted.
The finding matters because low Earth orbit is increasingly crowded. Imaging satellites, surveillance platforms, and internet mega-constellations like Starlink all operate between 400 and 2,000 kilometers altitude, sharing that band with thousands of debris fragments from old rocket stages and defunct satellites. A single collision can trigger a cascade of new debris, multiplying the hazard for everything else in orbit.
The solar cycle most recently peaked in late 2024. That means the conditions the study describes, where debris decay accelerates sharply, are playing out right now. Mission planners calculating collision risks and deorbit timelines need accurate models of how quickly debris will fall, and those models have to account for where the sun is in its cycle.
Ashruf and colleagues say the threshold they identified should hold for active satellites performing station-keeping as well, since those satellites must work against the same drag forces the debris experiences passively. The implication is that operators may need to budget more fuel for orbit maintenance during solar maximum periods than current planning models assume.
Robotic debris-capture missions are still in early development. For now, the main tool for managing the collision risk is precise tracking, which requires knowing not just where debris is today but how fast it is falling and how that rate will change as solar conditions evolve. The new threshold gives planners a more reliable marker for when to recalculate.
