Study Links Sun's 11-Year Cycle to Accelerated Orbital Decay of Space Debris

Researchers reported on May 6 that the sun's 11-year cycle accelerates the orbital loss of space debris, with objects falling faster as sunspot numbers approach their cycle peak. The study, published in Frontiers in Astronomy and Space Sciences, marks the first demonstration of this link between solar activity and orbital decay.

Ayisha Ashruf and colleagues at the Vikram Sarabhai Space Centre in Thiruvananthapuram, India, tracked the positions of 17 pieces of space debris in low Earth orbit for more than three decades, spanning from 1986 to 2024.

Orbital decay rates for these 17 objects rose sharply when sunspot numbers reached about 70 percent of their peak values. Each time a solar cycle passed this sunspot threshold, the space junk dropped a few kilometers in altitude. The objects circle Earth every 90 to 120 minutes at altitudes between 600 and 800 kilometers.

Science News reported that the altitude of a piece of space junk from a Delta 1 rocket showed a stepwise decrease through three solar cycles from 1986 to 2024. Peak solar activity in each of the three cycles corresponded to a sharp decline in the object's altitude. After each peak, the orbit leveled out until the next peak.

The sun’s radiation emission intensity waxes and wanes over a roughly 11-year cycle. Intense radiation heats and expands the thermosphere, an outer layer of Earth's atmosphere. Objects in low Earth orbit, a zone about 160 to 2,000 kilometers from Earth’s surface, experience denser atmosphere at cycle peaks, adding friction and slowing them down.

Ayisha Ashruf and her team identified a clear threshold beyond which increases in solar activity impacted the objects. That threshold pattern held true from cycle to cycle, though the exact sunspot threshold and the extent of altitude decrease also depended on the overall intensity of the solar cycle.

At the peak of each cycle, a surfeit of sunspots blooms on the sun’s surface, and more intense radiation streams toward Earth.

The study analyzed data from 1986 to 2024, covering three consecutive solar cycles. For the Delta 1 rocket debris, the black line in the altitude chart illustrated the stepwise drops, aligned with shaded blue, green, and red regions representing the cycles. Dashed red lines marked the peak activity points where sharp declines occurred.

Researchers noted that understanding the sun’s role in orbital decay could benefit space operations planning. This is particularly relevant as the proliferation of human-made space debris increases the threat of collisions with operational satellites and spacecraft. The 17 studied objects demonstrated consistent behavior, with decay accelerating once sunspot numbers hit the 70 percent mark.

Low Earth orbit objects, slowed by added friction during denser atmospheric conditions, descend out of orbit sooner than otherwise. The findings highlight how solar cycles influence the fate of space junk. Science News reported that identifying this pattern could help missions select optimal launch windows to avoid debris collisions, a need growing more acute as space junk accumulates.