DOES AN ACTIVE SUN BRING SATELLITES DOWN SOONER? SOLAR ACTIVITY AND THE RE-ENTRY OF UNCONTROLLED OBJECTS IN LOW EARTH ORBIT, 1990-2025

When the Sun is active, the upper atmosphere heats up and expands, and objects in low Earth orbit meet more drag. This study asks whether that effect is visible in the public catalogue of all tracked objects: do uncontrolled objects re-enter the atmosphere more often in years of high solar activity? Using the CelesTrak satellite catalogue and the yearly sunspot number of the Royal Observatory of Belgium (SILSO), the share of rocket bodies in low Earth orbit that re-entered in each year from 1990 to 2025 was compared with solar activity. Rocket bodies were chosen because nobody steers them down and they are rarely created in large bursts. The yearly re-entry share of rocket bodies was strongly correlated with the sunspot number (Pearson r = 0.80, Spearman ρ = 0.76, 36 years). On average it was 4.83% in the 11 years with a sunspot number of at least 100 and 2.10% in the 13 years below 30; an increase of 100 in the sunspot number was associated with a 1.76 times higher re-entry share (95% block-bootstrap interval 1.43-2.08). The raw number of re-entries followed solar activity more closely than the number of objects in orbit. For debris the link was weaker, because large break-ups dominate some years. Starlink satellites, which are controlled and can be lowered on purpose, re-entered at a median age of 2.5-4 years. The results agree with the physical picture of solar-driven drag and show that it can be seen in a simple yearly count.