Space & Science

Solar Activity Is Pulling Space Junk Back to Earth Faster — A 40-Year Study Reveals Why

Every piece of orbital debris above 200 kilometres experiences a tiny but constant drag from the tenuous upper atmosphere. Now researchers have shown that the Sun's 11-year activity cycle directly controls how fast that drag works — and during solar maximum, space junk falls back to Earth much sooner than expected.

• Scientists tracked 17 catalogued objects over nearly 40 years, covering solar cycles 22, 23, 24 and the early phase of cycle 25. They found that once sunspot numbers reach roughly 70 percent of their cycle peak, orbital debris begins losing altitude at a significantly accelerated rate.
• The mechanism works through solar extreme ultraviolet radiation: during periods of high sunspot activity, the Sun pumps more UV energy into Earth's upper atmosphere, causing it to heat up and expand. The expanded atmosphere creates more drag on orbiting objects, slowing them down and pulling them into lower orbits.
• Solar Cycle 25, which began in December 2019, has roared to life faster than forecasters predicted. Current sunspot numbers are around 70 — well above the threshold that accelerates debris descent — meaning more space junk is falling back to Earth than models based on calmer cycles would have predicted.

Earth's orbit is crowded. According to the European Space Agency, more than 40,000 pieces of debris larger than 10 centimetres are tracked routinely, and millions of smaller fragments are too small to monitor. These objects travel at speeds of roughly 7 to 8 kilometres per second, fast enough that a collision with a functioning satellite can be catastrophic. Understanding how long debris stays in orbit is essential for satellite operators, astronaut safety, and future space missions.

The new study, published in the journal Space Weather, analysed nearly four decades of orbital data on 17 specific pieces of debris, including spent rocket stages and defunct satellite bodies. The researchers compared the objects' altitude decay rates against records of solar activity across four complete solar cycles. The link was clear: during solar maximum, when the Sun's magnetic field is most active and sunspot numbers are highest, the upper atmosphere swells, and debris loses altitude between 1.5 and 2.5 times faster than during solar minimum.

The key factor is solar extreme ultraviolet (EUV) radiation. During solar maximum, the Sun emits more EUV photons, which are absorbed by Earth's thermosphere at altitudes between 100 and 600 kilometres. This absorption heats the atmosphere, causing it to expand outward. Satellites and debris orbiting at 200 to 500 kilometres then encounter more atmospheric molecules per second, creating greater aerodynamic drag. The increased drag slows the object, reducing its orbital energy, and it sinks into a lower orbit where the atmosphere is denser still — a self-reinforcing cycle.

Solar Cycle 25, the current cycle, has surprised forecasters with its intensity. The Sun's activity has ramped up much faster than the National Oceanic and Atmospheric Administration's original predictions, and sunspot counts have already crossed the 70-mark threshold that the study identifies as the acceleration trigger. For space agencies and commercial satellite operators, this means that debris removal timelines and collision avoidance calculations need to account for a dynamic, solar-driven atmosphere rather than assuming a static decay rate.

The findings also have practical implications for mega-constellations like Starlink, which operate thousands of satellites in low Earth orbit. During solar maximum, satellites experience more atmospheric drag, requiring more frequent station-keeping manoeuvres and shortening their operational lifetimes. Engineers must factor in the solar cycle when designing propulsion systems and planning satellite replacement schedules. What happens above our heads is literally shaped by what happens on the surface of the Sun.