Space debris and defunct satellites descend toward Earth significantly faster once solar activity crosses a specific intensity threshold, potentially aiding efforts to prevent catastrophic orbital collisions. Researchers from the Vikram Sarabhai Space Centre in India have demonstrated, for the first time, that solar emissions can predict the rate of orbital decay with sufficient precision to inform future satellite mission planning.
The study, published in Frontiers in Astronomy and Space Sciences, tracked 17 objects in low Earth orbit (LEO) over 36 years, covering three full solar cycles. Scientists identified a repeatable “transition boundary” occurring when sunspot numbers reach approximately two-thirds of their cycle peak. Beyond this point, increased Extreme Ultraviolet (EUV) radiation heats the Earth’s thermosphere, raising atmospheric density and creating “drag” that slows objects down, forcing them to lose altitude.
“For the first time, we find that once solar activity passes a certain level, this loss of altitude happens noticeably more quickly,” said Ayisha M Ashruf, scientist at the Space Physics Laboratory and the study’s corresponding author. “Our results imply that when solar activity passes certain levels, satellites – just like space junk – lose altitude faster so that more orbit corrections are required”.
Tracking the celestial incinerator
The research team utilised Two-Line Element (TLE) data for objects launched as far back as the 1960s. Because these objects lack active station-keeping manoeuvres, their orbital evolution reflects a purely natural response to thermospheric variability, making them ideal tools for tracing long-term solar effects.
Findings showed that peak decay rates declined progressively from solar cycle 22 through to cycle 24. This pattern closely mirrors the well-documented weakening of successive solar cycles in recent decades. While geomagnetic disturbances often grab headlines for disrupting satellites, the study found that they play only a secondary role in long-term decay, compared to sustained solar EUV forcing.
The identified threshold consistently sits between 67 per cent and 75 per cent of a solar cycle’s maximum activity. Identifying this point provides physical insight into the onset of rapid decay, offering a path toward more reliable reentry forecasting and fuel budgeting for high-value orbital assets.
Operational stakes for mega-constellations
The proliferation of internet “mega-constellations” like Starlink has made tracking debris more critical than ever, as even a single collision could trigger a Kessler Syndrome domino effect. The study’s results suggest that satellite operators must account for these solar thresholds when calculating fuel requirements and mission lifespans.
When solar activity crosses the identified boundary, satellites lose altitude faster and require more frequent orbit corrections to stay aloft. “This directly affects how long satellites stay in orbit and how much fuel they need, especially for missions launched near a solar maximum,” Ashruf said.
By turning 60-year-old “junk” into a valuable scientific instrument, the researchers say they have created a framework to help space scientists plan trajectories better and avoid the high-velocity debris currently clogging LEO. Improving these empirical atmospheric models remains a priority for the long-term sustainability of the near-Earth space environment.
