A low Earth orbit (LEO) between 400 and 2,000 km altitude is ideal for imaging and surveillance satellites and internet 'mega-constellations' such as Starlink. Unfortunately, these days it's also chock-full of 'junk' like old satellite debris and rocket stages, and these threaten new space launches. For example, even one collision may spread damage through a domino effect. Because missions to capture space junk with robots are still in their infancy, scientists today focus mainly on tracking debris more accurately to identify the most dangerous objects for future removal.
"Here we show that space debris around Earth loses altitude much faster when the Sun is more active," said Ayisha M Ashruf, a scientist and engineer at the Space Physics Laboratory, Vikram Sarabhai Space Centre, Thiruvananthapuram, India, and the corresponding author of a new study in Frontiers in Astronomy and Space Sciences .
"For the first time, we find that once solar activity passes a certain level, this loss of altitude happens noticeably more quickly. This observation is expected to be key for planning sustainable space operations in the future."
Chasing the Sun
The Sun has an 11-year cycle of active and quiet phases – correlated with the number of sunspots – which results in changes in the intensity at which it emits
UV radiation and charged particles, for example helium nuclei and heavy ions. When this outward stream peaks, like most recently in late 2024, solar emissions heat and expand upwards into the Earth's thermosphere (located between approximately 100 and 1,000 km, with a temperature between 500 and 2,500 °C). This in turns raises the atmospheric density around orbiting bodies (between 350 and 36,000 km) and increases the resistance or 'drag' on them, thus slowing them down and making them fall faster.
Ayisha and colleagues from the same institute followed the historic trajectory of 17 LEO space junk objects over a 36-year period since the 1960s, during the 22nd through 24th solar cycles. These objects orbit the Earth every 90 to 120 minutes at an altitude between 600 and 800 km, and are yet to reenter the atmosphere, where they will ultimately burn up.
Because space junk doesn't perform active station-keeping maneuvers like satellites do, changes in the speed of their descent ('orbital decay') only depend on fluctuations in thermospheric density. "This makes space debris an excellent tool for tracing long-term solar-activity effect on atmospheric drag," wrote the authors.
The scientists linked the trajectories to long-term data at the German Research Centre for Geosciences in Potsdam that track the number of sunspots and daily changes in the Sun's radio and Extreme Ultraviolet (EUV) emissions.
Crossing the threshold
The results showed that when the number of sunspots is higher than two-thirds of its maximum, space junk passes through a 'transition boundary' – a threshold beyond which it begins to fall much faster.
"This threshold doesn't seem to be tied to a fixed value of solar radiation, but rather to how close the Sun is to its peak activity. Around this point, the Sun produces more intense EUV radiation, which may be driven by changes in solar processes that become stronger near the peak," concluded Ayisha Ashruf.
The authors stress that their results are expected to help space scientists plan the trajectories of satellites better, avoiding collisions with space junk.
"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. This directly affects how long satellites stay in orbit and how much fuel they need, especially for missions launched near a solar maximum," explained Ayisha Ashruf.
"What is most interesting is that all of this information comes from objects launched back in the 1960s. They are still contributing to science, serving as valuable tools for studying long-term effects of solar activity on the thermosphere."
