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Low Earth Orbit (LEO) is cluttered with space junk, and the rate at which that junk falls into our atmosphere turns out to depend a lot on what the weather’s like up there.
We tend to think of weather as an atmospheric phenomenon, but space isn’t entirely empty. As sparse as the particles may be, they still exhibit variability, and conditions in space have important ramifications for satellites and space travelers.
The Sun is the driving force behind weather in space, just like on Earth. Yet, its effects are much more direct and predictable without the complexities of ocean and air currents. Magnetic activity in the Sun’s surface follows an 11-year cycle and influences the number and type of particles it throws off. When those ejections are at their peak, as they are now, objects just beyond the official “start” of space will lose altitude much faster.
The effect on all that LEO space junk is much like a gale blowing through an apple tree. Old rocket boosters and abandoned satellites all come tumbling down. Fortunately, most of these burn up in the atmosphere or splash harmlessly into the ocean. However, that trend has important implications for active satellites that we don’t want falling out of the sky quite yet.
A group of Indian astrophysicists published a paper yesterday quantifying this effect in greater detail than anyone had previously. Their study uses data collected across more than three decades and three complete solar cycles. Among the findings is that the rate of deorbiting junk doesn’t change smoothly with the solar cycle. Rather, there is a critical point at about two-thirds of maximum intensity, past which orbiting objects suddenly start dropping much faster.
The Sunspot Cycle and ‘Space Weather’
Astronomers have known about the solar cycle since the mid-19th century, because one of its effects is readily visible, as long as you use a smoked lens to dim the Sun’s light enough to look right at it. On its surface are tiny dark spots that appear and disappear. Sometimes you might see almost none of them, while at other times you might see many.
People counting them over time soon realized that it takes about five and a half years to go from almost none to the most you’re going to see, and then another five and a half to get back to the minimum again.
Figuring out the reason for the spots and the ramifications of the cycle took longer.
Sunspots are dark because they’re colder than the rest of the surface. Paradoxically, however, they’re also responsible for ejecting a disproportionate number of high-energy particles. That’s because they appear where loops of the Sun’s powerful magnetic field poke through the part of its atmosphere that emits visible light. The magnetic field suppresses the convection of hot material from deeper in the star, but can also fling ions up and away from the surface, helping them escape.
The reason they wax and wane is that the polarity of the Sun’s magnetic field is also reversing itself over a 22-year cycle—one big switcheroo every 11 years. Sunspots are at their peak when the field is at its most “twisted,” just before performing this stunt to get itself back in order. They hit that point most recently in October 2024 and are still close to the maximum today, though set to drop off soon.
Many of the particles ejected by the sunspots are helium nuclei and other heavy ions—the byproducts of the fusion reaction powering the Sun. These can wreak havoc on electronics, but they also have a dramatic effect on Earth’s thermosphere, where many of our satellites orbit.
How Sunspots Deorbit Satellites
Satellites are simultaneously in our atmosphere and in space. That may sound like a contradiction, but it’s just that the distinction between the two is a bit arbitrary.
“Outer space” is widely accepted to begin at the Kármán line, 100 km above the surface. That’s the point at which the physics of conventional air travel breaks down. The air is too thin for a wing to generate lift. So, we consider that space because you need a spacecraft to get there.
By the same token that you can’t fly there, drag is also very low. That makes it the closest distance at which you can orbit a satellite and have it stay there long enough to be useful.
But the drag isn’t zero, and they won’t stay there forever. Most LEO satellites have some kind of thruster on board to give them a boost when they need it.
When energetic particles from the Sun reach the Earth, they dump the bulk of their energy into this portion of the atmosphere just outside the Kármán line. That’s why it’s called the thermosphere, because it’s actually extremely hot due to this bombardment.
When sunspots are doing their 11-year thing, it gets even hotter. Gases expand when they’re hot, so the atmosphere gets bigger. That shifts the layers outwards like an inflating balloon, so a satellite orbiting at a constant distance from the surface is suddenly deeper in the atmosphere than it was previously. The deeper layers are denser, so drag goes up and the satellite starts to lose speed and see its orbit decay.
How big is that effect? Air pressure at a given point in the thermosphere can change more than tenfold between the sunspot minimum and the maximum. For comparison, the pressure difference between sea level and the peak of Mt. Everest is only a factor of three.
That’s some serious weather.
