Scientific Article for Students

Introduction What do you think of when you hear the term “climate change”? You probably think about global warming and greenhouse gases. But I bet you didn’t think about satellites in space! Satellites help us with things like communication, weather forecasting, and navigation. A lot of satellites, as well as the International Space Station, orbit in the thermosphere and exosphere. These are the uppermost layers of our atmosphere. Together, this region is called low Earth orbit. And these parts of the atmosphere are impacted by climate change. ****Explainer Box**** Greenhouse gases increase in all parts of the atmosphere because of climate change. Greenhouse gas molecules absorb heat energy from the sun. They also give off heat energy. Close to the Earth, this heat is trapped by other molecules. In the upper atmosphere, where there are a lot fewer molecules, heat is lost to space. This means that the upper atmosphere gets colder because of climate change. ****** Scientists know that climate change is cooling down the thermosphere. This changes the density of air and causes the thermosphere to shrink. When the thermosphere gets thinner, an orbiting satellite will stay at the same altitude, but the air it is moving through is then less dense. There is still enough air for a satellite to interact with and create drag, though. This will cause the satellite to lose energy and eventually fall to Earth (which is sometimes a good thing). If the air is less dense, then drag will also be less. Satellites could stay in orbit for longer. And this could become a problem in the future. Currently, the recommendation is for satellites to come out of orbit within 5 years of finishing their missions. Satellites can actively or passively lose energy and fall to Earth. But we keep adding more and more satellites into low Earth orbit. Unfortunately, there is also a lot of space debris in this region (see Figure 1 on p.2). It is from satellites that ran into each other in the past (and didn’t fall to Earth). Enough collisions could start a chain reaction and create a lot more debris. Scientists call this possibility the Kessler syndrome. If it happened, it would be hard to put any more satellites into low Earth orbit. We wanted to know how many satellites could be in orbit without triggering the Kessler syndrome. This number represents the carrying capacity of low Earth orbit. We also wanted to know how future climate change will impact this number. Methods We modeled changes in thermosphere density until 2100. We used models from the National Center for Atmospheric Research and the U.S. Naval Research Laboratory. We did this for three different climate scenarios. These were from the International Panel on Climate Change (IPCC). Then we used this information to help develop our own model. We calculated the satellite carrying capacity of low Earth orbit. We call this value the instantaneous Kessler capacity. We calculated this for our three climate scenarios until 2100. Then we compared them to a baseline estimate from greenhouse gas values in the year 2000. Results We found that thermosphere density decreased: at higher altitudes. during solar minimums. There is a natural 11-year cycle of solar activity that impacts the thermosphere. under more severe climate change scenarios. We found that the baseline instantaneous Kessler capacity was 34–72 million satellites (see Figure 2 on p.3). The instantaneous Kessler capacity was less during solar minimums. It was also less during more severe climate scenarios. By 2100 we could see a 50–66% overall reduction in capacity between 200 and 400 km. At 400–1000 km, this overall reduction jumps to 60–82%. Most (98–99%) of this capacity is available at lower altitudes. Discussion Our research shows that we need to think about our sustainable use of low Earth orbit. Climate change will decrease the density of the thermosphere over time. This reduces drag on satellites and will allow them to stay in orbit for longer. And the longer they stay in orbit, the greater the chance for a chain reaction of collisions. In the future it will be more dangerous to put new satellites into low Earth orbit. Our data show that we have room in low Earth orbit for millions of satellites. But there are some things to consider. Climate change will reduce the number of satellites we can fit into low Earth orbit. The vast majority of capacity for satellites will be at lower altitudes. Space debris falls to Earth quickly from this region because of drag. In our model, this means that lots of collisions can happen and not trigger the Kessler syndrome. But satellites are expensive and have decades-long lifespans. Not many people would want to put satellites at these altitudes if the chance of collision were high. Solar activity naturally changes the carrying capacity of low Earth orbit. At solar minimums, low Earth orbit can’t hold as many satellites safely. Any sustainability plans need to consider the 11-year solar cycle. There are lots of things that we can actively do to increase the carrying capacity of low Earth orbit. We can require that new satellites have technology to avoid collisions. We can also put in place space debris removal projects. Many governments track satellites and space debris. Improved tracking and coordination between governments could reduce collisions. Conclusion Climate change has the potential to impact many things in our world. And satellites in low Earth orbit are really far away from your daily life. But imagine if all those satellites ran into each other. You wouldn’t be able to get directions on your phone or watch your favorite TV show. And it would get worse. Planes wouldn’t be able to fly safely. We wouldn’t be able to predict the weather. And international communication would be limited. So what can you do? The biggest thing you can do is limit your production of greenhouse gases. This means reducing how much energy you use and how much stuff you buy (which takes resources to produce!) For example, avoid buying unnecessary items or see if you can get things secondhand. Also take public transportation, ride your bike, or carpool whenever you can. You can also write to your representatives about investing in green energy solutions and regulating local impacts on climate change. Every voice counts!