The number of satellites in near-Earth space has increased dramatically in recent years. What is the reason for this?
Karl-Heinz Glaßmeier: In the past, only the major nations sent satellites into Earth's orbit and to other planets. But then private companies like SpaceX began to launch tens of thousands of communications satellites into orbit. And we started to wonder: what happens to these satellites once they have served their purpose?
You describe three possible environmental problems that could arise from this space debris.
One is, of course, the so-called surface problem. The main concern is preventing debris from falling back to Earth and causing damage. So far, no humans have been hurt by falling parts but the risk is growing with the number of satellites. The second problem, which has been talked about for many years, is the remains of satellites in an orbit between 500 and 2000 kilometers. If this debris problem isn't brought under control, space travel in Earth’s orbit could eventually become impossible, as there would always be a risk of being hit by debris – and even small pieces can cause significant damage.
To get around these two problems, dead satellites are deliberately slowed down, then fall towards Earth and burn up in the atmosphere – but that's not without problems either?
It's certainly a good idea at first glance. But the idea that “it burns up and then it's gone” is wrong. In the coming years, we'll be introducing as much material into the atmosphere as naturally enters it via meteorites, and that's something we need to consider.
What is your biggest concern?
The man-made material burns up at an altitude of 70 to 80 kilometers in the mesosphere, but then the remains spiral down into the stratosphere above the polar regions, to an altitude of about 20 kilometers. Large quantities of aluminum and hafnium, for example, have already been measured there – these are clearly anthropogenic materials.
Are these quantities already a cause for concern?
Perhaps not in terms of mass, but aluminum oxide, for example, can act as a catalyst for chemical processes that produce chlorine. And a single chlorine atom can destroy up to 100,000 ozone molecules.
This immediately brings to mind the ozone hole over the poles – will it return due to space debris?
It could, but for now that's just a hypothesis. We are being cautious and don't want to sound alarmist at this stage. But we do need to investigate these processes closely.
How do you go about this?
We take measurements with aircraft in the polar stratosphere. Using lidar, we can send light beams into the stratosphere from the ground and detect elements via the reflected light. And in 2027, ESA plans to de-orbit a satellite packed with measuring instruments in a controlled manner during its Draco mission, to monitor the physical and chemical processes that occur as it disintegrates.
Are space researchers aware of the risks for the atmosphere?
My colleague Leonard Schulz and I organized a workshop on this topic at ESA in Noordwijk, Netherlands, in early 2024. We expected ten participants – but then 100 people showed up.
If your fears are confirmed, will we have to stop space exploration?
We can’t and we don’t want to do that. But we could, for example, do without certain materials. Today, companies are very reluctant to share information about what their satellites are made of, but we can only solve this problem by working together with them. Regulation alone is not enough. Near-Earth space has actually become part of our living environment.
Thee interview was conducted by Christoph Droesser