How powerful lasers can measure air pollution from space debris

The skies are crowded these days. Thousands of satellites circle the planet in low Earth orbit, less than 2,000 km from the surface. That number is growing rapidly as companies race to launch their own satellite constellations into space. Inevitably, the satellites, and the rockets that launched them into space, will eventually fall back to Earth. Researchers have now demonstrated that lasers can measure the pollutants released by this space debris as it burns up upon reentry (Commun. Earth Environ. 2026, DOI: 10.1038/s43247-025-03154-8).

In February 2025, atmospheric physicist Robin Wing spotted an interesting story in the morning news. The upper stage of a Falcon 9 rocket had made an uncontrolled reentry into Earth’s atmosphere and crashed into Poland the previous night. In Germany, Wing’s team at the Leibniz Institute of Atmospheric Physics was developing a lidar system to measure metals in the mesosphere—a region of atmosphere 60–100 km above the ground. This random event offered a perfect opportunity to test their instruments, he says.

The lidar the team developed uses a dye laser to detect elements in the upper atmosphere. Using dye as a lasing medium is “still one of the most flexible and dependable ways to get any frequency of light that you want,” Wing explains. In this case, the team tuned the laser to detect lithium atoms.

“Lithium is an excellent tracer for space debris,” Wing says. The outer layer of a rocket is often made of a lithium-aluminum alloy, and electronics are often powered by lithium-ion batteries. This makes the element abundant in human manufactured space debris, while it rarely occurs in natural meteors.

Lithium is also a good target from an instrumental perspective. It’s relatively easy to tune a laser so the energy of the photons matches that of the electronic transitions within elemental lithium, according to Wing. “We’re hitting a lithium atom with the right frequency of light and causing it to resonate and scatter back quite a lot of light,” he says. They then detect the scattered photons that make it back to the ground. “We’re putting up a lot of photons and getting one or two back,” he says.

In theory, of course, high-powered lasers capable of reaching the mesosphere could pose a risk to pilots flying in the troposphere, dozens of kilometers below. “We put a lot of work into our laser safety program,” Wing says. The team not only monitors transponder codes from commercial airplanes but tracks aircraft using radar pointed along each laser beam. If the radar detects something in the airspace above, it immediately shuts off the laser.

The evening after the rocket stage’s uncontrolled reentry, Wing and his team aimed their lidar system toward the cloud-free sky and started measuring lithium. The next day, the scientists processed the data and saw a 10-fold enhancement of lithium atoms at an altitude of 96 km. To determine where the excess came from, Wing asked his atmospheric modeling colleagues to simulate global mesospheric wind patterns for the previous two nights. “We did the back trajectory on the global simulation, and it intersected with the reentry trajectory of the rocket pretty closely,” Wing says. In other words, the lithium came from parts of the rocket burning up in the mesosphere as it plummeted back to Earth.

“I’m impressed with their measurement,” says Daniel Murphy, a physicist at the National Oceanic and Atmospheric Administration (NOAA) Chemical Sciences Laboratory who has studied stratospheric metal pollution from space debris. It’s exceptionally challenging to collect data in the mesosphere that shows how the debris impacts the atmosphere. “It’s far above airplanes, far above balloons,” Murphy says.

The number of satellites and rockets falling back to Earth will only increase as more satellite constellations are sent into low Earth orbit. It’s good to see that there is a viable way to monitor the pollution from those reentries, Murphy says. This is just a single measurement, he adds, “but it’s a demonstration that it can be done.”