A (somewhat dramatic) visualization of space debris of various sizes around Earth. Credit: NASA

Old satellites, used rocket parts: low Earth orbit is a dump of high-speed garbage, threatening the International Space Station and satellites. So when I read about the JAXA (the Japanese NASA) experiment to clean up space debris using an electrodynamic tether, I thought “Cool, but what the eff is an electrodynamic tether?”

As it turns out, the key principle is simple: when charged particles move in a magnetic field, they feel a force pushing them sideways in their motion.

In this case, a metallic tether orbits Earth, pointing straight out. Its electrons then travel across Earth’s magnetic field, which pushes them and creates a voltage along the tether.

Now comes the brilliant part: attaching a device that sucks in electrons from the environment on one end, and another that spits them out on the other end. A current can now flow through the tether, following the voltage.

The electrodynamic tether attached to a piece of space junk: where it flies, what’s the force pushing on it. Credit: dailykos.com

Since—repeat with me—electrons moving in a magnetic field feel a push sideways, the tether feels a push back.

Why back? Because otherwise we’d get a perpetual motion machine. But “in this house we obey the laws of thermodynamics”, so that doesn’t happen.

The JAXA mission aims at testing how such a tether works in practice. If all goes well, they plan to eventually attach tethers to pieces of space junk, to slow them down and make them drop off their orbits.

In the future, tethers could be just built in stuff to prevent it from becoming space debris. Tether devices are relatively light, and don’t need fuel: they are a great solution to get old stuff out of the way.

Electrodynamic tethers can work as fuel-less boosters, too. Create a current the other way (which needs power from somewhere), and the push from the magnetic field will accelerate you. For example, there’s a project to use excess power generated by the solar panels on the International Space Station to nudge it back in its orbit when needed (it slowly loses energy, as things do).

If you want more
  • Space junk crowding is less dramatic than some pictures lead to believe. And some people want to actually keep it there, as 99% invisible reported
  • JAXA’s mission wasn’t just about this. DailyKos has a good description of it all
  • How magnets, charges and currents interact ends up having to do with special relativity, as Veritasium and Minutephysics explain in this video


Cover photo: CC0 Lorri Lang/pixabay

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