The fourth meeting of the Conference of the Parties (COP) to the Minamata Convention on Mercury just wrapped up in Bali, Indonesia. COP meetings aren’t just about climate: The Minamata Convention is “a global treaty to protect human health and the environment from the adverse effects of mercury, a bio-accumulative neurotoxin.
One major success at the conference was the ban of mercury satellite propellants used in electric rocket engines after a motion from Canada, the European Union, and Norway. According to the Public Employees for Environmental Responsibility (PEER), which has been fighting to prevent mercury from being used as propellants since 2018:
I feel somewhat confident talking about electric propulsion systems because I tried to build one many years ago for a science fair project. All rockets work in much the same way, throwing stuff out the back at high speed to get an equal and opposite reaction that pushes the vehicle forward.
In an electric satellite propulsion system, you ionize the propellant, giving it an electric charge, and then accelerate it with magnetic fields and shoot it out the back. Mercury is great for this; it is heavy and it is easy to ionize. This only works in a serious vacuum, so there is a mercury diffusion pump in the background to evacuate the bell jar covering the engine. I never got a chance to actually demonstrate it; the judges were shocked that I was proposing to boil mercury and closed it down. They knew better than I how dangerous mercury was.
But what makes mercury great for an ion thruster makes it not so great for the planet that the satellites are orbiting. Some companies want to use it for low-orbit Internet satellites; mercury is really cheap compared to xenon or the krypton that is used in Elon Musk’s Starlink satellites. According to the Norwegian submission to the conference:
NASA tested ion drive engines in the ’60s. Mine was modeled on the little one in front of program manager Raymond Rulis. They dropped mercury because of its toxicity. According to the submission to the Convention:
But Bloomberg reports that Apollo Fusion, the maker of the xenon thruster in the top photo, has proposed using it: “Propulsion experts say mercury is a tempting choice, despite the safety hazards, because its performance is better than that of alternatives like xenon or krypton.”
Bloomberg continues:
No wonder the Minamata convention wanted to nip this in the bud. This thread of tweets from Kevin Bell of PEER is remarkable in its history of their work to bring this to fruition.
And I am very relieved those judges shut me down so many years ago.
