Minamata Convention Bans Mercury Satellite Propellants

Mercury is cheap, heavy, and works well in ion thrusters. But it is also poisonous.

An image of the Apollo Fusion ion thruster
The Apollo Fusion ion thruster.

R. Conversano / California Institute of Technology

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:

“The Minamata Convention on Mercury seeks to eliminate all mercury uses where technically-achievable non-mercury alternatives are available,” said Elena Lymberidi-Settimo, ZMWG [Zero Mercury Working Group] Coordinator at the European Environmental Bureau. “In the case of satellite propulsion systems, mercury-free alternatives have been available and used for decades.”
Lloyd Alter builds a rocket engine
Lloyd Alter builds a rocket engine.

The Globe and Mail

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:

“Mercury is one of the cheapest and easiest to store propellants for electric propulsion. While some mercury released in Low Earth Orbit may escape Earth’s gravitational field, mercury emissions originating from many common orbital maneuvers will return to Earth. The environmental and human health implications of such releases have not been evaluated. Using an atmospheric chemical transport model, we simulate global deposition of mercury released from satellite propulsion systems. We estimate that 75% of the mercury falling back to Earth will be deposited in the world’s oceans, with potentially negative implications for commercial fish and other marine life.”
SERT-1's Program Manager, Raymond J. Rulis examining the spacecraft
The SERT-1 Satellite and its program manager, Raymond J. Rulis.

NASA / Wikimedia Commons / CC BY-SA 4.0

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:

"Mercury was used as a propellant back in the 60s, primarily because of its high storage density (a lot of kilograms of mercury can be stored in a very small volume) which is attractive for the spacecraft. There were, however, a great deal of technical difficulties with mercury and health and safety concerns, for which NASA, the primary developer, abandoned the use of mercury and started using non-mercury alternatives."

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:

"A case study on Apollo’s website [now a dead link] that the company calls a 'representative configuration' ideal for a low-orbit satellite would carry 20 kilograms of an unnamed propellant. Multiply that by 1,000, and the constellation of satellites could use 20,000kg, or 20 metric tons, of mercury, which would be released over the satellites’ estimated five to seven years in orbit. By comparison, the entire U.S. emits about 50 metric tons of mercury each year; the entire population of the world generates about 2,000 metric tons."

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.