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UN ban on mercury as a propellant reveals gaps in 'Space Law'

When it comes to enforcing this, "Space Law is unclear."

UN ban on mercury as a propellant reveals gaps in 'Space Law'
A 3D illustration of a rocket flying through the clouds. Alexyz3d/iStock

In 2018, Kevin Bell, Staff Counsel at NGO Public Employees for Environmental Responsibility (PEER), received information from a whistleblower about a new satellite thruster under development that used mercury — a highly toxic liquid metal — as a propellant.

The company — Apollo Fusion — was in the process of selling its tech to space companies like SpaceX and OneWeb, who were in the planning stages of putting new mega-constellations of several satellites into orbit. The whistleblower was apparently motivated by concern that these systems would discharge mercury into the upper atmosphere, which would then make its way back down to Earth.

Mercury exhausted from plasma rocket engines on a satellite in space could return to Earth’s atmosphere, becoming airborne and settling into water bodies. Although cheap to use as a fuel component, it’s also a powerful bioaccumulative neurotoxin. This means that it can build up in living organisms and up move up the food chain, eventually resulting in damaging neurological symptoms in humans.

Thus, the negative repercussions of its use in satellite propulsion are potentially enormous. 

Surely, this was illegal?

The Minamata Treaty excludes space

Strangely, there was a regulatory gap in the international law governing the use of mercury - the Minamata Treaty.

While the Minamata Convention on Mercury, to which the US was the first signatory, in 2013, contains a long list of banned uses for mercury, its use in space was not one of these. In addition, the responsibility for enforcing the terms of the treaty falls on individual governments.

In the review process of applications for satellite space station authorizations, the Federal Communications Commission (FCC) solely enquires about "human exposure to [hazardous] levels of radiofrequency radiation" or high-intensity lighting.

No other environmental impacts, including mercury emissions or the presence of other hazardous materials in the launch payload, are currently reviewed by the FCC.

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Exposing cracks in regulation

PEER sent a request to the FCC in 2018 asking them to conduct an environmental review prior to approving communication satellite "mega-constellations". In their request, PEER pointed out that, "the FCC's regulatory approach of hands-off oversight which declines to examine the nature of the objects it authorizes to be fired into space has left a massive blind spot that should be corrected before the mega-constellations are launched and the mercury pollutants cannot be safely recovered".

PEER further pointed out that the use of mercury to power planned LEO satellite constellations could result in the release of around 20 tons of mercury each year, and that if imported mercury in the manufacture of the thrusters, this could be in clear contravention of the Minamata Treaty. Not only that, but by refusing to investigate the sourcing of the mercury, the FCC was in violation of the treaty.

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Fast forward to 2022. On March 28, the United Nations adopted a provision to phase out the use of mercury as a satellite propellant by 2025 during the Fourth meeting of the Conference of the Parties (COP) to the Minamata Convention on Mercury.

"To the best of my knowledge and research, our request to the FCC that they consider the environmental impacts of orbital activities was the first attempt to apply US environmental law in outer space," Bell told IE

What goes up comes back down - here's how

Brad King, the Richard and Elizabeth Henes Professor of Space Systems Engineering at Michigan Technological University and the CEO of Orbion Space Technology, Inc, told IE that mercury as a propellant can contribute to global mercury in three major ways.

The first is the possibility of airborne mercury.

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There are several scientific studies of this phenomenon, and "the consensus is that this mercury will, eventually, return to Earth even though it is emitted in space," said King.

He explained that this happens because the emitted mercury atoms will collide with the very sparse atmosphere of space, and these collisions will cause the mercury atoms to lose speed, eventually falling back into the denser atmosphere, where more collisions will cause them to descend even faster.

King stressed that though most attention was paid to the first possibility, the next two are not only far more deadly but also more likely to occur.

Satellite failure is a high possibility

Satellites that de-orbit from space could still have mercury in their propellant tanks. The mercury will be released when these satellites burn up in the atmosphere. Now, as per the Outer Space Treaty, operators of low-Earth-orbit satellites must intentionally de-orbit each satellite within 25 years of launch to avoid leaving space debris.

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"In a typical successful mission the satellite will have exhausted most of its onboard mercury before de-orbiting, so the residual mercury released into the atmosphere when the satellite burns up would be negligible. However, satellites frequently fail and de-orbit unintentionally and long before their mission is over," said King.

He illustrated this with a recent situation.

In February 2022, more than 40 SpaceX Starlink satellites fell out of orbit only weeks after launching.

"An unexpected solar storm caused the satellites to lose some functionality, and they plunged to a fiery death with their propellant tanks still full. These SpaceX satellites did not use mercury, however. They used the much more environmentally friendly krypton for their thrusters. Krypton is an inert, harmless gas that is already present in our atmosphere and so there was no environmental risk from this expensive accident. However, had these satellites been fueled with mercury this event would have resulted in 100s of kilograms of mercury being either released into the atmosphere or deposited on the Earth's surface, or in the ocean if the tanks survived re-entry," he said.

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The third prospect includes rockets that carry satellites into orbit. Such launch vehicles have a possibility of failure.

"In recent months there have been notable rocket explosions by companies such as Astra, RocketLab, Firefly, and even SpaceX. Needless to say, when a rocket launched out over the ocean from, say Florida, explodes and that rocket is carrying 100 satellites - each of which has a full tank of mercury - that mercury will all end up in the ocean," explained King. "Or, if there is an explosion on the launchpad, that mercury will end up in the sensitive environment around Cape Kennedy."

Total destruction once released back to Earth

A fisherman on board sorting the fish freshly fished.
Mercury eventually makes its way into aquatic ecosystems through atmospheric deposition or point-source discharges. Sourcenito100/iStock

Though mercury disseminates between air, water, and soil in different forms, its ability to build up in living organisms and thereby move along the food chain is perhaps its most harmful impact on the environment.

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"Mercury circulates in the atmosphere and ultimately makes its way into humans by moving up the aquatic food chain," Michael Bender, Director of the Mercury Policy Project and International Coordinator of the Zero Mercury Working Group, told IE

In a research paper titled Are mercury emissions from satellite electric propulsion an environmental concern?, co-author Dan Fourie noted that 75 percent of the mercury falling back to Earth will be deposited in the world’s oceans. This could constitute a greater magnitude of mercury than the land-based anthropogenic mercury emissions that are eventually deposited in the oceans.

Once in the water, aquatic micro-organisms and vegetation convert the mercury into deadly methylmercury. As fish ingest the contaminated organisms, the mercury becomes concentrated in their tissues in a process called biomagnification

Once absorbed, the mercury is bound tightly to fish protein and even the most robust cooking methods, such as boiling and deep-frying, cannot remove it. Eating contaminated fish can thus have severe health consequences for humans.

This is the situation that occurred in Minamata, Japan, in the 1950s, when large amounts of mercury were absorbed into the bay's marine ecosystem. Fish from the bay were eaten by local residents, resulting in the deaths of more than 1,000 people, with many more affected with severe neurological symptoms, including brain damage, and a large number of cases of fetal abnormalities in pregnant women who ate the fish. 

And the damage caused to the central nervous system is irreversible.

Later investigation found that a local chemical plant had discharged untreated effluent containing methyl mercury chloride into the bay, which was absorbed by the fish and eventually passed on to the villagers. 

"Health agencies worldwide—including the U.S. Food and Drug Administration and all 50 states—warn pregnant women to avoid or limit consumption of certain fish because of mercury exposure risks, particularly for the pregnant women and the developing fetus," added Bender. 

A cheaper, but deadly alternative

A 3D image of satellite.
A 3D image of satellite. Source: enot-poloskun/iStock

The reduction of satellite launch costs is leading to the creation of a new generation of smaller, cheaper satellites for Earth observation and communications. This is resulting in the Earth's lower orbit becoming cluttered with satellites. And mercury happens to be one of the cheapest and easiest to store propellants for electric propulsion for these satellites.

"A few kilograms of mercury can be stored in a very small vessel - which is important because space is a premium on satellites that strive to be as compact as possible," King told us.

Nevertheless, Joseph Latrell, the founder of Quub, a satellite development company, told IE, that the use of mercury as fuel is a bad idea.

"Working with the substance is hazardous, [it] has the potential for environmental leaks either during loading or in transit to space," he told IE.

Satellites are required to undergo two tests - the design must go through a qualification test, and each satellite that is to be launched must undergo acceptance testing. "During those tests, there it is possible that an issue would develop and mercury could contaminate everything. Not a good look," said Joseph.

There are better solutions. 

"Our engine manufacturer uses bismuth, titanium, or aluminum as fuel. The advantages are they are solid to start with so you can get a lot of use our of it. The electronics of the engine will fail before the fuel runs out," said Joseph.

A mercury ban is 'vital'

King highlighted that mercury has not been used as a rocket propellant in some 50 years. 

"Because of the dangerous health and environmental implications of mercury, the space industry moved away from it and instead uses xenon or krypton in its place," he said. 

Having said that, the recent ban on mercury as a propellant will ensure that the industry doesn't return to mercury in the future. (Apollo Fusion has since abandoned the use of mercury.) 

"At least one company recently 'rediscovered' the benefits of mercury and was seeking to exploit its properties in the commercial space industry to gain [an] advantage over other companies using xenon and krypton. The recent Minemata ban makes sure we can’t turn back the clock and make bad decisions all over again," added King. 

King sure is right.

The efforts of PEER, from Bell's tweets, reveal how essential the ban is to avert another Minamata-like situation. Or a disaster of more epic proportions. 

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