Mercury Thrusters: A Global Catastrophe Averted Just in Time

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The field of spacecraft design is obsessed with efficiency out of necessity. The cost to do anything in space is astronomical, and also strongly related to the weight of the launch. So any technology or technique that can bring those numbers down is critical to the operation.

In recent years, mercury propellants have promised to be one such technology. The only catch was the potentially ruinous environmental cost. Today we are going to look at the benefits of mercury propellants and how they were banned in a short time.

Electric thrust

As we explored in our previous in-depth explainer, ion thrusters have proven useful in countless space missions. Rather than using chemical reactions to generate thrust, they instead use electric fields to accelerate ions. Compared to traditional rockets, they cannot generate so much thrust. However, they are much more fuel efficient. This means they can generate a lot more delta-v (shift) with the same amount of fuel.

NASA has experimented with mercury-based ion thrusters on SERT-I (pictured) and SERT-II spacecraft. However, the mercury was deemed too toxic to be used in future missions. Credit: NASA, public domain

Although their thrust is so weak that you could never use one to launch a vehicle into orbit, they find their primary application in station-keeping satellites, helping them maintain their position over time against drag forces in the upper atmosphere. They can also be used to propel long-range probes that have no gravity to fight.

Most thrusters these days use inert gases like xenon or krypton as fuel. However, these gases are expensive and their molecules are relatively light. Mercury, on the other hand, is much heavier, still very easy to ionize, and easy to store on a spacecraft in liquid form. It’s also very, very cheap. Because of its toxicity, many industries often have to pay to dispose of mercury as a by-product. The old adage that “you can’t even give it awayreally applies here.

The problem

Mercury has a multitude of uses, like the thermometer seen here. However, liquid silver metal is now used less often due to knowledge of its negative health effects. Credit: CambridgeBayWeather, public domain

While mercury makes an excellent fuel for ion thrusters on paper, its toxicity is too potent to ignore. Causing deleterious effects on the nervous system and the brain, its presence in the environment can have major negative effects on human populations. Whether it’s lowering IQ or damaging memory, it’s all bad all the way. It is a toxin that accumulates in the body over time and often enters the human body through the food chain. Indeed, the mercury levels in many sea creatures mean that pregnant women are specifically advised to avoid many types of seafood.

For this reason, NASA abandoned the use of mercury as a propellant after the first experiments in the 1970s. In addition to contaminating the atmosphere, mercury also carries other risks. There are occupational hazards for crews working on thrusters. In addition, explosions on the launch pad or collisions would spread the toxic material into the surrounding environment.

For these reasons, mercury was quickly deemed a “dead fuel” by NASA, simply too dangerous to use despite the benefits.

About the developments

NASA switched to xenon-powered Hall-effect thrusters after mercury was deemed too dangerous to use. Credit: NASA JPL, public domain

As is often the case, however, a Silicon Valley startup would have “disrupted” an established industry by rehashing an old idea. Bloomberg published a story in 2018, regarding the activities of startup Apollo Fusion. Industry insiders told the outlet that the startup is researching a new propellant technology using mercury as a propellant.

This quickly set off alarm bells for many around the world. With SpaceX planning to launch more than 10,000 satellites over a span of a few years, and many other companies rushing to establish their own massive satellite fleets, the outlook was terrifying. If Apollo Fusion won a contract to equip thousands of satellites with mercury thrusters, widespread pollution of the entire Earth was suddenly on the table.

A scientific paper showed that a constellation of 2,000 satellites with 100 kg of propellant on board would deposit 20,000 kg of mercury into the upper atmosphere every year for a decade. Due to the weight of mercury ions, the majority would eventually fall back to Earth and account for 1% of existing global mercury emissions. Modeling suggested that 75% of this mercury would end up in the world’s oceans, with negative impacts on marine life and fishing operations.

60 Starlink satellites seen before deployment in 2019.
Concerns abounded that if mercury thrusters were used for the upcoming constellations of thousands of satellites, it could spread significant pollution into the atmosphere and around the world. Credit: SpaceX, public domain

Great efforts have been made over the decades to reduce the amount of mercury in the environment. The Minimata Convention on Mercury, a United Nations treaty, provided a framework to control the use of mercury by signatory countries. 128 countries have signed the treaty, involving restrictions on the use of mercury in everything from batteries to lamps, soaps and cosmetics.

At the time of signing in 2013, the idea of ​​a return to mercury propulsion was simply not on the table. Apollo Fusion wasn’t created until 2016. Worse still, US regulations meant there were very few downtimes for any company wanting to launch mercury into space. Communications satellites fall under the jurisdiction of the Federal Communications Commission, which has allowed satellite operators to self-certify that their craft have no deleterious impact on humans or the environment.

A safe resolution

Fortunately, the hard work of scientists lobbying against the technology paid off. In March this year, the UN held a meeting regarding the Minamata Convention on Mercury and passed a resolution to phase out all use of mercury as a satellite propellant by 2025.

With most space nations being signatories to the convention, this makes the business case for mercury propellants virtually unviable. As for Apollo Fusion, the company has stuck to working in the world of ion propulsion, though it may have ditched mercury thrusters for now. The company, which was acquired by US space launch company Astra, has since flown a xenon booster into space as part of SpaceX’s Transporter-2 mission last year.

In any case, it seems that the thousands of satellites that will be put into orbit in the coming years will go into space without mercury-spewing thrusters on board. This should be a big relief to all of us here on Earth, where there is already more than enough mercury pollution.

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