The space vehicle design field is necessarily obsessed with efficiency. The cost of doing anything in space is astronomical and also heavily tied to launch weight. So any technology or technique that can bring these numbers down is best for exploitation.
In recent years, mercury thrusters promised to become such a technology. The only catch was the potentially devastating environmental costs. Today we will look at the benefits of mercury thrusters and how they were banned in a short time.
electric thrust
As we explored in our previous in-depth explanation, ion thrusters have proved valuable in countless space missions. Instead of using chemical reactions to generate thrust, they use electric fields to accelerate ions. Compared to traditional rockets, nowhere near as much thrust can they generate. However, they are much more economical. This means they can generate much more delta-v (change in speed) with the same amount of fuel.
NASA experimented with mercury-based ion engines on the SERT-I (pictured) and SERT-II spacecraft. However, mercury was considered too toxic to use in future missions. Credit: NASA, Public Domain
While their thrust is so meager that you could never use one to launch a vehicle into orbit, their primary application is in stationing satellites, allowing them to track their position over time. retained against the forces of the upper atmospheric resistance. They can also be used to propel long-range probes that have no gravity to fight against.
Today, most thrusters use inert gases such as xenon or krypton as fuel. However, these gases are expensive and their molecules are relatively lightweight. Mercury, on the other hand, is much heavier, still very easy to ionize, and easy to store in liquid form on a spacecraft. It’s also very, very, cheap. Because of its toxicity alone, many industries often have to pay to remove mercury as a by-product. The old adage “you can’t even give it away” really applies here.
The problem
Mercury has a multitude of uses, such as the thermometer seen here. However, the silvery liquid metal is now less commonly used due to knowledge of its negative health effects. Credit: CambridgeBayWeather, Public Domain
While mercury is an excellent fuel for ion thrust on paper, its toxicity is too potent to ignore. Because it has harmful effects on the nervous system and brain, its presence in the environment can have major negative effects on the human population. From lowering IQs to damaging memory, it’s all bad. It is a toxin that accumulates in the body over time and often enters the human body through the food chain. Indeed, mercury concentrations in many marine animals 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 initial experiments in the 1970s. In addition to polluting the atmosphere, mercury poses other risks. There are occupational hazards for the crews working on the bow thrusters. In addition, explosions on the launch pad or crashes would spread the toxic material into the environment.
For these reasons, mercury was soon considered a “dead fuel” by NASA, simply too dangerous to use, despite its benefits.
Regarding developments
NASA switched to xenon-powered Hall effect thrusters after mercury was deemed too dangerous to use. Credit: NASA JPL, Public Domain
However, as is so often the case, a Silicon Valley startup was reported to be “disrupting” an established industry by rehashing an old idea. Bloomberg did a story in 2018 about the activities of startup Apollo Fusion. Industry insiders told the outlet that the startup was looking for a new thruster technology that uses 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 period of a few years, and many other companies rushing to set up their own massive satellite fleets, the prospects were terrifying. When Apollo Fusion was awarded a contract to equip thousands of satellites with mercury thrusters, widespread pollution from all over the Earth suddenly appeared 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, representing 1% of existing global mercury emissions. Modeling suggested that 75% of this mercury would end up in the world’s oceans, negatively impacting marine life and fisheries.
60 Starlink satellites seen prior to deployment in 2019.
There was a lot of concern that if mercury thrusters were used for future constellations of thousands of satellites, it could spread significant pollution into the atmosphere and around the world. Credit: SpaceX, Public Domain
Major efforts have been made in recent 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 the signatory countries. 128 countries signed the treaty, which restricted 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 was only founded in 2016. Worse, US regulations meant there was little stopping any company that wanted to launch mercury into space. Communications satellites fall under the jurisdiction of the Federal Communications Commission, which allowed satellite operators to self-certify their craft as having no harmful effects on humans or the environment.
A safe solution
Fortunately, the hard work of scientists lobbying against the technology has paid off. In March of this year, the UN held a meeting on the Minamata Convention on Mercury and passed a resolution to phase out the use of mercury as a means of propulsion for satellites by 2025.
Since most space-faring countries have signed the convention, it makes the business case for mercury thrusters virtually unachievable. As for Apollo Fusion, the company has continued to work in the world of ion propulsion, although it may have given up on mercury propellants at this point. The company, which was acquired by US space launch company Astra, has since flown a xenon propeller into space as part of SpaceX’s Transporter-2 mission last year.
In any case, it seems that the thousands of satellites orbiting the Earth in the coming years will go into space without mercury-spitting thrusters on board. That should be a great relief to all of us here on Earth, where there is already plenty of mercury pollution.
This post Mercury Thrusters: a global disaster averted just in time
was original published at “https://hackaday.com/2022/04/18/mercury-thrusters-a-worldwide-disaster-averted-just-in-time/”
