This episode is sponsored by Coursera: one hundred percent online learning from the world's best universities and companies. [♪ INTRO].

Exploring space has arguably been one of humanity's greatest achievements. We've landed people on the Moon, put hardware on and around other planets, and launched all kinds of powerful space telescopes and satellites.

That work has helped us learn more about the history of the universe and our place in it, but it also comes at a cost: In striving to learn about other worlds, we've actually been damaging our own. Today, building and launching rockets often involves using toxic cleaning solvents and dumping tons of greenhouse gases into the atmosphere. And while reusable rockets and boosters are a great step, we're going to have to do a lot more if we really want to make the space industry Earth-friendly.

That's where these four technologies can help us out. The environmental engineering part of space exploration isn't something that's talked about a whole lot, but the science is really fascinating. Engineers are approaching it from a bunch of angles, but for now, we're going to focus on two main research areas.

First, scientists are investigating propellants, including a type called hypergolic propellants, which are used in things like rockets and satellites. Unlike other propellants, these ignite spontaneously when their fuel and oxidizer come together, without the need of something like a spark. Most commonly, they're made with some variety of hydrazine.

Hydrazine has been used in everything from the Space Shuttles to Mars missions, so it's great for exploration, but it's also incredibly toxic and corrosive. So it's no surprise that researchers are looking for alternatives. And the good news is that they've found some!

One possible alternative to hydrazine-bearing propellants is a salt called ammonium dinitramide, or ADN for short. When it's heated, ADN breaks down into just nitrogen, oxygen, and water, so it's pretty environmentally clean. It's also significantly less dangerous to handle and work with.

The problem is, ADN is normally a solid and isn't very reactive. It can be dissolved in things like methanol or ammonia, but even then, it can take temperatures of more than 1600 degrees Celsius to ignite it. So recently, scientists have been looking into ways to make ADN ignition more spontaneous.

And in 2018, a team in Germany published some exciting new results. Their trick was to make a really good catalyst. Catalysts help increase the surface area available for a reaction to take place, and they reduce the amount of heat needed to get things started.

Traditionally, catalysts have been a bunch of tiny pellets that the propellant has to move through, and that's worked pretty well. Using something like this, you can get ADN to ignite at around 350 degrees Celsius. But that still means you have to preheat the mixture before you can use it, which takes energy and, maybe more importantly, it takes time.

And if you need to move your spacecraft in an emergency, time just isn't something you have. So, this team wanted to see if they could reduce ADN's ignition temperature even more by using a better catalyst. And to do it, they turned to 3D printing.

In this new study, they modeled and 3D printed a honeycomb structure called a monolith. It's basically a ceramic framework covered in a metal catalyst. Compared to the pellets, this complex structure has an incredibly high surface area, which massively increases the speed and efficiency of the reaction.

Using monoliths, the scientists were able to reduce the ignition temperature of ADN from 350 degrees to just 100 degrees Celsius. Which is amazing progress! Their next challenge is to get the reaction started at room temperature.

Because of the chemistry involved, that will probably take more than just a fancy catalyst, but cold-starting green propellants are finally on the horizon. Now, ADN isn't the only green solution out there. Scientists are also researching another new kind of propellant, called a metal-organic framework.

These are the solid structures made of groups of metal ions and an organic molecule called a linker. They're normally stable, and they've traditionally been used as catalysts to help with gas separation and storage in various industries. But now, scientists are trying to figure out if these frameworks could do more.

They're investigating if they could release huge amounts of energy in the same way that hydrazine does. And they've found that they can, with some adjustments. By adding simple chemicals like vinyl or acetylene to the metal-organic framework, researchers have found that the stable, solid structure can become incredibly reactive.

On contact with an oxidizer like nitric acid, it ultimately breaks down and releases vast amounts of heat. And some fire, for good measure. In April 2019, scientists at McGill University in Canada reported that they had used this system to get their metal-organic frameworks to ignite at room temperature.

Some of them even ignited just two milliseconds after contact with an oxidizer. Which, like, is pretty close to “instantaneous”. So far, though, these compounds have only been tested in the lab, and plenty more research is needed before they can be incorporated into any rocket thrusters.

But if they can be, they would be a huge step forward in the green propellant world. Then again… it's not just the rockets that can damage the environment. Space hardware can also have a negative impact on Earth even before it gets into space.

That's because spacecraft components have to be ultra-clean. A speck of dust, or even the oil from a fingerprint, could be enough to damage or interfere with the sensitive instruments on-board space-bound machinery. You also have to make sure a bunch of bacteria doesn't hitch a ride; otherwise, you could contaminate whatever world you're going to study.

So all that to say, cleaning spacecraft is a really important job. To remove any possible contaminants, spacecraft components have been cleaned with everything from the highly toxic trichloroethylene to various alcohols. These solvents are often aggressive.

They have to be, to get rid of the most stubborn substances. But they can also be dangerous for humans and the environment. So, like with propellants, scientists are looking into alternatives.

One possibility is supercritical carbon dioxide. Solid and liquid carbon dioxide are already used for cleaning, but you can unlock more of CO2's powers by making it supercritical. By keeping it above 31 degrees Celsius, and at pressures of more than 7.3 megapascals, CO2 becomes a supercritical fluid with properties of both a liquid and a gas.

More specifically, it flows and fills its container like a gas, but has a density like a liquid. That means it can penetrate into porous solids, and dissolve small, lightweight molecules. Back in 2014, NASA research showed that it can be used to remove sticky greases and clean small, delicate spacecraft parts with about 90% effectiveness.

For comparison, a simple rinse with the most common solvents managed to shift just 77% of the grease. As kind of a cool bonus, this carbon dioxide could be pulled straight from the atmosphere, which would mean no net greenhouse gases. And finally, we have one more cleaning option for you, and it might seem a bit more futuristic: plasma.

When a gas like oxygen or hydrogen is exposed to a strong electric field, the electrons are ripped from the gas' atoms, and that creates a high-energy mix of ions and electrons: a plasma. These particles have enough energy to attack any contaminants on a surface, like by knocking them off or by breaking them down into smaller pieces. So by sticking something in a chamber and blasting plasma at it, even oddly-shaped surfaces can be made squeaky clean.

Admittedly, the plasma can attack the outermost layers of whatever it is you're cleaning, but it's not enough to be actually noteworthy. Like with supercritical carbon dioxide, plasma cleaning is also pretty eco-friendly, since when you're done and the electric field is turned off, you're left with just the neutral gas you started with. Both of these cleaning methods have already been embraced by NASA at places like Kennedy Space Center.

But while the science is pretty well understood, the need for special equipment and controlled conditions means it's still pretty costly. At the moment, these green solutions to cleaning and propulsion aren't fully developed, or economically appealing, but they're a really promising step in the right direction. And as we keep learning, new innovations like these could really help shape the future of green space exploration.

Talking about subjects like this is a good reminder that there's no one, standard job in the space industry. It takes all kinds of skills to help us explore the universe. And if you want to brush up on some of your skills, you can check out a course at Coursera.

Coursera was founded in 2012 by two computer science professors from Stanford University. They wanted to share their knowledge with the world, so they put their courses online for anyone to check out. Today, they have over 3,000 courses covering everything from astronomy to artificial intelligence to data science.

So whether you're looking to make a career change or just grow in a skill you already have, there's something there for you. They even have a course about science writing and communication. Which, y'know, we're big fans of around here.

Right now, more than 35 million people around the world are learning on Coursera. And if you want to check them out for yourself, you can click the link in the description to learn more. [♪ OUTRO].