Space Exploration Isn’t Great for the Earth (But It Could Be)


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]

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