Space dust

15. SpaceX's unusual carbon problem

Jan 31, 2023

Note: The views expressed here are the author’s own and do not reflect the views of Energy Impact Partners.

Second note: We typically follow a certain format around here. We jump between a few different topics in climate, maybe related, maybe not. We have a good time. Today, we’re going to do something different: a single deep dive. As I got deeper into this topic, it felt wrong to do anything else. Let me know what you think about it.

Are launch emissions a big deal?

“When you’re getting ready to launch into space, you’re sitting on a big explosion waiting to happen.”
–Sally Ride

In climate tech, we tend to focus on reducing existing emissions sources, because, you know, they’re the ones causing climate change today. But the world doesn’t stand still. New emissions sources emerge from time to time. Three that emerged over the past decade include shale gas, Bitcoin mining, and permafrost methane. In this newsletter, we’ve covered a few others that have flown under the radar, such as nitrous oxide, carbon monoxide, and even hydrogen.

But what if I told you there was another new industry, worth $469B and growing 9% per year, with a large and poorly understood emissions footprint?

Today we’re going to spend some time in space.

SpaceX created a space renaissance

Since the space race of the ‘50s and ‘60s, humanity has steadily launched 100 to 200 objects per year into space. This turned out to be good enough for all sorts of things: GPS, Google Maps, satellite TV, and so on. But then in the 2010s, something changed. Space suddenly went exponential. By 2021, we were launching an eye-popping 1,800 objects into space per year, with no signs of slowing down.

This explosion is the result of two things:

We’ve made it easier to squeeze more objects into a rocket. With the popularization of micro- and nanosatellites, the size of the average object launched into orbit has gotten smaller.
We’ve made it easier to launch rockets, more than doubling the number of launches since the mid-2010s.

*Source: Wikipedia, analysis by author.*

Although this explosion of space activity has been driven by the US, the UK, and China, when you get down to it, it’s really been driven by one company: SpaceX. From a recent New York Times piece:

Mr. Musk had aspirations to land humans on Mars. Finding American launch vehicles absurdly expensive, he established Space Exploration Technologies Corp, better known as SpaceX. The key to undercutting competition, he eventually realized, was rocket reusability. In the concept SpaceX would achieve, the rocket would fly its payload to space, then land on Earth upright — essentially a launch in reverse.
By 2012, SpaceX had become the first company to dock a private cargo spacecraft with the International Space Station. Then, in 2015, it landed a Falcon 9 booster for the first time. In 2020, the company flew two astronauts to the space station and returned NASA to human spaceflight for the first time in nine years.
Today, Falcon 9 rockets and Dragon capsules are the workhorses of NASA, the Defense Department and private spaceflight. Many SpaceX employees are animated by a fervor for Mr. Musk’s vision: They want to play a role in permanently putting humans on Mars and making humanity a “multiplanetary species,” as Mr. Musk puts it. SpaceX launches and landings, once bordering on magic, have become predictable, if not monotonous.

SpaceX’s vision – lower costs from reusability – has worked out beautifully. In particular, the Falcon 9 and Falcon Heavy boast the lowest cost per kilogram of any launch vehicle in history.

*Source: CSIS Aerospace Security Project*

SpaceX is the primary beneficiary for now; most objects in low-earth orbit are now Starlink satellites. But more broadly, SpaceX has made a scarce resource abundant. What happens when you do that? Lower and lower value uses for that resource start to make sense, so people use more of it. SpaceX’s economies of scale have enabled new commercial applications in areas as diverse as high-speed internet, remote sensing, pharmaceuticals, semiconductors, carbon nanotubes, 3D-printed organs, and fiber-optics. Let’s take a quick look at one of them, courtesy of venture capital firm Fifty Years:

Pharmaceutical research, development, and production is a prime example. There are a myriad of therapeutic and bioengineering applications that could benefit from the microgravity of space: better immune response understanding for vaccine development; better protein crystallization for structure-based drug development; high quality protein assembly; better production of amorphous therapeutic compounds -- and the list goes on! In-space research and manufacturing may help treat diseases from cancer to Alzheimer’s. Some of the more promising research so far has focused on taking advantage of the absence of gravity-driven phenomena like convection and sedimentation to make larger and better ordered space-grown protein crystals to enable a structure-based approach to fighting disease. There’s a reason big pharmaceutical companies like Eli Lilly and Merck have already partnered with the International Space Station (ISS) to pursue better drug development.

This is unbelievably cool. I won’t keep listing things to do in space, but suffice it to say that there are more we know about and likely many more that we don’t. Satellites and space factories will get smaller and smaller, and launch costs will continue to go down. Using our newfound launch abilities to prove out a fraction of these things would represent important steps forward for humanity. It’s an amazing time to work in space.

We’re launching lots of black carbon into the atmosphere

That being said, it’s unfortunately time to start thinking about launch emissions – but not for the usual reasons.

To launch things into space, we load rockets with fuel mixtures known as propellants. Propellants release enormous amounts of energy very quickly, which is necessary to generate thrust and escape the Earth’s atmosphere. Only a few propellants can do the job; here are the main ones:

N2O4/UMDH: Dinitrogen tetroxide and unsymmetrical dimethylhydrazine. Insanely reactive and quite toxic. Soviet scientists called it “Devil’s venom.”
Solid propellants: Mixtures of inorganic oxidizers and fuels. “Particle factories that produce acid rains, ozone holes”. Most well-known in NASA’s Space Shuttle.
Hybrid propellants: Liquid or gaseous oxidizers with solid fuels. Similar issues to solid propellants.
LOX/RP-1: Liquid oxygen and highly refined kerosene. Reliable and popular, but sooty. In use by SpaceX’s Falcon 9 and Falcon Heavy.
LOX/liquid methane: Liquid oxygen and methane. Thought to be a cleaner-burning version of LOX/RP-1. Planned for use in several future rockets.
LOX/liquid hydrogen: Liquid oxygen and hydrogen. According to Space.com, “Green but weak fuel that can't do it on its own.”

This is a diverse set of fuels that will release a cocktail of emissions when burned. So what exactly do propellant emissions look like? As far as I can tell, studying their emissions in the context of climate change is a relatively new effort.

We’ll look at two studies, by Ryan et al. and Ross et al., both in the journal Earth’s Future. Here are the major emission they find:

Carbon dioxide: For once, not a very big deal. About 1% of emissions compared to aviation.
Water: Also not a big deal.1
Ozone, NOₓ, chlorine, and hydrochloric acid: These cause atmospheric and environmental issues, but not big global warming issues.
Aluminum oxide: A wildcard. Ross et al. found that alumina emissions caused some global warming, but Ryan et al. concluded that its effects were not well understood and so didn’t model it in.
Black carbon: Also known as soot. This is the main problem. We’re going to dig in here.

Black carbon is a familiar pollutant. We’ve been emitting it as long as we’ve had fire at our disposal, and only in the 20th century did we get good at scrubbing it out of our exhaust. It’s known to cause health issues, but it isn’t generally thought to be one of the top few climate threats. So why should it suddenly be a big deal when rockets emit it?

There are two big reasons. First, black carbon has a large global warming potential (GWP). For such a foundational pollutant, we don’t seem to understand its GWP very well, but estimates range from about 460 to 4,470 times that of carbon dioxide. For the sake of argument, Let’s take a middle-of-the-road estimate of 2,500.2 That’s bad. But we don't emit very much of it, and it doesn't last very long in the atmosphere. Not a huge deal on its own.

But second, oops: when emitted by rockets high up in the atmosphere, Ross et al. found that it’s about 500 times more warming than it is at ground level. Now we’re talking about a seriously high GWP:

Effective GWP = 2,500 × 500 = 1,250,000

This is a big deal if true. I’ve never seen a GWP anywhere near that high, and I’m a climate weirdo.3 A GWP of 1,250,000 means that in order to have the same effect as gigatons of carbon dioxide, you need to emit only kilotons of black carbon in the upper atmosphere. It doesn’t take a lot to do that.

To this point, Ryan et al. estimated that in 2019, the world’s rockets emitted about 500 tons of black carbon. We're almost there. With an effective GWP of 1,250,000, that would represent the equivalent of 625 megatons of carbon dioxide, or about 1.3% of total greenhouse gas emissions.4

That was 2019, and when you’re on an exponential curve, three years is an eternity. So if you make the same assumptions as Ross et al. and scale up to 2022’s 186 launches, you get about 910 tons of black carbon. On top of that, the fleet is shifting toward the LOX/RP-1 propellants driving these emissions, so it’s likely even higher. Even if this math is off by a factor of 10, it’s not looking good.

There are reasons to be skeptical

I have this nagging feeling that that this isn’t as big a deal as it seems on paper. We’ve probably emitted much more black carbon in the past without causing extreme warming. Am I missing something? Here are two things I can’t quite square:

A 2021 study found annual black carbon emissions to be about 4-8 megatons in 2017. A ground-level GWP of 2,500 would suggest that this is equivalent to 10-20 gigatons of carbon dioxide. This is out of step with most recent climate assessments. This seems possible, but it also seems unlikely that we’d have missed that given how much work has gone into characterizing climate change.
In times when everything was powered by burning coal and wood with little scrubbing, I'd be surprised if we weren't emitting more than 500 times as much black carbon than the space industry is now emitting. In many developing economies, home cooking is still a major source of black carbon. A 2023 study found that to be more or less true, estimating that single-digit megatons of black carbon have been emitted since the mid-1800s. Similarly, it seems unlikely that we’ve just missed how big a deal that much black carbon was.

That being said, that same 2023 study found that historic measurements of black carbon emissions, and therefore many of the inputs we now use in climate models, were wrong. It seems that we just don’t understand these emissions very well.

What can we do about black carbon?

Rocket soot is not like other greenhouse gases. For one thing, it’s not even a gas. So if we look to fix this issue, the usual tools in the environmental toolkit won’t apply:

Sustainable fuels: Out. They offset the carbon dioxide, but carbon dioxide isn’t the problem.
Hydrogen: Out. Liquid oxygen and liquid hydration aren’t a powerful enough propellant on their own for widespread use.
Particle scrubbing: Out. Where would you even place a scrubber?

It seems like there are three options left:

LOX/liquid methane: Methane burns more cleanly than kerosene, while putting out almost the same amount of power. However, things behave differently under extreme conditions; I think it remains to be seen how much black carbon methane eliminates. Keep an eye on Stoke Space, which is developing a rocket based on liquid methane and liquid hydrogen propellants in different stages.
Chemical additives: Reactive fuel additives that either prevent soot formation or quickly burn up soot after it forms. But how do you pull this off? Whatever chemical you choose has to be A) stable enough to survive rocket flames, B) reactive enough to chew up soot, and C) safe enough to avoid causing unintended impacts in the atmosphere.
Entirely new propellant formulations: What else can pack enough power to launch heavy objects into space?

Any one of these is a tall order, but the payoff could be high. The flip side of the coin is black carbon’s is short lifetime in the atmosphere. If we suddenly cut emissions, its effects would go away within weeks, not years.

Where do we go from here?

In cheaply launching things into space, we’ve found a new valuable thing to do. It requires some fuel and causes some pollution. We now have to ask ourselves some tough questions: how much do we like the new thing, and how bad is the pollution? It’s hard to answer those questions honestly when we can’t yet know the magnitude of the upside and can only guess at the severity of the downside. So while entrepreneurs work to discover what space can do for us, we need the scientific community to study its effects on the climate in greater detail. So are launch emissions a big deal? I don’t know, but it’s time to find out.

“Fix your little problem and light this candle.”
–Alan B. Shepard

Special thanks to Erika Reinhardt and Spark Climate Solutions for helpful discussions.

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