Pioneering Sustainable Space Exploration, pt.1: CO2 emissions
A single Starship launch emits as much CO2 as 583 average US cars in a year. Is it time to start worrying about the environmental cost of Space tech?
Issue 94: Astronauts: 11 585.
Space exploration has long captivated humanity's imagination, but as we reach for the stars, the sustainability of our methods comes under scrutiny. Traditionally, the environmental impacts of spaceflight were thought to be negligible, similar to how airplane emissions were once considered inconsequential before air travel became commonplace. However, there is now a movement to reduce CO2 emissions in aviation and promote easy, regular human space travel, which is significantly more CO2 intensive. If we envision a future space economy that is both cheap and sustainable, we must consider space sustainability.
The environmental impact of launching and maintaining spacecraft is significant, involving high resource consumption, potential harm to Earth’s ecosystems, and the growing problem of space debris.
Sustainability in space activities means ensuring that our celestial endeavors do not compromise the ability of future generations to explore or utilize space. This encompasses everything from minimizing the greenhouse gas footprint of launches to designing missions that leave minimal debris in orbit (we have discussed space debris in a couple of articles: one and two).
The Ecosystem Impact of Rocket Launches
Rocket launches are more than just visually spectacular; they also have tangible impacts on the environment. A notable concern is carbon dioxide (CO2) and other GHG emissions. For instance, a single Falcon 9 launch produces approximately 425 metric tons of CO2, equivalent to about 73 cars driving for one year. This figure increases significantly up to 2,683 tons with larger rockets like the Starship.
Additionally, the ozone layer is at risk. Rockets can emit chlorine and other chemicals that potentially deplete ozone, though the full impact is still under investigation.
Rocket production also contributes a significant carbon footprint, a topic that remains controversial and complex due to varying methodologies used to measure these emissions. It's crucial to consider not just the fuel burned but also the broader implications of building and reusing rockets, which can help reduce the overall environmental impact.
In this article, we assume that the carbon footprint of rocket production in an ideal world is minimized by reusing all rocket stages so that we can focus on the CO2 emissions from the fuel burned. By considering only this narrow aspect of sustainability and by omitting the harm calculations to the ozone layer, by rocket production footprint, space debris, and more, we aim to demonstrate how broad and nuanced this topic is even in this narrow aspect. We will cover other ecosystem impact topics in our forthcoming articles.
Carbon Inefficiency of Rocket Engines
Rocket engines are carbon inefficient, a fact that becomes apparent when observing the extensive exhaust trail they leave in the sky, which conjures images of substantial gas emissions. To substantiate this numerically, we will compare rocket engines to airline engines (or, generally, air-breathing engines) throughout this article. A key metric for this comparison is specific impulse, which measures how effectively fuel is utilized to generate thrust. Aircraft engines, such as turbofans, have specific impulses near 6,000 seconds (60,000 m/s in metric units). In contrast, engines like the Raptor used in Starship achieve specific impulses of about 380 seconds in vacuum (3,800 m/s).
This inefficiency arises partly because rockets must carry their oxidizer, increasing the weight and reducing overall efficiency. In contrast, aircraft engines are air-breathing and utilize the oxidizer from the atmosphere. Furthermore, although nitrogen does not participate in combustion, it constitutes a significant portion of the expelled mass, enhancing the specific impulse of air-breathing engines.
Additionally, rockets exhibit lower-than-theoretical efficiency in the lower atmosphere, where they cannot fully expand their exhaust against atmospheric pressure, which is higher, than the exhaust pressure. This further reduces their specific impulse.
Comparative Emissions: Rockets vs. Aviation
We can see significant differences in CO2 emissions between rockets and aircraft, not only in terms of specific impulse but also in terms of integral emissions. Rockets like the Falcon 9 emit approximately 425 tonnes of CO2 per flight, while a large aircraft emits about 80 g of CO2/passenger/km. For example, a flight from New York to Los Angeles (4000 km) with a Boeing 747 (416 passengers) will add 128 metric tonnes of CO2 to the atmosphere (use this ICAO calculator to estimate your flight’s carbon footprint).
Despite the high emissions per flight, the overall environmental impact of rockets is currently less significant than that of the aviation industry, mainly due to the much lower frequency of rocket launches. For instance, rocket launches accounted for only 0.0000059% of global CO2 emissions in 2018 (112 launches, growth to 212 in 2023), whereas the airline industry contributed about 2.4% of global CO2 emissions that same year (37.8 million departures). This disparity shows that while individual rocket launches are more polluting, the total impact of the space industry is minor compared to aviation, due to the vastly greater number of airplane flights. It needs an astonishing ~ 20 million rocket launches a year to generate just 1% of global CO2 emissions.
Looking into the future, particularly with projects like SpaceX's Starship which aims for high-frequency launches possibly comparable to airline flights, the environmental impact could become more significant. A single launch of SpaceX's Starship, when considering both the Super Heavy booster and the Starship itself, emits approximately 2,683 tonnes of CO2. It is as much as 583 average cars would emit in the US a year. If Starship launches were to become as frequent as commercial airline flights, it would dramatically increase the space industry's share of global emissions. This highlights the importance of continuing to develop and implement cleaner technologies in rocket design and operation.
Innovations Driving Sustainability
The push for more sustainable rocket technologies is led by advancements in reusable rocket technology. Companies like SpaceX and Blue Origin lead this effort, successfully landing and reusing rockets multiple times, which drastically cuts the need for new materials and reduces the energy consumed in manufacturing.
In propulsion, green alternatives are gaining ground. Hydrogen is seen as a cleaner alternative to carbon-based fuels such as methane and kerosene (RP-1), emitting virtually only water as exhaust. However, the reality is more complex; the hot exhaust, when interacting with atmospheric air, reacts with nitrogen to produce nitrous oxide, thereby contributing to greenhouse gas emissions. Additionally, while water vapor is environmentally friendly in the lower atmosphere where it quickly cycles into oceans, in the upper atmosphere, it can linger longer and contribute to the greenhouse effect. Precise climate models are still needed to describe this process numerically.
Methane, though less green than hydrogen, is still considered a cleaner alternative, producing fewer soot particles and other pollutants than kerosene. It is notably used in SpaceX's Raptor engines and is recognized for its lower carbon footprint compared to other rocket fuels like RP-1. The relatively low carbon footprint of methane is attributed to its chemical structure, having only one carbon atom for every four hydrogen atoms. It is therefore the greenest possible hydrocarbon (see the image caption below for details).
Future Directions and Technologies
For the next couple of decades, avoiding solid-state boosters to save the ozone layer, prioritizing reusability, and utilizing reliable, cheap, and efficient methane fuel will suffice and cause minimal harm to the atmosphere, given the relatively low number of rocket launches.
Looking ahead, the next major step toward sustainability in space exploration will be necessary when the number of rocket launches approaches that of aircraft flights. This evolution could involve more radical innovations, such as air-breathing engines for the first stage of rockets, which would dramatically reduce the need for heavy chemical propellants. Currently, this approach is seen in air launches—using a regular aircraft as the first stage. So far this is not economical, a point on which we can agree with Tim Dodd; see his video where he explains in detail why. Virgin Orbit’s bankruptcy supports his points. But why is it not economical? Special aircraft optimized for rocket launches do not yet exist. Present aircraft are optimized for many hours of relatively slow horizontal flight at an altitude of about 10 km. What we need is an aircraft capable of a short, mostly vertical flight, where only takeoff and landing are horizontal, and the aircraft’s body, not its wings, is used to host the second stage and payload. The air-breathing engine should be optimized for high-altitude, high-speed flight in lower-density air. Here we come to the concept of an aircraft close to that of Skylon.
Additionally, the use of nuclear or electric propulsion for 3rd stage could increase efficiency and reduce emissions, making long-duration interplanetary missions more feasible. Yes, they can not be used at the 2nd stage where high trust is needed, but they reduce the integral mass of the payload and the 3rd stage, making the 1st and the 2nd stages lighter as well.
The journey towards sustainable space exploration is complex and challenging, involving numerous technological and economic factors. The space industry needs to continue pushing the boundaries of innovation and sustainability, ensuring that as we advance our capabilities to explore new worlds, we also preserve our own.
While rocket technologies may not be the most efficient, it's important to recognize that their overall environmental impact remains relatively minor compared to other industries, due to the limited number of launches. Currently, the payloads we send into space often provide significant environmental benefits. These include monitoring greenhouse gas emissions and optimizing routes, etc. Thus, when discussing the negative effects, we must also consider the substantial benefits these technologies provide to both the economy and humanity, although that's a topic for another discussion.