There are a growing number of fans of space exploration and colonization, but they seem to be totally clueless about the potential environmental problems from rocket emissions. Many of these space fans turn to Tim Dodd, who is known as the “Everyday Astronaut” on the internet, to get the latest news from SpaceX, Blue Origin, ULA, Virgin Galactic, Boeing, Rocket Lab, etc. Unfortunately, people like Tim Dodd are not reliable sources of information when it comes to the environmental impact of rockets.
Dodd recently produced an article and video about the emissions from rockets. Dodd should be commended for trying to calculate the emissions for a number of different rocket models. However, he either hasn’t read the papers by critical investigators such as Dr. Martin Ross or his boosterism of space exploration makes it difficult from him to understand the potential problems with a massive increase in rocket launches.
In response, to Dodd’s article, I posted this critical comment, pointing out what he missed in his article:
Tim, I appreciate the time that you took to calculate the emissions for each type of rocket, but you did a real disservice to the community by not clearly explaining how rocket emissions threaten the environment. M. Ross et al (2009) estimate that rocket emissions only account for 0.03% of ozone depletion, but they predict that rocket emissions will deplete the ozone layer more than CFC emissions by the year 2050. See:
The big threat to the ozone layer is the growing use of rockets that are fueled by ammonium perchlorate and alumina, which will be driven by the future boom in the space tourism industry. Virgin Galactic’s Unity uses 15,000 lbs of fuel, which we can guesstimate is 68% NH4ClO4, 18% Al and 14% HTPB. Considering that Virgin Galactic already has reservations for 600 passengers / 100 flights, it is likely that Virgin Galactic will be launching several thousand times per year by 2040.
Your article focuses on CO2 emissions which is totally wrongheaded, because the vast majority of radiative forcing (RF) from rockets emissions comes from black carbon, alumina and water vapor. Unlike in the troposphere, where black carbon and alumina particles quickly disappear, these particles stay in the troposphere for roughly 4 years on average, so they cause a lot more warming over time. Ross and Sheaffer (2014) calculate that rocket emissions produce about 16 mW/m2 of RF, which is about a fourth of the RF of the global aviation industry. See: https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2013EF000160
The amount of RF is probably higher today than what Ross and Sheaffer calculated, because of the increasing number of kerosene rocket launches by SpaceX. Ross and Sheaffer believe that black carbon has a global warming potential (GWP) of 50,000 due to how long it lasts in the stratosphere, making it the most potent warming substance produced by humans. Ross and Sheaffer calculate that a kerosene rocket produces 30 times the radiative forcing of a comparable hydrolox rocket. If the use of solid and kerosene rockets keeps growing, the rocket industry could produce more radiative forcing than the aviation industry in the future.
We don’t know exactly how long particles stay in the stratosphere at altitudes of 20-30 km where most of the ozone layer is located. We need a lot more study of the impact of black carbon, alumina particles, methane and water vapor in the stratosphere. Despite that uncertainty, I believe that rockets using ammonium perchlorate and alumina fuel should banned (except for non-regular uses such as emergency abort systems and ballistic missiles). If we look at the amount of black carbon that kerosene rockets produce, we need to move to methalox and hydrolox rockets as quickly as possible.
If the plans of SpaceX, Blue Origin and ULA come to pass, there will be massive numbers of methalox rocket launches in the future. I haven’t found any papers that calculate the radiative forcing and ozone depletion from methalox rockets. They seem to be better for the environment than solid and kerosene rockets, but there are still serious questions about whether the water vapor, NOx, CO2 and unburned methane that they emit will have serious effects in the stratosphere. What percentage of methane isn’t burned in the Raptor and BE-4 engines? I can’t find any estimates. In the troposphere, methane converts to CO2 after an average of 9.1 years, so it is calculated that 1 gram of methane has the same global warming potential (GWP) as 33 grams of CO2, but if methane doesn’t convert to CO2 very quickly in the stratosphere, then its GWP could be much higher. The water vapor from rocket emissions will stay much longer in the stratosphere than in the troposphere, so the warming effect from rocket contrails could be much larger. If the growing emissions from rockets heat up the stratosphere, then the chemical reactions that destroy ozone could also speed up, and we could potentially destroy the ozone layer.
We don’t have to worry too much today, but if Virgin Galactic has thousands of solid rocket launches per year and SpaceX is launching StarShip tens of thousands of times per year, then we really do have to worry. Elon Musk is planning to send a million colonists to Mars, which would mean 10,000 passenger flights and 100,000 equipment flights of StarShip to Mars. If we guesstimate 5 launches to fuel up StarShip for each flight, then that means a total of 550,000 launches. We had better be really sure that we know the impact of methalox rockets in the stratosphere before we start colonizing space, because we can’t take the risk of destroying the ozone layer.
It is clear that Dodd did a lot of research for his article on rocket emissions, so he probably ran into some of the Martin Ross’s work, but he seems to have either ignored it or done mental gymnastics to dismiss it. Sadly, this is the response of many smart people when they find out that their favorite activity has large environmental impacts. We can always find ways to justify what we appreciate, whether it be red meat, private cars, air flight or space flight. The challenge is trying to be honest with ourselves about the impacts of what we humans do to our planet.