With the cost of launching a rocket into space falling, the number of rocket launches is, well, taking off. Last year, governments and companies across the world successfully launched 133 rockets into orbit, breaking a record that stood for 45 years.
But there's a catch. Breaking free from Earth's gravity requires a rocket to release a tremendous amount of energy in a short period of time. As a rocket leaves Earth, it produces hot exhaust that changes the physics and chemistry of the atmosphere as it passes through. In a paper published Tuesday in the peer-reviewed journal Physics of Fluids, a pair of physicists simulated the launch of a SpaceX Falcon 9 rocket blasting into space.
They found several reasons to be concerned.
The carbon footprint isn't the problem
Rockets aren't responsible for putting that much carbon dioxide into the atmosphere. A typical launch burns roughly the same amount of fuel as a day-long commercial flight but produces seven times as much CO2 — between 200 and 300 tons — as the airliner. That's far more carbon than the average person will generate in their lifetime, but it's a rounding error compared to the 900 million tons of CO2 the aviation industry was spewing annually before the pandemic.
But that's not the whole story. “We don't care about a rocket's carbon footprint. That's irrelevant," says researcher Martin Ross. For him, it's the particles contained in rocket exhaust — chiefly alumina and black carbon — that really matter. “These particles scatter and absorb sunlight. They change the temperature and circulation of the stratosphere,” Ross says.
Unfortunately, scientists only have a faint understanding of the total environmental impact of a rocket launch. “The current level of data about rocket emissions does not provide researchers with enough information to fully assess the impact of launches on the global environment,” Ross says.
The effect of carbon emissions high in the atmosphere is uncertain
The researchers behind the new study are bringing the problem into sharper focus by modeling the exhaust from the nine nozzles of a Falcon 9 rocket as it launches into space. These simulations incorporate data about the rocket and its propellant (RP-1) with equations that describe how gases behave under various conditions. Thanks to some serious computing power, the researchers were able to predict how exhaust behaves after exiting the nozzles, at increments of roughly 0.6 miles (1 km) in altitude.
The researchers analyzed the launch by comparing the volume of exhaust released during one kilometer of upward travel through a certain band of the atmosphere (e.g. between 2 km and 2.99 km) with the properties of the atmosphere at that specific altitude. They had to adopt this somewhat confusing method because the physical and chemical makeup of the atmosphere is different at different altitudes.
They found that the amount of total exhaust is "negligible" compared to the air around it, even at high altitudes. That's a surprise because the atmosphere is much less dense at higher altitudes. According to their calculations, the amount of exhaust released by a Falcon 9 as it travels between 70 km and 70.99 km (roughly 43 miles) is just one-fourteenth the amount of mass found in one cubic kilometer (roughly .25 mi3) of air at that altitude. (This is conveyed by the blue line in the chart below.)
What isn't negligible is the amount of CO2 that a Falcon 9 introduces into higher levels of the atmosphere as it passes through (represented by the dotted red line in the figure above). Once it passes an altitude of 27 miles (43.5 km), a rocket starts emitting more than one cubic kilometer's worth of CO2 for each kilometer it climbs. By the time it reaches 43.5 miles (70 km), a Falcon 9 releases more than 25 times the amount of CO2 found in a cubic kilometer of air at that altitude.
And rocket exhaust contains more than carbon
It's more than CO2. "Perhaps even more crucially, the [amount of] carbon monoxide (CO) and water (H2O) [in rocket exhaust] are of a similar order as carbon dioxide," the authors write. That's a concern because there's hardly any carbon monoxide or water high in the atmosphere. "Therefore, these compounds’ emissions at high altitudes introduce an even more significant contribution/rise to the existing, if any, trace amounts already present."
Water vapor immediately freezes at that altitude, but researchers have no idea where those ice crystals end up. Carbon monoxide reacts with hydroxide (O2) to form even more CO2. The researchers also discovered that dangerous exhaust emissions called thermal nitrogen oxides (NOx) can stick around for a long time in hot rivers before dispersing throughout the atmosphere, especially at lower altitudes.
The future is uncertain, but researchers and regulators are paying attention
With just more than 100 launches per year, some say that pollution from rockets isn't an issue. "One of the arguments that people have used in the past was to say that we don't really need to pay attention to rockets or to the space industry, or the space industry is small, and it's always going to be small," Ross says.
He doesn't agree. "I think the developments that we're seeing the past few years show that … space is entering this very rapid growth phase like aviation saw in the '20s and '30s."
The authors behind the new study feel the same way. "We believe that the problem of atmospheric pollution caused by rocket launches is vital and needs to be addressed appropriately as commercial space flights, in particular, are expected to increase in the future," they write.
The problem of pollution from rockets slowly coming into clearer focus, and it's being taken seriously in high places. Later this year, the World Meteorological Organization and the UN Environmental Program will release new a report that summarizes how rocket emissions deplete ozone. With any luck, this attention will cause atmospheric pollution to become a key factor in the design of future rockets.