Physicists have figured out how launching a Falcon 9 changes the atmosphere


With the falling cost of launching a rocket into space, the number of rocket launches is taking off. Last year, governments and corporations around the world successfully launched 133 rockets into orbit, breaking a record that stood for 45 years.

But there is a catch. To free itself from Earth’s gravity, a rocket must release an enormous amount of energy in a short period of time. When a rocket leaves Earth, it produces hot exhaust gases that change the physics and chemistry of the atmosphere as it passes. In an article published Tuesday in the peer-reviewed journal Fluid Physicsa pair of physicists simulated the launch of a SpaceX Falcon 9 rocket blasting into space.

They found several reasons to worry.

The carbon footprint is not the issue

Rockets are not responsible for emitting as much carbon dioxide into the atmosphere. A typical launch consumes roughly the same amount of fuel as a one-day commercial flight, but produces seven times as much CO2 — between 200 and 300 tons — as an airliner. That’s far more carbon than an average person will generate in their lifetime, but that’s a rounding error compared to the 900 million tonnes of CO2 the aviation industry spat every year before the pandemic.

But that’s not the whole story. “We don’t care about a rocket’s carbon footprint. It’s irrelevant,” says researcher Martin Ross. alumina and carbon black — which really matter.” These particles scatter and absorb sunlight, he says.

Unfortunately, scientists have little understanding of the total environmental impact of a rocket launch. “The current level of rocket emission data does not provide researchers with enough information to fully assess the impact of launches on the global environment,” Ross said.

The effect of high carbon emissions in the atmosphere is uncertain

The researchers behind the new study shed more light on the problem by modeling the exhaust from the nine nozzles of a Falcon 9 rocket as it launches into space. These simulations integrate data on the rocket and its propellant (RP-1) with equations describing the behavior of gases under various conditions. Using significant computing power, the researchers were able to predict the behavior of the exhaust gases after they exit the nozzles, at increments of about 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 (eg 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 total amount of exhaust is “negligible” compared to the air around it, even at high altitudes. This is 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 when it travels between 70 km and 70.99 km (about 43 miles) is only one-fourteenth the amount of mass found in a cubic kilometer (about 0 .25 miles3) of air at this altitude. (This is indicated by the blue line in the table below.)

Amount of mass contained in one cubic kilometer of exhaust relative to ambient air.

What is not negligible is the amount of CO2 that a Falcon 9 introduces into the upper layers of the atmosphere during its passage (represented by the red dotted line in the figure above). Once it passes an altitude of 27 miles (43.5 km), a rocket begins emitting over a cubic kilometer of CO2 for every 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 one cubic kilometer of air at this altitude.

And rocket exhaust contains more than carbon

It’s more than CO2. “Perhaps more importantly, the [amount of] carbon monoxide (CO) and water (H2O) [in rocket exhaust] are of the same order as carbon dioxide,” the authors write. This is concerning because there is virtually no carbon monoxide or water in the atmosphere. “Therefore, emissions of these compounds at high altitude introduce an even greater contribution/increase of the existing, if any, minute amounts already present. »

Water vapor freezes immediately at this altitude, but researchers don’t know where these ice crystals end up. Carbon monoxide reacts with hydroxide (OH) to form even more CO2. Researchers have also found that dangerous exhaust gases called thermal nitrogen oxides (NOx) can linger in warm rivers for a long time before dispersing into the atmosphere, especially at low altitudes.

The future is uncertain, but researchers and regulators are watching

With just over 100 launches a year, some say rocket pollution is not a problem. “One of the arguments people have used in the past was that we don’t really need to pay attention to rockets or the space industry, or the space industry is small, and it always will be small. “Ross said. said.

He does not agree. “I think the developments we’ve seen in recent years show that…space is entering this phase of very rapid growth like aviation saw in the 1920s and 30s.”

The authors of the new study agree. “We believe that the problem of air pollution caused by rocket launches is vital and must be addressed appropriately, as commercial spaceflight, in particular, is expected to increase in the future,” they write.

The problem of rocket pollution is slowly becoming clearer, and it is being taken seriously in high places. Later this year, the World Meteorological Organization and the United Nations Environmental Program will release a new report that summarizes how rocket emissions deplete the ozone layer. Hopefully, this attention will make air pollution a key factor in the design of future rockets.

Fixed: A previous version of this article incorrectly states that a launch releases approximately the same amount of CO2 like a one-day flight. In fact, a typical launch consumes the same amount of fuel as a one-day flight, but produces about seven times as much CO2 emissions.


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