Rocket launches used for space exploration continue to gain popularity as the human population grows and technology advances toward the future. Large increases in rocket launch demand could come about for a variety of reasons, including national decisions regarding security, space exploration, significant reductions in launch costs, or the emergence of new markets such as space tourism, manufacturing, or solar power. Rocket launches can affect the natural environment, most specifically the composition of the atmosphere as they travel through it. A major environmental problem faced today is the depletion of the ozone layer in the atmosphere by rockets and other molecular species such as chlorofluorocarbons. Up to forty percent of the total ozone present in the wake of a rocket is destroyed from rocket plumes. Ozone concentrations are calculated using ultraviolet and visible light radiometers attached to rockets. Calculations analyzed from these radiometers are where scientists first noticed the depletion of ozone caused by rockets.
When a rocket launches, it sends out large amounts of gas called a rocket plume. A rocket’s plume is made of various gases that are a result of the combustion of the fuel source used to propel the rocket into space. There are two main types of rocket engine fuels: solid rocket motor fuels and liquid rocket engine fuels. Solid rocket fuels are considered to be more harmful to ozone than liquid rocket fuels. It is estimated that global ozone depletion results from a mixture of solid and liquid rocket fuel emissions. Solid rocket fuels are more durable and do not have to be stored under certain conditions. Liquid rocket fuel is better in use because the rocket thrust can be adjusted mid-flight unlike solid rocket fuel that burns at a fixed rate without any mechanism to alter it.
Solid rocket motors account for approximately one-third of the stratosphere propellants in the atmosphere from rockets while liquid propellants account for the other portion. Solid rocket fuel is composed of aluminum, ammonium perchlorate, and a polymer matrix. The combustion of this solid fuel gives rise to chlorine in the exhaust that originates from the ammonium perchlorate.
Hydrochloric acid is the main volatile compound created by combustion in solid rocket motor plumes. Hydrochloric acid is what accounts for majority of the chlorine in rocket plumes. The process called afterburning accounts for conversion of hydrochloric acid into pure chlorine molecules. Afterburning occurs when rocket engines burn propellants with an excess of fuel compared to the oxidizer. This conversion takes place due to the high concentration of oxygen and hydrogen molecules. A possible reaction for this occurrence is OH- + HCl (g)-> H2O(l) + Cl-.
A chlorine mechanism such as the one below is the main cause of ozone destruction.
2(Cl + O3 -> ClO + O2)
ClO + ClO + M -> Cl2O2 + M
Cl2O2 + Cl -> Cl2 + ClOO
Cl2 + hv -> 2Cl
ClOO + M -> Cl + O2 + M
Net: 2O3 -> 3O2
What occurs is chlorine (Cl2) combines with ozone (O3) after exposure to light to form a hypochlorite (ClO) molecule and diatomic oxygen (O2). This results in affecting the oxygen available to breathe in. The hypochlorite (ClO) then combines with another hypochlorite in the presence of a third body (M), which is just a solid surface that allows the reaction to take place. The reason this reaction is so harmful to ozone is the chlorine is able to be recycled and be eligible to destroy another ozone molecule again in a continuous cycle. On average, over five to ten molecules of ozone are consumed by every chlorine atom deposited in a rocket’s plume originating from the rocket fuel. Chlorine was confirmed to be the greatest contributor to ozone loss resulting from rocket plumes due to data collected in night rocket launches. This is due to the need of sunlight to activate the chlorine in order to initiate the destruction of ozone cycle by chlorine molecules.
Other rocket fuels used in rockets are liquid rocket fuels. Combustion of this type of fuel was once thought to cause some ozone destruction by a nitrogen mechanism. Nitric oxide is a known mechanism to destroy ozone and was once present or created from combustion of liquid rocket fuels; however this has mostly been eliminated. The nitric oxide mechanism is not as ozone depleting as solid-fueled rockets. Destruction of ozone by nitric oxide is short and immediate after launch, minimizing the time window for destruction. This seems to not allow enough time for the rocket to reach the stratosphere, where ozone is most abundant. The nitric oxide mechanism also has a slower catalytic cycle compared to the hypochlorite mechanism. Overall, liquid rocket engines are less harmful to stratospheric ozone than solid rocket motors.
On a global scale today, rockets have a negligible impact on destruction of the ozone layer. With the increase in rocket launches predicted to occur in the future, rockets may have a larger impact on the ozone layer. It is important to consider changes in fuel sources to minimize the effects. Advancements have been made to make liquid rocket fuel non nitrogen-based to reduce the effects of nitric oxide caused ozone loss. Using chlorine-free rocket fuel is another future approach to help eliminate the effect of chlorine radicals destroying ozone. Chlorine is shown to cause the most damage to the ozone layer from rocket fuel as well as overall due to it being used in a catalytic cycle of destruction. Eliminating chlorine from rocket fuel could have a large impact on protecting the ozone layer from future rocket launches. Solid fueled rockets contribute negligibly to ozone depletion on a global scale due to the understanding of its harmful affects on the ozone layer. With the increasing abundance of space exploration, the use of these solid fueled rockets should be taken into consideration, as they will increase the impact on the global ozone layer. Solid rocket motors cause over sixty times more damage to the ozone layer compared to liquid rocket engines. It is shown to be more efficient to use liquid rocket fueled compared to the traditionally used solid rocket fuel. Rocket emissions have remained unregulated. Rocket launches will continue to increase in abundance into the future and will cause future problems when it comes to the earth’s precious ozone layer. The chlorine-free rocket fuels will help decrease the destruction of ozone molecules in the atmosphere due to rocket launches.