Spacecraft re-entry in the atmosphere has consequences. Many survive intense heat and friction through ablation, drawing heat away from the craft's surface. This generates byproducts like nitrogen oxides (NOx).
Emissions contribute to atmospheric pollution and affect aerosol properties in the stratosphere. The complex cocktail of chemicals includes metallic oxides and NOx. Lithium, aluminum, silicon, copper and lead are also shed.
According the the European Space Agency (ESA):
"We measured metals that vaporized during spacecraft reentries in stratospheric sulfuric acid particles. Over 20 elements from reentry were detected and present in ratios consistent with alloys used in spacecraft. The mass of lithium, aluminum, copper and lead from the reentry of spacecraft was found to exceed the cosmic dust influx of those metals. About 10% of stratospheric sulfuric acid particles larger than 120 nm in diameter contain aluminum and other elements from spacecraft reentry."
Ablation occurs during re-entry as materials are subjected to extreme heat and pressure. As a spacecraft descends through the atmosphere, it deals with temperatures up to 3000°C.
The intense heat causes intended erosion or vaporization of components, which lift the heat away from the craft. These are often heat shields made from metals like titanium, aluminum and alloys. Tungsten, a heavy metal, can survive temperatures up to 3422°C.
Titanium, tungsten and aluminum oxidize by reacting with atmospheric oxygen, to generate metallic oxides. An example is aluminum oxide (Al2O3), which contributes to changes in atmospheric aerosol composition.
The amount of metallic oxides generated are directly linked to the type of materials used in spacecraft construction, as well as specific conditions during re-entry. Re-entry also produces nitrogen oxides (NOx).
NOx is a collective term for nitrogen monoxide (NO) and nitrogen dioxide (NO2), both of which are potent atmospheric pollutants. NOx are already a major concern, active in smog formation, acid rain, and ozone depletion.
Production of NOx during re-entry happens when heat generated during re-entry causes the direct thermal dissociation of nitrogen and oxygen molecules in the air, followed by their recombination into NOx.
The ablative materials themselves may contain nitrogen-bearing compounds, releasing NOx upon decomposition. The major concern with NOx is deposition in the stratosphere.
The stratosphere includes the ozone layer, which shields the Earth from ultraviolet radiation. NOx in the stratosphere catalyzes reactions which destroy ozone molecules.
NOx influences stratospheric aerosol properties. Stratospheric aerosols are particles suspended in the upper atmosphere, needed to regulate global temperatures.
They reflect sunlight back into space, helping cool the planet; or they absorb heat, causing the atmosphere to warm up. NOx reacts with other atmospheric components to form nitric acid (HNO3) and condense to create new aerosols or alter size and composition of existing ones.
Changes in stratospheric aerosol properties can affect precipitation patterns and temperature distributions. Once NOx are in the atmosphere, they trigger chemical reactions causing ground-level ozone formation.
Ground-level ozone is a harmful pollutant. It causes respiratory problems in humans and harms plant and wildlife growth. NOx emissions contribute to approximately 40% of the formation of atmospheric ozone.
The frequency of space launches and re-entries is projected to increase dramatically, driven by the burgeoning commercial space industry and renewed interest in lunar and Martian exploration.
While pollution from spacecraft re-entry is currently small compared to industrial emissions, it is a growing concern as space missions increase.
Emissions from spacecraft re-entries will account for 2% of global nitrogen oxides by 2035.
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