Rockets, reentry, and the stratosphere: A clearer picture of how spaceflight pollutes the air above our weather

Background

For decades, the environmental footprint of rockets was treated as a rounding error. Launches were rare, the Space Shuttle’s chlorine‑rich plumes were exceptional but episodic, and global climate policy focused on the colossal emissions from ground transport, power generation, and aviation. That calculus is changing. The satellite economy has exploded, reusable boosters have pushed costs down and flight rates up, and hundreds of spacecraft now deorbit every month. What used to be an occasional experiment has become a weekly industrial activity that reaches deep into layers of the atmosphere where aircraft simply do not go.

Why this matters has less to do with total mass and more to do with where and what rockets emit. The stratosphere—roughly 10 to 50 kilometers above Earth, depending on latitude—acts like a semi‑sealed attic above the weather. Particles and gases lofted there persist far longer than they would in the turbulent lower atmosphere. That persistence, combined with the chemistry of rocket plumes and the byproducts of spacecraft reentry, means a kilogram of exhaust injected into the stratosphere can have an outsized effect on ozone and climate compared to the same kilogram released at Earth’s surface.

Several lines of research have converged in recent years:

• Direct sampling flights and lidar observations that detect unusual metal‑rich particles in the stratosphere, a fingerprint of modern reentries.
• Updated chemistry‑climate models that simulate how different propellants—kerosene, methane, hydrogen, and solid fuels—perturb ozone and temperature when burned at high altitude.
• Emissions inventories that now include not only launches but also the full arc of reentry: upper stages, satellites, and debris burning up.

The result is a more complete picture: while rockets still contribute a tiny share of global greenhouse gases, they have a disproportionate influence on a part of the atmosphere we rely on to shield life from ultraviolet radiation and to buffer the climate system.

What happened

A new peer‑reviewed study synthesizes the latest observations and modeling to quantify how present‑day—and near‑future—launch and reentry activity affects the stratosphere. The authors tackled three intertwined questions:

1. What are the dominant pollutants from modern rockets, and at what altitudes do they accumulate?
2. How does the mix of propellants and hardware choices (liquid kerosene, methane, hydrogen, solid boosters) translate into ozone loss and climate forcing?
3. What is the added impact of the rapidly rising number of reentries that inject metallic vapors and oxides high in the atmosphere?

Key elements of the analysis include:

• A current emissions inventory for major launch systems and upper stages, with updated “emission indices” (how many grams of soot, water, NOx, chlorine, etc., per kilogram of fuel burned) appropriate to modern engines.
• Chemistry‑climate simulations that track those emissions in three dimensions, including seasonal cycles and polar dynamics where ozone is most vulnerable.
• An explicit treatment of reentry ablation, which releases aluminum, magnesium, and other metals that seed new particles and enable ozone‑destroying reactions on their surfaces.

The headline conclusions fit a pattern that atmospheric scientists have suspected but not quantified this comprehensively until now:

• Today’s global mass of rocket emissions is small, but most of it is injected at altitudes where it is highly effective per kilogram at modifying ozone and absorbing or reflecting radiation.
• Black carbon (soot) from hydrocarbon engines is a dominant driver of warming in the upper stratosphere because those dark particles absorb sunlight efficiently and can linger for months.
• Solid rocket motors emit alumina particles and hydrogen chloride (HCl); both are potent actors in stratospheric chemistry. HCl contributes reactive chlorine that catalytically destroys ozone, and alumina provides surfaces that accelerate ozone loss.
• Hydrogen‑fueled engines emit primarily water vapor. That avoids soot and chlorine but still adds stratospheric moisture—a greenhouse gas that can also encourage the formation of ice particles that host ozone‑depleting reactions under cold conditions.
• The newest ingredient—massive growth in satellite and upper‑stage reentries—injects metal vapors and metal‑oxide nanoparticles that were historically dominated by meteoric dust. The anthropogenic share is rising fast and changing the composition and behavior of stratospheric aerosols.

Importantly, the study doesn’t stop at a snapshot of today. It runs plausible growth scenarios for the 2030s, including weekly heavy‑lift flights, frequent rideshare launches for megaconstellations, continued use of solid boosters for some vehicles, and escalating reentries from short‑lived satellites. In these scenarios, regional ozone reductions intensify, stratospheric heating from soot increases, and the metal fraction of the stratospheric aerosol burden shifts decisively toward a human‑made signature.

Key takeaways

• The stratosphere amplifies small inputs. A few thousand tons per year of rocket‑related emissions—soot, alumina, HCl, and water—have far more leverage on ozone and temperature than the same mass near the surface.

• Propellant choice matters:

  • Kerosene (RP‑1) engines tend to produce the most soot. Even low percent‑level soot yields can drive significant stratospheric heating.
  • Methane (CH4) engines generally emit much less soot than kerosene if tuned for complete combustion, reducing their warming footprint. They still produce CO2 and water.
  • Hydrogen (H2) engines emit no carbon and essentially no soot, but they do inject water directly into the stratosphere.
  • Solid motors (ammonium perchlorate with aluminum powder) emit alumina and chlorine, a problematic mix for ozone.

• Reentry is no longer an afterthought. As thousands of satellites deorbit over the next decade, ablated metals and oxides will increasingly seed stratospheric particles that:

  • Provide surfaces for heterogeneous chemistry that accelerates ozone loss.
  • Alter how the stratosphere scatters and absorbs sunlight, with knock‑on effects on temperature and circulation.

• Global averages obscure local peaks. While the study finds the global mean climate forcing from rockets remains small compared with aviation, regional and seasonal ozone impacts—especially at high latitudes during spring—could become policy‑relevant under high‑growth scenarios.

• Data gaps are shrinking but still real. Soot emission indices for modern methalox engines, chlorine activation rates on alumina surfaces at realistic plume conditions, and the microphysics of metal‑rich particles from reentry are all areas where targeted measurements would sharpen forecasts.

• “Cleaner” does not mean “impact‑free.” Hydrogen and methane engines avoid the worst soot and chlorine issues, but they do not eliminate stratospheric perturbations—particularly water vapor increases for hydrogen and reentry‑driven metal aerosols that are independent of launch propellant.

How rockets perturb the air above the weather

Soot (black carbon) and shortwave heating

• Source: Incomplete combustion in hydrocarbon engines (kerosene > methane).
• Altitude: Plume rise and upper‑stage burns inject soot into the middle and upper stratosphere.
• Effect: Strong absorption of sunlight warms the stratosphere locally, changing winds and mixing. Warming also shifts chemical reaction rates that set ozone concentrations.
• Persistence: Months to over a year aloft, versus days for tropospheric soot.

Chlorine and alumina from solid motors

• Source: Ammonium perchlorate oxidizer and aluminum fuel.
• Altitude: First‑stage plumes can cross the tropopause; upper‑stage solids (or kick motors) act higher.
• Effect: HCl releases reactive chlorine; alumina particles provide surfaces for reactions that speed ozone loss, especially under cold conditions that form polar stratospheric clouds.
• Local deposition: Near launch sites and downrange, acidic fallout from HCl can cause short‑term air and water quality issues.

Water vapor from hydrogen engines

• Source: LOX/LH2 combustion products.
• Altitude: Direct injection into the stratosphere and mesosphere on ascent and during upper‑stage burns.
• Effect: Water is a greenhouse gas aloft and can help form ice particles that host ozone chemistry. However, the absence of soot and chlorine makes hydrogen comparatively benign for ozone relative to kerosene and solids.

Reentry metals and novel aerosols

• Source: Ablation of aluminum tanks, solar panels, wiring alloys, and thermal protection materials from satellites and upper stages.
• Altitude: Ablation peaks in the mesosphere and upper stratosphere; products settle downward.
• Effect: Metal vapors oxidize and combine with sulfuric and nitric acids to form mixed metal‑sulfate or metal‑nitrate particles. These act as reactive surfaces for ozone chemistry and modify how sunlight is scattered.

How big is the problem—really?

Context is essential. On CO2 alone, rockets are tiny: annual launch emissions are a sliver of one percent of global fossil CO2. But focusing on CO2 misses what makes spaceflight distinctive. Above 15–20 kilometers, aircraft are largely absent, thunderstorms struggle to loft pollution, and the air is exceedingly dry. In that environment, even modest injections of soot, chlorine, water, and metal‑bearing particles are chemically and radiatively potent.

The study emphasizes two scales:

• Global mean climate forcing and ozone change: still small compared to aviation or surface sources, but rising. Under aggressive growth, rocket‑induced forcing could move from “negligible” to “non‑trivial” in climate assessments.
• Regional/seasonal peaks: can be orders of magnitude larger than the global mean. For example, polar late‑winter/early‑spring conditions are primed for ozone loss; additional chlorine or surface area from rocket byproducts can make a measurable dent there.

Technology and policy levers that matter now

• Prefer low‑soot propulsion where it works:

  • Transitioning from kerosene to methane or hydrogen for high‑altitude burns reduces black carbon injection substantially.
  • Engine tuning and oxidizer‑to‑fuel ratios can further minimize soot.

• Rethink solid motor use:

  • Where solids are operationally essential (e.g., heavy‑lift boosters), minimize their role in high‑altitude phases or explore alternative oxidizers with less chlorine, recognizing today’s trade‑offs in performance and manufacturability.

• Manage reentry as a pollutant source:

  • Accounting for reentry emissions in environmental reviews, not just launch plumes.
  • Designing satellites and stages to reduce metal mass ablated or to reenter in ways that limit high‑altitude injection, where feasible.

• Measure what we model:

  • Coordinated campaigns using high‑altitude aircraft, balloons, and lidar to pin down soot yields from modern engines and the composition/size of reentry‑derived particles.

• Build governance that fits the physics:

  • Aviation has ICAO to standardize emissions accounting and technology goals. Spaceflight lacks an equivalent global framework. National launch licensing (e.g., FAA in the US) can incorporate stratospheric impacts, but orbital reentry is international by nature and will benefit from common metrics and reporting.

What to watch next

• The propulsion mix of next‑gen fleets:

  • Methane engines (e.g., BE‑4, Raptor) are displacing kerosene at the heavy‑lift end. Their real‑world soot indices will determine how much black carbon reaches the stratosphere as flight rates surge.
  • Hydrogen‑fueled upper stages remain attractive for performance and low soot; expect renewed interest as climate accounting tightens.

• The cadence of mega‑constellation logistics:

  • Frequent batches up, frequent satellites down. Watch monthly reentry totals and whether operators adopt designs that reduce ablation yields.

• Solid booster reliance in key programs:

  • Vehicles that keep large solid boosters in their architecture will anchor the chlorine and alumina contribution. Monitoring launch timing relative to polar winter/spring could matter.

• Observations of metal‑rich aerosols aloft:

  • Balloon and aircraft sampling campaigns are poised to provide year‑over‑year trends in the anthropogenic metal fraction of stratospheric particles.

• Regulatory pilots:

  • Expect early moves to fold stratospheric impacts into environmental impact statements for launch sites, and perhaps voluntary reporting of soot and chlorine indices by engine families.

• Surprises from reusability and reentry profiles:

  • Reuse cuts manufacturing emissions and can reduce the number of upper stages built and disposed of, but it doesn’t eliminate upper‑stage reentries. New recovery or disposal profiles could change where and how material ablates.

FAQ

• Aren’t rockets a rounding error compared to planes and cars?

  • For CO2, yes. For stratospheric soot, chlorine, and reentry‑derived metals, rockets loom much larger because they inject directly into layers of the atmosphere where persistence and chemistry magnify their effects.

• Do methane or hydrogen engines solve the problem?

  • They substantially reduce the soot issue (hydrogen eliminates it), which is a big win for climate forcing aloft. But hydrogen adds stratospheric water vapor, and neither fuel addresses the growing impact of reentry aerosols.

• Are solid rocket motors uniquely harmful?

  • They are the primary source of chlorine and alumina at high altitude, both of which accelerate ozone loss. Their operational advantages are real, but from an atmospheric perspective, minimizing their high‑altitude use is beneficial.

• How bad is reentry for the atmosphere?

  • The study highlights reentry as an emerging driver of metal‑rich particles in the stratosphere. The chemistry is complex, but the direction of travel is clear: more reentries shift aerosol composition toward anthropogenic metals that can hasten ozone loss and alter radiative balance.

• Does reusability help?

  • It can. Reusing first stages means fewer new boosters and potentially fewer disposed upper stages over time. But the upper stages that reach orbit still need to be deorbited, and the satellites themselves will reenter. Reusability isn’t a silver bullet, but it’s part of a lower‑impact trajectory when combined with cleaner propellants.

• Can we capture or scrub rocket emissions?

  • In‑flight scrubbing is not practical for rockets. Ground‑level measures can mitigate local acid deposition from solids. The most effective levers are propellant choice, engine design for low soot, and reentry mass/trajectory management.

• Will regulations limit launches?

  • The study doesn’t prescribe policy, but it underscores the need for transparent accounting and standards. Expect incremental moves—emissions reporting, technology incentives—rather than hard launch caps in the near term.

Bottom line

Spaceflight’s environmental footprint is still modest in mass but muscular in influence where it counts: the dry, thin air above the clouds that guards our ozone layer and shapes climate. The new analysis makes clear that choices we make now—about fuels, engines, boosters, and how we retire space hardware—will determine whether rocket pollution remains a manageable niche issue or grows into a notable slice of the stratospheric budget. The tools to steer toward the better outcome are already on the table.

Source & original reading: https://arstechnica.com/space/2026/02/study-shows-how-rocket-launches-pollute-the-atmosphere/