Yet SCoPEx was halted in 2024 before completing its core mission, terminated not by scientific failure but by governance challenges. This makes analyzing its lessons—both what the science revealed and what the governance process exposed—critically important for anyone considering future atmospheric research.
TLDR: Key Takeaways
- SCoPEx aimed to measure aerosol behavior, turbulent mixing, and calcium carbonate particle chemistry at 20 km altitude
- Governance failures and indigenous community opposition halted the project — no particle-release flight ever took place
- The Advisory Committee produced a five-part governance framework spanning safety, finance, legal review, scientific merit, and community engagement
- Community engagement must begin before site agreements are signed — not after — and financial transparency matters equally
- No direct measurement data was collected, but the project yielded modeling outputs, instrument designs, and the most comprehensive governance framework produced for a stratospheric aerosol experiment
What Was SCoPEx and Why Did It Matter?
SCoPEx (Stratospheric Controlled Perturbation Experiment) represented one of the first proposed real-world outdoor experiments in solar geoengineering. Almost all prior research had relied on laboratory studies or computer models, creating a critical data gap.
The core concept: a high-altitude balloon would lift a 600 kg propeller-equipped gondola to approximately 20 km altitude, release 100 g to 2 kg of calcium carbonate (or other aerosols), then maneuver back through the resulting plume to take measurements. Those measurements would capture:
- Turbulent mixing behavior at stratospheric altitudes
- Aerosol coagulation rates under real atmospheric conditions
- Chemical interactions that lab studies and volcanic eruption data cannot replicate
These are precisely the sub-grid scale processes that current global climate models depend on—but cannot validate without field data.
The funding context puts the data gap in perspective. Global solar geoengineering research received approximately $95 million from 2008 to 2021, with most of that going toward modeling and interdisciplinary work rather than field experiments. Harvard's Solar Geoengineering Research Program launched in 2017 with $23 million in committed funding through 2024. Despite that investment, models still depended on unverified assumptions about the very processes SCoPEx was designed to measure.
SCoPEx was more than a science experiment. When Swedish authorities halted the planned 2021 test flight following pressure from indigenous Sámi communities and environmental groups, it forced a concrete question: what governance structures need to exist before any atmospheric field experiment proceeds? That question—still unresolved—is central to understanding both the experiment's legacy and its limits.
The Science Behind SCoPEx: Experimental Design and Data Goals
Balloon Platform and Instrumentation
The SCoPEx gondola integrated three primary systems for precise stratospheric maneuvering:
- Frame and altitude control: Aluminum/carbon fiber structure with ballast hopper for coarse altitude adjustments
- Ascender system: Variable 0–150 m rope (13 mm diameter, 10 m/min max speed) for fine altitude control
- Twin propellers: 1.88 m diameter, 32 N thrust each, enabling 3 m/s maximum airspeed for horizontal flight through the plume
Core instruments included:
- LITOS-style constant temperature anemometer for stratospheric turbulence measurement
- POPS (Portable Optical Particle Spectrometer) with detection range 0.13–3 μm for aerosol size distribution
- Scanning LIDAR for plume tracking and navigation (with 150 m blind zone)
- Solid aerosolizer for dispensing ~0.5 μm calcium carbonate particles
- Radiometer for light scattering comparison
Three Scientific Goals
Goal 1 — Turbulent Mixing
Stratospheric turbulence is highly intermittent and poorly characterized at the 10–500 m scales most relevant to aerosol plume evolution. SCoPEx aimed to provide horizontal kinetic energy spectrum measurements at float altitude, complementing existing vertical-only LITOS balloon data. Understanding turbulent dissipation rates at these scales is essential to predicting how aerosol plumes disperse in actual stratospheric conditions.
Goal 2 — Aerosol Microphysics
The experiment would measure how calcium carbonate monomers (~0.275 μm radius) coagulate into fractal dimers and trimers as a function of injection rate and turbulence. CFD modeling by Golja et al. (2021) showed that at 0.1 g/s injection, 99% of mass stays in monomer form across the final 100 m of a 3 km plume.
At 100 g/s injection, fractal dimers and trimers dominate — a three-orders-of-magnitude shift in behavior that CFD alone couldn't confirm without flight data.
Goal 3 — Stratospheric Chemistry
Beyond microphysics, understanding how these particles interact chemically with the stratosphere was equally critical. Calcium carbonate offered theoretical advantages over sulfate aerosols: roughly 10-fold less stratospheric heating, acid-neutralizing reactivity that could reduce ozone depletion, and rapid dissolution in water.
In practice, laboratory research revealed that CaCO3 surfaces rapidly passivate under stratospheric conditions, sharply reducing reactivity over time. The SCoPEx team ultimately de-prioritized this goal, as validating chemical behavior required a longer experimental timeline than the platform's initial flights could support.
Key Findings and Data Analysis: What SCoPEx Actually Revealed
SCoPEx cleared both its review gates — three independent engineers found no significant safety concerns, and five scientific reviewers concluded the design was sufficient to proceed. The particle-release flight never occurred, though, so no direct stratospheric measurement data was collected.
Two distinct reviews were completed before the program stalled:
- Engineering safety review: Three independent reviewers found no significant safety concerns
- Scientific merit review: Five experts concluded the experimental design was sufficient to proceed
What the Scientific Review Process Revealed
Peer reviewers flagged several design challenges:
- Raised high risk of failing to meet objectives, given the complexity of operating a novel platform and payload simultaneously
- Flagged a 150 m LIDAR blind zone that could miss critical near-field plume dynamics entirely
- Questioned whether the POPS detection range (0.13–3 μm) would adequately capture how aerosol size distribution evolves over time
- Found insufficient detail on how the gondola would reliably re-enter the dispersing plume after initial sampling
Computational Modeling as Surrogate Data
The Golja et al. CFD plume model provided predictions that still need field data to confirm. At 0.1 g/s injection, 99% of calcium carbonate mass remains monomeric in the final 100 m. At 100 g/s injection, fractal aggregates dominate. Until a sensor-equipped gondola actually flies through an injected plume, neither scenario can be validated against real stratospheric conditions.
Remaining Data Gaps
The most consequential gaps include:
- Turbulent dissipation rates at 10–500 m scales have never been measured under controlled stratospheric horizontal-flight conditions
- The coagulation kernel for solid aerosol particles under realistic stratospheric turbulence remains unvalidated
- Calcium carbonate's heterogeneous chemistry with stratospheric HCl and NOx still relies entirely on laboratory parameterizations — none tested in situ
The Governance Controversy: Why the Experiment Was Halted
Chronology of Delays
The project shifted launch sites from Tucson to Sweden. In February 2021, the Saami Council and Swedish environmental organizations formally opposed the planned platform test in Kiruna, citing absence of engagement with indigenous communities and "slippery slope" concerns about eventual deployment. The Swedish Space Corporation and Harvard subsequently canceled the flight. In March 2024, lead investigator Professor Frank Keutsch formally terminated the project.
The Structural Governance Failure
The Saami Council's opposition letter put the problem plainly:
"We find it remarkable that the project has gone so far as to establish an agreement with SSC on test flying without having applied for any permits or entered into any dialogue with either the Swedish government, its authorities, the Swedish research community, Swedish civil society, or the Saami people."
The research team had signed a formal site agreement before conducting any public engagement, community consultation, or stakeholder notification. The Advisory Committee's final report identified early engagement as the single most critical principle for future experiments — and the absence of it is precisely what the five-element framework below was designed to prevent.
Five-Element Governance Framework
The Advisory Committee developed a five-element framework covering:
- Engineering safety — Technical risk assessment and flight safety review
- Financial review — Transparent disclosure of all funding sources
- Legal review — Regulatory compliance and permitting requirements
- Scientific merit — Independent peer review of research design and objectives
- Societal review — Community engagement and stakeholder consultation
By 2023, only the societal engagement step remained incomplete. The project's termination before completing that final step leaves the framework itself as SCoPEx's most transferable contribution — a checklist that any future stratospheric research program will need to work through before a balloon ever leaves the ground.
Lessons Learned: Science, Governance, and Community Engagement
Governance Principles That Must Lead
The Advisory Committee agreed on four core principles for societal engagement:
- Start engagement as early as possible — before site agreements are signed
- Include social scientists with engagement expertise on research teams during design
- Don't presuppose what communities will be concerned about — listen first
- Develop a concrete plan to respond to community input throughout the experiment lifecycle
Scientific Integrity and Transparency
SCoPEx established important precedents that have since influenced geoengineering research norms:
- Transparent financial disclosure of all funding sources
- Commitment to open-access data (raw data to be published within 60 days of flight)
- No intellectual property filing by core team members
- Independent peer review at multiple stages
Taken together, these commitments address a core challenge in contested science: building enough public trust to keep the research process open to scrutiny.
The Unresolved Tension Between Research and Governance Scale
The Advisory Committee identified a central dilemma: a small-scale experiment with negligible physical impact (less than 2 kg of material) required the same level of global governance deliberation as a much larger intervention. This raises open questions about what threshold of physical effect should trigger formal engagement — a question the Advisory Committee explicitly called on policymakers to address.
What's Next for Stratospheric Research?
Following SCoPEx's closure, the landscape has fragmented. Harvard continues the Solar Geoengineering Research Program under the Salata Institute. Meanwhile, other actors have proceeded without the governance framework SCoPEx worked to establish. In 2022, startup Make Sunsets conducted unregulated SO2 balloon releases in Mexico, prompting the Mexican government to ban geoengineering and the U.S. EPA to issue a demand for information under the Clean Air Act in 2025.
This illustrates the governance vacuum that responsible programs like SCoPEx were designed to prevent.
That gap shapes what comes next for stratospheric research infrastructure. Key considerations for future programs include:
- Balloon platform reuse: The SCoPEx balloon system remains available for stratospheric science outside solar geoengineering
- Launch cadence limitations: Balloon platforms can't support rapid, sequential data collection — a constraint for time-sensitive sampling campaigns
- Alternative delivery methods: Propulsion-based systems offer a complementary pathway where high-cadence access matters
Green Launch's hydrogen light-gas combustion technology addresses that last point directly. Its system can deliver research payloads to stratospheric altitudes with a re-launch window of roughly every 60–90 minutes, supporting the kind of sequential sampling that single-balloon missions cannot replicate.
Frequently Asked Questions
What was the Harvard SCoPEx experiment designed to measure?
SCoPEx aimed to measure aerosol microphysics, stratospheric turbulence, and chemical interactions of calcium carbonate particles at 20 km altitude. A balloon-borne propeller gondola would release and then fly back through a small aerosol plume to capture data on turbulent mixing and particle coagulation rates.
Why did Harvard cancel the SCoPEx experiment?
Opposition from the Saami Council and Swedish environmental groups over lack of indigenous community engagement led to cancellation of the 2021 Sweden test flight. The project was formally terminated in March 2024 after lead investigator Professor Frank Keutsch concluded it was time to pursue other research directions.
What is stratospheric aerosol injection and how does it relate to climate change?
Stratospheric aerosol injection (SAI) is a proposed solar geoengineering technique that releases reflective particles into the upper atmosphere to scatter sunlight and reduce global warming. It cannot substitute for emissions reductions and carries significant uncertainties, including regional precipitation changes, ozone effects, and termination shock risks.
What scientific data did SCoPEx actually produce?
No particle-release flight was completed, so direct stratospheric measurement data was not collected. However, the project produced significant computational modeling outputs, instrument designs, peer-reviewed literature on plume dynamics, and the field's most comprehensive governance framework for controversial atmospheric research.
What are the main risks associated with solar geoengineering research?
Key risks include regional precipitation changes, ozone layer effects (particularly for sulfate aerosols), and termination shock if deployment were suddenly halted, per the IPCC AR6 Working Group I report. Moral hazard and governance challenges around unilateral deployment without international consensus are additional concerns.
What governance lessons from SCoPEx apply to future atmospheric research experiments?
Four core principles emerged: begin community engagement before site agreements are signed, include social scientists from the start, avoid presupposing stakeholder concerns, and build a credible plan to respond to input throughout the experiment lifecycle. Technical planning cannot proceed ahead of social engagement — sequence is as important as substance.
