The interaction between dust particles and low-pressure gas is at the heart of many astrophysical and planetary phenomena. Notably, the activity of comets, and their presumed formation history, involves a deep understanding of dust–gas transport processes at low gas pressures. The TEMPus VoLA project was designed to replicate and study these interactions experimentally, in microgravity.
The origins of the Solar system
Formation processes of the early Solar System that are investigated in the context of the TEMPusVoLA project.
Comets and asteroids are planetesimals that remain from the formation of the Solar system. As we unravel their properties and dynamical behaviour, we also deduce the conditions in which they formed. Experiments in low-gravity environments allow direct investigations of the presumed mechanisms that enabled the first stages in forming planets (Capelo et al., 2026) and the ongoing processes on primordial bodies such as comets and asteroids (Capelo et al., 2024).
A novel Zero-G experimental facility
TEMPus VoLA: The Timed Epstein Multi-pressure Vessel at Low Accelerations flew aboard three parabolic flight campaigns on Air Zero-G, operated by Novespace. See apparatus details in (Capelo et al., 2022).
TEMPus VoLA (the 'Timed Epstein Multi-Pressure Vessel at Low Accelerations') is a dedicated microgravity facility designed to investigate gas–dust interactions at low pressures. The systems enable direct observation of dust-driven hydrodynamic instabilities, drag reduction phenomena, and dust-regolith mechanical properties under near-weightless conditions.
For photos of the pilot measurement campaign, see images from the Swiss Space Museum Album 1, and Album 2. Links to further media coverage can be found on the outreach page.
This work was initiated within the framework of the Swiss National Center for Competence in Research (NCCR) PlanetS. It involves researchers from the University of Bern, University of Zürich, and ETH Zürich. The project has benefitted from continuous support from the UZH Spacehub, the European Space Agency (ESA), and the Swiss Space Office (SSO).
References
2026
Experimental evidence for granular shear-flow instability in the Epstein regime
Holly L. Capelo, Jean-David Bodénan, Martin Jutzi, and 7 more authors
Stability analysis of two-fluid protoplanetary disc models has enriched our understanding of how solids can grow into larger bodies called planetesimals. Dust particles entrained in a gas stream modify the flow, creating shear layers prone to instability. In such environments, drag occurs in the free-molecular (Epstein) regime. Recreating these two-phase flows on Earth is difficult due to gravity-driven buoyancy. Here, we use particle image velocimetry to study a low-pressure dust-gas mixture at Knudsen numbers up to 10 in microgravity. We observe a granular shear flow instability, characterized by a periodic velocity field, which can be modeled to first order as a Kelvin-Helmholtz (KH) instability. This behavior resembles a Kelvin-Helmholtz instability and provides a benchmark for two-fluid theories relevant to planet formation.
2024
Gas permeability and mechanical properties of dust grain aggregates at hyper- and zero-gravity
Holly L. Capelo, Jean-David Bodénan, Martin Jutzi, and 13 more authors
Monthly Notices of the Royal Astronomical Society, Sep 2024
Particle-particle and particle-gas processes significantly impact planetary precursors such as dust aggregates and planetesimals. We investigate gas permeability in 12 granular samples, mimicking planetesimal dust regoliths. Using parabolic flights, this study assesses how gravitational compression - and lack thereof - influences gas permeation, impacting the equilibrium state of low-gravity objects. Transitioning between micro- and hyper-gravity induces granular sedimentation dynamics, revealing collective dust-grain aerodynamics. Our experiments measure across Knudsen number (Kn) ranges, reflecting transitional flow. Using mass and momentum conservation, we derive and calculate pressure gradients within the granular matrix. Key findings: (i) As confinement pressure increases with gravitational load and mass flow, and average pore space decrease. This implies that a planetesimal’s unique dust-compaction history limits subsurface volatile outflows. (ii) The derived pressure gradient enables tensile strength determination for asteroid regolith simulants with cohesion. This offers a unique approach to studying dust-layer properties when suspended in confinement pressures comparable to the equilibrium state on planetesimals surfaces, which will be valuable for modelling their collisional evolution. (iii) We observe a dynamical flow symmetry breaking when granular material moves against the pressure gradient. This occurs even at low Reynolds numbers, suggesting that Stokes numbers for drifting dust aggregates near the Stokes-Epstein transition require a drag force modification based on permeability.
2022
TEMPus VoLA: The timed Epstein multi-pressure vessel at low accelerations
H. L. Capelo, J. Kühn, A. Pommerol, and 12 more authors
The field of planetary system formation relies extensively on our understanding of the aerodynamic interaction between gas and dust in protoplanetary disks. Of particular importance are the mechanisms triggering fluid instabilities and clumping of dust particles into aggregates, and their subsequent inclusion into planetesimals. We introduce the timed Epstein multi-pressure vessel at low accelerations, which is an experimental apparatus for the study of particle dynamics and rarefied gas under micro- gravity conditions. This facility contains three experiments dedicated to studying aerodynamic processes: (i) the development of pressure gradients due to collective particle-gas interaction, (ii) the drag coefficients of dust aggregates with variable particle-gas velocity, and (iii) the effect of dust on the profile of a shear flow and resultant onset of turbulence. The approach is innovative with respect to previous experiments because we access an untouched parameter space in terms of dust particle packing fraction, and Knudsen, Stokes, and Reynolds numbers. The mechanisms investigated are also relevant for our understanding of the emission of dust from active surfaces, such as cometary nuclei, and new experimental data will help interpreting previous datasets (Rosetta) and prepare future spacecraft observations (Comet Interceptor). We report on the performance of the experiments, which has been tested over the course of multiple flight campaigns. The project is now ready to benefit from additional flight campaigns, to cover a wide parameter space. The outcome will be a comprehensive framework to test models and numerical recipes for studying collective dust particle aerodynamics under space-like conditions.