VATMOS-SR : A Venus atmospheric sample return mission

Science

Venus and Earth share similar physical properties and likely formed through similar accretion mechanisms. Yet, the two planets followed very different evolutionary paths, leading to Venus being uninhabitable today, while Earth has remained habitable. Planetary atmospheres hold crucial clues about the geological and atmospheric evolution of a planet. Returning Venusian atmospheric samples to Earth for detailed analysis is crucial to understand why Earth and Venus evolved so differently. Furthermore, understanding the origin and evolution of our Solar System’s planetary atmospheres is key to studying exoplanets.

Noble gases in planetary atmospheres are unique tracers of the geological evolution of the planet as a whole. They carry the fingerprints of processes driving atmospheric composition, including the original supply of volatiles from the solar nebula, the delivery of volatiles by asteroids and comets, the escape rate of planetary atmospheres, the degassing of the interior through magmatic activity, and the timing of these processes throughout the planet’s history. Constraining these parameters is also essential for building consistent climate models of early Venus. However, the elemental and isotopic patterns of noble gases and other volatile elements such as C, N, S and O in Venus’ atmosphere are poorly known, representing a significant missing link in our understanding of Venus’ evolution [1]. Alongside NASA's upcoming DAVINCI in-situ measurements [2], a sample return mission will provide the unambiguous data required to answer long-standing questions about Venus’ origin and evolution. This is the primary objective of the Venus Atmospheric Sample Return (VATMOS-SR) mission.

Constraints

A key science requirement is collecting well-mixed atmospheric samples slightly below the homopause (~120 km at Venus, depending on latitude and solar time). Factoring in a two-scale-height safety margin, 110 km is the target altitude. To ensure sufficient volume for terrestrial laboratory measurements and redundancy, a minimum of two distinct cylinders must capture at least one liter (1000 cm³) of gas collected at an altitude of 110 km to conduct repeated measurements of all observables targeted by the mission. This estimate assumes a minimum atmospheric density [3], with only 50% of the gas sample used for immediate measurements, and 50% curated for future studies.

Additionally, sampling a planetary atmosphere at high speed (>10 km.s^-1) induces an elemental and isotopic fractionation of volatile elements in the sampled gas due to differential diffusion of species through the strong compression layer. The speed of diffusion depends on species’ molecular weight. As a result, collected gas samples will present a mass-dependent elemental and isotopic fractionation relative to the starting composition. Fractionation during sampling can be accurately modeled using a Direct Simulation Monte Carlo (DSMC) method [4].

Skimming through even just the upper Venus atmosphere presents significant engineering challenges. A phase 0 study was recently conducted by CNES to support the science team in defining this innovative Venusian atmospheric sample return mission in the framework of the ESA F3 call. This phase 0 study involved the design of a novel interplanetary probe capable of withstanding the unique conditions of a double atmospheric entry: first into Venus's dense atmosphere and then back into Earth's.

Mission Design

A Vega-C launch vehicle injects the payload into an elliptical orbit with a periapsis altitude of approximately 1550 km. The payload consists of three elements: a propulsion module (similar to that of Lisa Pathfinder), a probe responsible for the Venus flyby and Earth return, and an Earth Return Capsule (ERC) to host atmospheric samples. After launch, the propulsion module raises the orbit perigee to about 30,000 km before separation. The probe then provides the remaining ΔV to escape Earth’s gravity and initiate the interplanetary transfer. Following a ballistic trajectory, the probe takes approximately four months to reach Venus. Designed as a free-return trajectory, the probe naturally returns to Earth after the Venus flyby, accounting and correcting for the deceleration induced by the Venusian atmosphere. The scientific sampling occurs at the minimum flyby altitude (periapsis), and trajectory correction maneuvers are performed on the outgoing leg of the Venus escape trajectory.

The magnitude of deceleration due to Venus atmospheric flyby is a major driver for the overall mission design. This deceleration is subject to two major uncertainties: atmospheric density (highly dependent on atmospheric modeling) and the minimum flyby altitude (which can deviate from the planned periapsis altitude). To ensure mission success, the probe must carry sufficient propellant to correct any ΔV discrepancies post-flyby.  Using a statistical approach, a ΔV budget of 800 m/s was computed to cover 100% uncertainty in atmospheric deceleration estimates and 4 km uncertainty in flyby altitude. Following these corrections, the eight-month return trajectory to Earth is purely ballistic.

Finally, the probe’s design must account for the extreme Venusian atmospheric conditions, ensuring integrity during the high-speed sampling pass and during Earth's atmospheric re-entry. The thermal protection of the spacecraft uses silicon carbide (SiC) for the top of the Earth re-entry capsule, and carbon felt / phenolic resin (e.g., ASTERM or PICA) for the remaining surface of the spacecraft. During the vehicle’s <250-second transit through the Venusian atmosphere at altitudes below 200 km, aerothermal numerical simulations suggest a peak heating load of 1.6 to 1.8 MW/m² at the nose of the vehicle. A layer of 80 mm of protective material for the nose is required to maintain internal surface temperatures below 400 K. Lower heating fluxes of 25 to 70 kW/m² are estimated for the aft section, requiring less than 30mm of protective material.

Although VATMOS-SR was not selected for further development in the recent ESA F-call, the mission design presented in this study successfully demonstrates the technical feasibility of returning an atmospheric sample from Venus within one year.

[1] Avice et al. (2022), SSR, 10.1007/s11214-022-00929-9, [2] Garvin et al. (2022), PSJ, 10.3847/PSJ/ac63c2, [3] Mahieux et al. (2012), JGR Planets, 10.1029/2012JE004058, [4] Borner et al. (2025), Icarus, 10.1016/j.icarus.2025.116800