Ice Giant Exploration: Advancements in Atmospheric Entry Technology
The outer gas planets Jupiter and Saturn and especially, the outer ice giants of Uranus and Neptune have long been acknowledged as a target for a high priority exploration mission. These cold, dense bodies are unlike any other planets found in our Solar System, the last to be explored, and the least understood. Their atmosphere consists mainly of hydrogen (H2), helium (He) and in the case for Neptune traces of methane (CH4). A mission to one or more of these icy giant bodies will provide a significant contribution to furthering our understanding on the formation and evolution of the solar system and other exoplanetary systems. ESA is preparing the aerothermodynamic testing technologies for high-speed entries into the outer, giant, ice and gas planets for such future missions.
The scientific merit of icy giants has been recommended internationally within the NASA 2023-2032 Planetary Sciences Decadal Survey and the ESA Voyage 2050 Programme. An atmospheric probe with in-situ instrumentation to any one of the ice giants has been marked as a high priority mission, and could be envisioned within an ESA (M)edium class science mission.
Two ESA CDF studies performed in 2019 examined the potential ESA contributions to a NASA-led mission to either Uranus or Neptune and the gas giant Saturn. Similar to the partnership for the Cassini-Huygens mission, where ESA provided the Huygens probe, the mission would have a significant impact for the European planetary science community as a whole. A potential launch opportunity exists in the early 2030s where a Jupiter swing-by would allow access to multiple planets. Recently planetary scientists at NASA also expressed that a mission to Uranus is a priority future opportunity.
Before any mission can be considered, further investigation is required to understand the aerothermal environment of an ice giant entry. Any descending spacecraft would be subjected to intense heating as it plunges into the cold and dense atmosphere at entry speeds of around 23 km/s for a Uranus or Neptune mission, and around 27 km/s for Saturn. The spacecraft’s thermal protection system would need to protect the precious payload from the extreme heating effects. The heating rate would be orders of magnitude larger than any mission currently undertaken by ESA. “The aim of the activity was to adapt current ground-based facility to simulate relevant H2/He/CH4 atmospheric condition on the probe in ground test facilities, which were not yet available in Europe and no plasma facility exists to simulate a H2/He/CH4 environment,” explains Louis Walpot, Technical Officer of this activity.
Through a combined German, British and ESA GSTP funded De-Risk activity, the High Enthalpy Flow Diagnostics Group (HEFDiG) at the University of Stuttgart Institute of Space Systems (IRS), and the University of Oxford hypersonic group adapted their respective ground test facilities.
The Oxford T6 Stalker Tunnel, located at Oxford University, simulated the high speed aerothermodynamic gas radiative dynamics, and investigated the convective heat fluxes in a representative H2/He/CH4 environment. It is Europe’s fastest wind tunnel facility, providing a hypersonic, multi-mode and aerothermodynamic testing facility, based on the design by the late Professor Ray Stalker.
“The tunnel is capable of measuring both convection and radiative heat flux, and critically provide the required flow speeds for the replication of ice giant entry, with traces of CH4. The tunnel itself operates with a free-piston driver, which can be coupled to several different components downstream to become a shock tube, reflected shock tunnel or an expansion tube. This adaptability allows for a wide range of testing from subscale modelling testing to the exploration of fundamental high speed flow processes,” adds Louis Walpot.
Similarly, the gas surface interactions on ablators are studied in the plasma wind tunnel facility PWK1 at IRS. PWK1 is currently the only plasma facility with the required hydrogen capabilities in the world to study the interaction of pyrolysis and ablation on a spacecraft’s thermal protection system.
The newly developed expansion nozzle was used to measure the convective heat flux on a scaled 1:10 model of a 45 degree sphere cone probe like the Jupiter Galileo probe.
Various entry conditions were tested, where test flight equivalent test flow velocities up to 19 km/s were reached under nominal composition (85% H2, 15% He), Stalker substituted (an aerothermodynamically similar environment at lower velocities) and nominal composition with methane at 0.5% and 5%. Each test response was recorded with high-speed cameras, coaxial thermocouples and compared against known and comprehensive numerical data sets.
A new steel tertiary barrel was used for the very first time, to measure the radiance via emission spectroscopy of the H2, He and CH4 test gas. A peak shock speed of 18.9 km/s was achieved. The presence of small amounts of CH4 was shown experimentally for the first time to strongly affects the spectral radiance in the post-shock region dominated by C2 and atomic hydrogen and carbon.
The contract closed in 2022 after successfully reaching technology readiness level 6, demonstrating the ability of model testing in a relevant environment. The investigation into icy giants continues within the GSTP and TDE technology programmes. Two TDE funded activities are ongoing to develop 1) an entry and descent instrument sensor suite for ice giant entries and 2) new state-of-the-art state CFD code for the characterisation of the aerothermal environment of ice giants planets entry capsules.
Furthermore, a proposed GSTP follow-on activity will enable the expansion of the T6 facility, reaching higher speeds of up to 25 km/s to simulate the entry conditions of Saturn. The expansion will also enable the exploration of flow and heat transfer over a more realistic, representative geometry and trajectory points. High resolution measurements of the heat flux on the model, including non-intrusive measurements of the shock layer including the shock stand-off, radiative emission and electron density would be possible, as well as the effects of smaller trace molecules/atoms possibly present in the planetary atmospheres.
ESA and NASA have also expressed their mutual interest in understanding the fundamental aerothermodynamic principles of future ice giant missions through a signed Memorandum of Understanding. Understanding the aerothermodynamic heating sensitivity during entry with respect to the atmospheric composition of minor species in giant planet could lead to significant increases in hypervelocity shock-layer radiation during entry. Only by further understanding these effects can engineers design safer missions to these cold and unknown planetary bodies.