Although Uranus and Neptune are full of gas, their compositions present important differences in relation to Jupiter and Saturn, although they are quite similar to each other. Planetary scientists are interested in send a spacecraft to at least one of them, but funding is uncertain and it will take many years to get there even if it is approved. Meanwhile, the European Space Agency (ESA) has attempted to replicate their atmospheres to simulate what a probe would experience upon entering the atmosphere of any of the Ice Giants.
Like their larger counterparts, Uranus and Neptune are composed mainly of hydrogen and helium. However, although their methane concentrations are low (2.3 and 1.5 percent, respectively), they are 3 to 8 times higher than what the Galileo and Cassini spacecraft encountered when entering Jupiter and Saturn. All other gases are traces.
At least in the near future, any mission to any of the ice giants will likely be led by NASA, but ESA is interested in participating.
In the T6 Stalker hypersonic plasma tunnel at the University of Oxford and the plasma wind tunnels of the High Enthalapy Flow Diagnostics Group at the University of Stuttgart, suitable gases were fired at a solid object, producing these images.
The shapes chosen are likely intended to model what a future spacecraft will look like when it makes its final splash, rather than something designed to appeal to those already amused by the name of a planet.
A wind tunnel reveals the heat experienced when a model space probe experiences the shock of encountering the atmosphere of Uranus or Neptune.
Image credit: High Enthalpy Flow Diagnostics Group at the University of Stuttgart
Along with their external gases, Uranus and Neptune are believed to have giant oceans of supercritical liquids. No probe will last long inside, just as Cassini didn’t when it found Saturn, but we want the spacecraft to survive as long as possible to maximize the information it can obtain.
“The challenge is that any probe would be subject to high pressures and temperatures and would therefore require a high-performance thermal protection system to resist its atmospheric entry over a useful period of time,” ESA’s Louis Walpot said in a statement. “To begin designing such a system, we first need to adapt current European testing facilities in order to reproduce the atmospheric compositions and velocities involved.”
Walpot previously noted; “The objective of the activity was to adapt current land-based facilities to simulate relevant Htwo/He/CH4 atmospheric condition on the probe in ground test facilities, which were not yet available in Europe and there is no plasma facility to simulate a Htwo/He/CH4 environment.”
A scaled-down model of the Galileo probe that entered Jupiter entering an atmosphere more similar to Uranus or Neptune
Image credit: University of Oxford
Unless substantially altered by rockets – which would mean carrying a lot of extra fuel – a future spacecraft would enter either planet at a speed close to its orbital speed, or about 24 km/s (54,000 mph). No installation has been able to achieve this yet. However, by pushing the plasma past the model probe at 19 km/s (43,000 mph), the research team is getting closer.
The facility can measure the heat the probe experiences through both convection and radiation. The experiments revealed that even small amounts of methane alter the radiation spectrum around the region being shocked, compared to conditions dominated by hydrogen and carbon.