Take a Plunge Into the Ice Giants

Our Solar System’s ice giants, Uranus and Neptune, have been largely left out of the planetary probe game. While all of the other planets—including even the demoted Pluto—have been the subjects of dedicated missions, the ice giants have not. In fact, the only spacecraft to ever even fly by Uranus and Neptune was Voyager 2 in the late 1980s.

But the lack of dedicated missions isn’t because they’re inconsequential worlds with nothing to teach us about nature. It’s because it’s difficult. Sure, distant Pluto received a dedicated visit, but it was just a flyby mission. Sending orbiters to distant planets is difficult because they need engines and fuel to enter orbit. They also need to be travelling slower, so the mission takes longer to arrive. And then there’s the fact that solar energy is not a thing that far away from the Sun.

That doesn’t mean we’ll never send one. It just explains why they’ve been left out of the planetary probe game so far. Uranus and Neptune are both fascinating worlds that deserve to be explored more thoroughly. Atmospheric probes will likely be the first step.

Just because they haven’t had dedicated missions doesn’t mean we haven’t studied the ice giants. We know quite a bit about their internal structures and their atmospheric compositions. Enough that we can simulate them and what it might be like for probes sent into their atmospheres and how they can manage the heat.

Scientists know enough about the ice giants’ atmospheres to simulate atmospheric probe entry. Image Credit: (L) By Uranus-intern-de.png: FrancescoAderivative work: WolfmanSF (talk) – Uranus-intern-de.png, Public Domain, https://commons.wikimedia.org/w/index.php?curid=14583180. (R) By Lajoswinkler – Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=121263027

That’s what the ESA has done as part of an effort to simulate atmospheric probes that we’ll no doubt send to these planets one day.

The ESA’s simulations took place at two of their facilities: the hypersonic plasma T6 Stalker Tunnel at Oxford University in the UK and the University of Stuttgart’s High Enthalpy Flow Diagnostics Group’s plasma wind tunnels in Germany. Both of these facilities are designed to test and understand how probes travel through different planetary atmospheres.

This is the PWK1 Plasma Wind Tunnel at Stuttgart University in Germany. It’s the only facility in the world with the required hydrogen capabilities to study the interaction of pyrolysis and ablation on a spacecraft’s thermal protection system. Image Credit: ESA

One of the difficulties in simulating probe entry into the ice giants’ atmospheres is velocity. The T6 Stalker Tunnel achieved speeds of 19km/second, and it’s the fastest wind tunnel in the UK. That’s a start, but ice giant probes will travel even faster than that, which means there’ll be more simulations in the future.

In the meantime, the simulations can still generate valuable data. “The tunnel is capable of measuring both convection and radiative heat flux and critically provides the required flow speeds for the replication of ice giant entry, with traces of CH4,” said ESA Aerothermodynamics Engineer Louis Walpot.

Uranus and Neptune are similar to Jupiter and Saturn, but there are critical differences. The pair of ice giants contain heavier elements in supercritical liquid oceans well below the surface clouds. These oceans make up a sizable portion of the planets’ masses. Both planets also have methane in their atmospheres, which makes them appear blue. This means that atmospheric probes would face a number of severe challenges.

“The challenge is that any probe would be subject to high pressures and temperatures and therefore would require a high-performance thermal protection system to endure its atmospheric entry for a useful amount of time,” explained Walpot. These simulations will help engineers understand the challenges and how to design and build a probe that can withstand the entry.

This image shows one of the atmospheric entry tests at Oxford’s T6 Tunnel. It shows the luminosity during a test of the scaled 1:10 Galileo probe model with a 45-degree sphere cone. Image Credit: University of Oxford.

“To begin designing such a system, we need first to adapt current European testing facilities in order to reproduce the atmospheric compositions and velocities involved,” Walpot said.

This image shows one of the atmospheric entry tests at PWK1. Image Credit: ESA

Both NASA and the ESA are considering missions to the ice giants, though they’re decades away. Testing at both these facilities is laying the groundwork for the ESA’s potential mission. The ESA has given high priority to an atmospheric entry probe to either of the ice giants.

But there’s lots of time for engineers to work on their simulations and develop probes that can handle the ice giants’ heat. These missions are decades away.