How UQ researchers are helping to protect the spacecraft that will explore Uranus and Neptune

Tucked away under UQ’s Hawken Engineering Building, an unassuming-looking metal tube is playing an important role in exploring our solar system’s most distant planets.

One of the challenges of exploring the planets in our solar system is working out how to safely get spacecraft into their atmospheres.

A project led by Professor Richard Morgan and Dr Chris James from the School of Mechanical and Mining Engineering, aims to overcome this challenge.

Dubbed the Advancing the Science of Giant Planet Atmospheric Entry project, the team was awarded $585,000 from the Australian Research Council in 2022.

The project will focus on improving the models used to test the heat shields that protect spacecraft entering the atmospheres of the giant planets, particularly Uranus and Neptune.

But how do you design a heat shield on Earth that will work when it's in the atmosphere on an ice giant like Neptune, over 4.5 billion kilometres away? That's where UQ's expansion tube comes in.

Measuring 20 metres long and with an internal diameter of around 20 centimetres, the expansion tube functions a little bit like a wind tunnel.

Instead of using a fan to generate constant wind, the expansion tube uses a piston to compress the gas to extremely high pressure and temperature – around 3000°C, or just over half the surface temperature of the sun, and around 100 times the pressure of the Earth’s atmosphere.

This pressure and heat is then released into the tube in extremely short bursts, where the researchers can monitor how it affects model spacecraft and heat shields inside the tube.

“For a very short period of time, about a 10,000th of a second, a little miniature spacecraft feels like it’s flying,” says Dr James, a research fellow at the School of Mechanical and Mining Engineering.

The project aligns well with plans organisations like NASA have for space exploration projects to planets such as Uranus and Neptune, which are likely to launch in the late 2020s and early 2030s.

“The launch window when the planets are all aligned correctly is about 2030,” explains Dr James.

“So they’re going to want to be doing the design work over the next five years, to perfect the heat shield of the probe to get it on the spacecraft to launch around 2030.”

Entering a planet’s atmosphere is one of the most challenging aspects of space exploration.

“When you want to explore the atmosphere of a planet, you can’t just orbit around it, you’ve got to enter the planet,” Dr James says.

Planetary entry is one of the most severe environments spacecraft encounter, and the harshest planetary entries are on the giant planets.

Neptune, for example, is four times larger than Earth, weighs sixteen times as much and spins almost six times as fast. This means the entry speeds for Neptune are around 25km/s, compared to 8km/s for Earth re-entry from the International Space Station, or 11km/s from the moon.

As a result, probes entering giant planet atmospheres encounter extremely high levels of heat and pressure. To overcome this, agencies such as NASA have spent decades inventing heat shields to protect spacecraft during atmospheric entry.

“The materials themselves are a low-density composite, usually some form of carbon fibre impregnated with a glue to give it rigidity,” Dr James says.

“PICA (Phenolic Impregnated Carbon Ablator), the material used on Mars Science Laboratory and Mars 2020, is just chopped carbon fibres impregnated with phenolic resin, which is basically a glue.”

Although the spacecraft used in the experiments are obviously scaled-down versions of entry vehicles, which measure around a metre in diameter, the outcomes of the research conducted using the expansion tube are still applicable at larger scales.

“We mainly try to study the phenomena, because the phenomena are energy dependent,” Dr James says. “So if you’re entering at 25 kilometres a second into Neptune, certain phenomena are going to occur based on that speed.

Researchers are able to place model spacecraft and heat shields in the expansion tube to directly measure how they're affected by factors such as heat and pressure.

They can also replace the air in the expansion tube with gases that are similar to the atmospheric composition of planets like Neptune.

“This allows us to study the interaction of the gas with a model,” Dr James says, explaining that even NASA’s facilities don’t allow them to do these types of experiments.

This is important research, as there are many complex aspects to planetary entry that affect how the heat shields function.

The first aspect the researchers will be examining is the post-shock gas flow. As a spacecraft enters the atmosphere of a planet, it moves at hypersonic speeds. This causes a shockwave to form off the bow, or front, of the spacecraft.

“Blunt re-entry vehicles ("blunt bodies"), like Apollo, the SpaceX Crew Dragon, Mars exploration vehicles, and giant planet entry vehicles, are designed specifically to have a really strong shock form off that bow and to just basically be a ‘flying brick’,” Dr James says.

This shockwave is beneficial for two reasons: it generates a high amount of drag force to help slow and eventually stop the spacecraft, and it causes a chemical reaction in the gas that flows over the spacecraft, which helps to direct heat away from the spacecraft itself.

“You get nitrogen and oxygen molecules, if you’re using air as an example, and you break them apart into atoms, and that takes up a lot of energy,” Dr James explains. “And the benefit is that if you break up those molecules, and they just get thrown out the back of your spacecraft, that energy doesn’t transfer to the surface of the vehicle.”

This project will examine how the shockwave functions in a giant planet atmosphere, and whether the gas molecules will break up in the way that they expect.

Another aspect of the project will also examine the effects of the energy absorbed by the heat shields, which are designed to burn away during atmospheric entry.

“We want to study the reaction of a carbon surface that’s burning away with the hot gas flow,” Dr James says, explaining that the information they gain from this might help to reduce the amount of heat shielding  required for the spacecraft, freeing up more room for other equipment.

The third aspect the research will examine is how the gas flow functions close to the spacecraft.

“The problem is, you can have a situation where the gas particles get caught in the rough surface of the heat shields, and they recombine into molecules, and then that recombination just releases the energy again.” Dr James says.

“Ideally we don’t want that to happen – we want it to stay dissociated and flow over the vehicle and be lost in the wake behind it.”

It’s this ability to physically test the heat shields that makes UQ’s research stand out.

“The first expansion tube was built at UQ in 1988,” says Dr James, “so we’ve been doing this kind of research for over 30 years.