On December 7, 1995, a NASA probe entered Jupiter’s atmosphere and immediately started to burn. It had been hatched six months earlier by the orbiting Galileo mission, and now, 80 million miles later, it was ready to sample the thick layers of hydrogen and helium surrounding the solar system’s largest planet.
The spacecraft, called the Jupiter Atmospheric Probe, had been carefully designed to withstand the soaring temperatures it would encounter on contact with Jovian air. It had a huge carbon-based heat shield, comprising about 50 percent of the probe’s total weight, which had been designed to dissipate heat by wearing away as the probe descended. This controlled process, called ablation, had been carefully modeled back on Earth—NASA had even built a special test lab called the Giant Planet Facility in an attempt to re-create the conditions and test the design.
As the probe descended through the clouds at more than 100,000 mph, friction heated the air around it to more than 28,000 degrees Fahrenheit—splitting atoms into charged particles and creating an electric soup known as plasma. Plasma accounts for natural phenomena like lightning or the aurora; the sun is a giant burning ball of it. It is often referred to as the fourth state of matter, but really it’s the first: In the moments after the Big Bang, plasma was all there was.
The plasma ate through the Jupiter probe’s heat shield much faster than anyone at NASA had predicted. When the agency’s engineers analyzed the data from sensors embedded in the heat shield, they realized that their careful models had been way off the mark. The shield disintegrated much more than expected in some areas, and much less in others. The probe barely survived, and the only reason it did was that they had built a margin for error into the design by making it extra thick. “This was left as an open question,” says Eva Kostadinova, an expert on plasma from Auburn University. “But if you want to design new missions, you have to be able to model what’s going on.”
After the Galileo mission, scientists used the data from the probe to tweak their models of ablation, but they still faced a big problem: It’s very difficult to precisely re-create the conditions of a high-speed entry to a dense atmosphere, so it’s hard to test those models for accuracy. That also poses a barrier for new heat shield materials that could be lighter or better than the carbon-based ones used right now. If you can’t test them, it’s very hard to be confident they’ll work when attached to a billion-dollar spacecraft.
Past testing efforts have used lasers, plasma jets, and high-speed projectiles to simulate the heat of entry, but none of them are quite right. “No aerospace facility on Earth can reach the high heating conditions that you experience during atmospheric entry into something like Jupiter,” says Kostadinova.
Now, new research by Kostadinova and collaborator Dmitri Orlov from UC San Diego has demonstrated a potential alternative—the fiery innards of an experimental nuclear fusion reactor.
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There are a few hundred such reactors, known as tokamaks, in state-funded research facilities around the world, including the Joint European Torus in the United Kingdom, and ITER, the International Thermonuclear Experimental Reactor, a 35-nation collaboration in southern France. For decades, researchers have been using them to grapple with the challenges of nuclear fusion, a potentially revolutionary technology that could provide essentially unlimited power. Inside a tokamak, powerful magnets are used to hold whirling plasma at a high pressure, enabling it to reach the tens of millions of degrees required for atoms to fuse together and release energy. Cynics argue that nuclear fusion is doomed to forever remain the energy source of the future—right now, fusion experiments still consume more electricity than they generate.
But Kostadinova and her collaborator Dmitri Orlov were more interested in the plasma inside these reactors, which they realized could be the perfect environment to simulate a spacecraft entering the atmosphere of a gas giant. Orlov works on the DIII-D fusion reactor, an experimental tokamak at a US Department of Energy facility in San Diego, but his background is in aerospace engineering.
Together, they used the DIII-D facilities to run a series of experiments on ablation. Using a port at the bottom of the tokamak, they inserted a series of carbon rods into the plasma flow, and used high-speed and infrared cameras and spectrometers to track how they disintegrated. Orlov and Kostadinova also fired minuscule carbon pellets into the reactor at high speed, mimicking on a small scale what the heat shield on the Galileo probe would have encountered in Jupiter’s atmosphere.
The conditions inside the tokamak were remarkably similar in terms of the temperature of the plasma, the speed it flowed over the material, and even its composition: The Jovian atmosphere is mostly hydrogen and helium, the DIII-D tokamak uses deuterium, which is an isotope of hydrogen. “Instead of launching something at a very high velocity, we instead put a stationary object into a very fast flow,” Orlov says.
The experiments, which were presented at a meeting of the American Physical Society in Pittsburgh this month, helped to validate the models of ablation that were developed by NASA scientists using data sent back from the Galileo probe. But they also serve as a proof of concept for a new type of testing. “We’re opening this new field of research,” says Orlov. “Nobody has done it before.”
It’s something that’s sorely needed in the industry. “There’s been a lag in new testing procedures,” says Yanni Barghouty, founder of Cosmic Shielding Corporation, a startup building radiation shields for spacecraft. “It allows you to prototype a lot faster and more cheaply—there’s a feedback loop.”
Whether nuclear fusion reactors will be a practical testing ground remains to be seen—they’re incredibly sensitive devices that have been designed for another purpose entirely. Orlov and Kostadinov were given time at DIII-D as part of a special effort to use the reactor to expand scientific knowledge, utilizing a port built into the tokamak for the purpose of safely testing new materials. But it’s an expensive process. Their day on the machine cost half a million dollars. As a result, this kind of experiment will likely be done sparingly in the future, when the opportunity arises, to tweak and improve computer simulations.
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With further experiments, Orlov and Kostadinova hope that the models can be improved and used to optimize heat shield design for future missions—putting more material where it’s needed, but also removing it from where it’s not. NASA’s DAVINCI+ mission, scheduled to launch toward Venus near the end of the decade, could be the first to take advantage. It comprises an orbiter and a descent probe, which will need powerful shielding as it falls through the hot, thick Venusian atmosphere. The Galileo probe taught scientists much about the formation of the solar system, but with a better heat shield, it could have done much more. “Half of the payload is something that’s just going to burn,” says Kostadinova. “You’re limiting the number of scientific instruments you can really fit in.”
Beyond that, the technique could be used to test new materials, such as silicon carbide, or new forms of heat shield that use a mixture of passive materials that ablate and other components that don’t. Engineers will need those for future missions—the Galileo probe took the slowest, flattest trajectory possible to limit ablation, and still stretched the limits of what was then possible.
The research could also help in the design of fusion reactors themselves. Until now, most research has understandably focused on the core plasma reactions inside a tokamak. But as nuclear fusion inches toward commercialization, more attention will need to be paid to the construction of the reactors and the design of materials that can contain the fusion reaction and safely dissipate the energy if things go wrong.
Kostadinova and Orlov are calling for more collaboration between the fusion and space research communities, which both have an interest in understanding plasma reactions—and in developing substances that can contain them. “The future is to make better materials, and new materials,” Kostadinova says.
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