Finally, a Practical Use for Nuclear Fusion

By Amit Katwala

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 Dimitri Orlov from UC San Diego has demonstrated a potential alternative—the fiery innards of an experimental nuclear fusion reactor.