In December 1995, the Galileo probe made its dramatic descent into Jupiter’s atmosphere at a velocity of more than 47 km/s. In 30 seconds, it decelerated from Mach 50 to Mach 1, undergoing incredible heating as it did so. Anytime an object moves through a fluid faster than the local speed of sound, it creates a leading shock wave that compresses the fluid, heats it, and redirects it around the object. The faster the speed, the hotter the fluid will be after passing through the shock wave.
Above about five times the speed of sound, the heating effect is so strong that it’s able to rip molecules apart, creating a chemically reactive mixture that will ablate away material from the object. For this reason, Galileo and other planetary entry vehicles carry heat shields made to sacrifice themselves while protecting the cargo and (in some cases) crew onboard. Data from Galileo showed that, although the heat shield survived the brunt of its descent, it experienced worse conditions than expected. Near the heat shield’s shoulder, almost all of its material ablated away.
Scientists continue to study Galileo’s descent even now, using it to test and inform their models of the flow and chemistry that occurs at these hypersonic speeds. The better we can understand and predict these flows, the better our designs will become. Mass that’s currently spent on overly-conservative heat shields can instead go toward additional instruments or supplies. (Image credit: Chop Shop Studio; research credit: L. Santos Fernandes et al.; via AIP)