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Use cases for smart materials in space exploration keep cropping up everywhere. They are used in everything from antenna deployments on satellites to rover deformation and reformation.

One of the latest ideas is to use them to transform the solar sails that could primarily be used as a propulsion system for a mission into a heat shield when that mission reaches its final destination. A new paper by Joseph Ivarson and Davide Guzzetti, both of Auburn University's Department of Aerospace Engineering, in Acta Astronautica, describes how the idea might work and lists some potential applications for exploring various parts of the solar system.

The concept, which they call the Shape Shifting Sailer (3S), is simple—have a thin sheet of material that acts as a solar sail for most of a craft's journey to its destination. Once it reaches that destination, switch the sheet's orientation so that it can act as a heat shield and drag device for the probe entering the target world's atmosphere or aerobraking into its orbit.

That switch could be as simple as a set of shape memory alloy (SMA) hinges that fold the usually flat solar sail into more of a cone or shield shape to assist with drag, which would slow the probe down, and also to assist with deflecting some of the heat itself, essentially acting as a—admittedly only partially effective—heat shield.

Before they tried to build such a system, though, the engineers did what all good engineers would—they modeled the system first. In this case, they separated their modeling work into two different phases—a "design space" study and a feasibility study.

In engineering language, a "design space" doesn't have anything to do with outer space—it's a term for trying to capture all the different factors that could affect a given metric, such as the weight or peak temperature a probe entering Mars's atmosphere would experience. By varying those factors in simulation, engineers can develop a sense of the most important design decisions they have to make, especially regarding tradeoffs like lowering a sail's weight or increasing its thermal shielding effect.

In their paper, the authors looked at case studies for five potential target worlds—Earth, Mars, Titan, Uranus, and Neptune. They then turned to a semi-autonomous algorithm known as a genetic algorithm that optimized the tradeoff of the sail's peak temperature and its peak pressure. They found those two metrics to be opposed, as the physical shapes that minimize one of those two characteristics tend to maximize the other.

To minimize the pressure, it's best to be shaped like a leaf—large surface area, but very light weight, whereas to minimize temperature, it's best to be shaped like a cannonball—very small, thick, and dense, which typically implies high thermal inertia—meaning how much overall heat a material can take.

In the next phase of their study, they opened up potential flight paths into the atmosphere and orbit around the various worlds in the study. Around Earth, they found the material could at least somewhat help with the overall thermal load, decreasing the peak heating rate by 20–25%, but only if the sail were jettisoned during reentry. Mars was the best case scenario for using the 3S idea, as it showed—again, during a jettison scenario—that heating on the entering probe could be reduced by as much as 40%.

Unfortunately, the results weren't as great for Titan, Uranus, and Neptune. On the gas giants, using the system at all proved infeasible, as the entry speeds required for aerobraking in their atmospheres were too high and would burn up any feasible material that could be potentially manufactured in the near future. On Titan, the 3S system would be feasible, but would require about as much mass in the sail as there would be in the payload in order to be effective. Given that getting mass to Titan is expensive in itself, it seems unlikely that the idea will take off—or land, for that matter.

But even if the idea is only applicable to Mars exploration, that will be the focal point of a lot of future missions as NASA continues its path from the moon to Mars. Given the promising results of the simulations shown in the paper, it might be worth spending some money developing a prototype of the 3S system to see if there are any kinks in the plan. But, given the current state of space exploration funding, that might have to wait for some time yet.