The clever shapes originated in the 1950s at Ames Research Center, the same place where Hwang would later work to develop the MSL heat shield. Harry Julian “Harvey” Allen, who headed the Ames High-Speed Research Division during the early 1950s, devised the so-called blunt body, which would have a flat, broad side to take the brunt of the heat. Allen and a colleague worked on the theory over the next year. They realized that a blunt body would create a strong shock wave in front of it, which deflected much of the heat away from the vehicle. They then put together the second piece of the puzzle: ablation. This means using materials that are designed to decompose and erode on entry, creating a charred layer that effectively pushes heat away from the vehicle.

The blunt-body concept was initially met with skepticism, and it remained classified until 1957. But by May 1961, when Alan Shepard became the first American to visit space, his Friendship 7 capsule used a conical blunt face to safely return to Earth.

Because of the Apollo program, new ablative materials were a very active research area in the 1960s. For Apollo, NASA turned to a company called Avco, which specialized in materials for long-range missile warheads. A 2.7-inch-thick layer of “Avcoat,” a heat-shield material made of epoxy resin in a fiberglass matrix, absorbed the worst of the heat on Apollo’s reentry.

HEEET is intended for entry into extreme environments, like those on Saturn or Neptune. Jessica Chou

For the Viking missions—which would launch the first successful Mars landers in the 1970s—NASA used a new material called SLA-561V. Like Avcoat, SLA (for “super-lightweight ablator”) is based on a honeycomb structure filled with gobs of ablative resin. But the engineers at Martin Marietta, the company that devised the material, also integrated lighter constituents, such as silicon and cork, to reduce its density.

The space shuttles, first launched in the 1980s, needed an entirely new approach. The shuttles were meant to be reusable, and that went for the heat shields as well. Instead of a substance like SLA, the shuttles were protected with reinforced carbon-carbon on the nose cap and the leading edges of the wings, and by ceramic tiles on their belly.

Hwang, who grew up in a small town in Iowa, remembers handling a space shuttle tile in a school presentation. The experience planted the desire to one day work on heat-shield technologies. After earning her doctorate in plasma physics at the University of Illinois, Urbana-Champaign, she took a job at Ames Research Center, but one that had nothing to do with heat shields. For several years, she worked on using plasmas to etch circuits in microchips. When funding ran short, she switched to heat shields, realizing her childhood ambition.

When Hwang was given the task of creating a heat shield for the MSL project in 2006, she initially turned to SLA. But it became clear pretty fast that SLA wasn’t going to work. “We were never really able to isolate what was causing the failure,” Hwang says, “but the failure was repeatable; we tested in many different facilities, and we saw the same failure in different conditions.”

There weren’t many other options, though. The only viable choice was something called phenolic-­impregnated carbon ablator (PICA), which had been developed at Ames in the 1990s for the Stardust mission—the first to return samples from a comet, and the fastest atmospheric reentry in history. Stardust had used one continuous piece of PICA, but MSL was too large for that approach to be practical. They instead had to create tiles of the material and designed the Mars probe to be covered with them, doing so in a way that didn’t allow the streamlines of gas to flow along the potentially vulnerable seams between the tiles. It was the first tiled ablative heat shield, and the largest aeroshell ever flown. (The same solution is now being used by SpaceX for its Dragon capsule. NASA loaned Dan Rasky, one of the designers of PICA at Ames, to SpaceX to help design the Dragon’s heat-shield material, known as PICA-X.)

“I want to explore our solar system. We’ve only been to a handful of destinations. I want to go to all of them.”

As the MSL launch deadline loomed, Hwang and her team blasted PICA samples in the Arc Jet Complex at Ames, improving their understanding of the material and gap fillers with each new test. They perfected their shield in time for the 2009 launch—only to see the mission delayed until 2011 to make sure other systems were ready. The MSL eventually landed on Mars in August 2012. Curiosity is still active on Mars, and has been so successful that NASA is now developing another mission, the Mars 2020 rover, based on a similar design. Hwang has reprised her role managing the thermal protection system, which will again use PICA to shield the spacecraft as it descends to Mars in early 2021.

Jessica Chou

One of the Mars 2020 rover’s most important duties will be gathering samples that may one day be rocketed back to Earth by a future lander. Even as scientists learn how to land the next generation of spacecraft on other worlds, they are also working out how to bring tantalizing alien environments back to Earth.

If humans want to land on Mars, it will require heat shields at least four times the diameter of the one on MSL. That’s why NASA is now developing concepts for expandable aeroshells that can be tucked inside the launch vehicle shroud and deployed into a larger shield in space. Much of that work is being done at NASA’s Langley Research Center in Virginia. On the morning of July 23, 2012, a sounding rocket blasted off from NASA’s Wallops Flight Facility, across the Chesapeake Bay from Langley, on Virginia’s eastern shore. The rocket carried a deployable aeroshell known as a hypersonic inflatable aerodynamic decelerator (HIAD), a broad, shallow cone consisting of an inflatable structure of doughnut-shaped tubes. The HIAD was less than a half meter in diameter, but once in space it deployed to three meters. Making the shield wider spreads the heat of reentry out across a larger area.

The rocket went 290 miles up—well above the boundary of space—and then the HIAD inflated to its full size. Onboard cameras captured a view of the Atlantic Ocean as the structure dropped through the atmosphere. The HIAD concept has performed well in these flight tests, but some people still balk at the idea of protecting Mars-bound astronauts with a blow-up aeroshell. “A lot of people say: ‘Oh, you have an inflatable structure—it’s going to bend up like a pool toy,’ ” says Robert Dillman, an aerospace engineer at Langley and a member of the HIAD team. “This thing is pretty solid. It rings when you tap it.”

Larger aeroshells push shock waves farther away from the spacecraft, providing more protection from entry heat. The remaining heat is warded off by a flexible thermal protection system that covers the inflatable structure with durable outer fabrics and insulation.

The next HIAD scheduled to fly will reach low Earth orbit and expand to six meters. But these inflatable concepts are not the only expandable aeroshells in the works. A team at Ames is developing a foldable shield called the Adaptable, Deployable Entry and Placement Technology. Made from flexible 3D-woven carbon fibers, the shield pops open like an umbrella and is held steady by metal struts.

Hwang is also involved with the development of something called the Heat Shield for Extreme Entry Environment Technology (HEEET), which could accommodate missions to Venus, Saturn, Uranus, and Neptune. HEEET is far more robust than PICA and SLA-561V, and thus better suited for dense atmospheres. Traditionally, each mission has had a unique heat shield, but this makes things more expensive. Hwang hopes to achieve economies of scale—a sort of Ford Model T of reentry.

“I want to explore our solar system,” she says. “We’ve only been to a handful of destinations. I want to go to all of them.”

Becky Ferreira is a science reporter based in Ithaca, New York. Her work has appeared in Wired and the New York Times.