For months on end, the samples kept melting. This wasn’t exactly surprising—the cork-filled fiberglass honeycomb was being subjected to a blast of heat four times more intense than what the space shuttle’s leading edge withstood on reentering Earth’s atmosphere. It was like putting the world’s hottest oven in the middle of its most powerful wind tunnel.
The same materials had already protected all America’s previous Mars landers from the heat of hitting the Martian atmosphere at nearly 10,000 miles (16,000 kilometers) per hour. But that wasn’t going to be good enough anymore. The shield for the Mars Science Laboratory (MSL) would need to withstand about 250 watts of energy per square centimeter—about 10 times the heat experienced by the Viking, America’s first Mars lander, which touched down on the planet in 1976. That’s because MSL, scheduled to launch in August 2009, would be three times heavier than the Viking. The Curiosity rover that MSL would carry was about five times heavier than the Spirit and Opportunity rovers, which had landed safely on Mars in 2004. MSL’s size and weight weren’t insoluble problems in themselves. But computer simulations showed that the probe’s huge weight would result in heavy turbulence, leading to more severe conditions than any previous Mars entry heat shields would have endured. And when they turned the heat-shield material sideways to the oncoming flow of hot air to simulate turbulence, honeycomb cells in it would “pop,” leading to a chain reaction of failures. “The test looked unlike anything we had ever seen before,” remembers Helen Hwang, a researcher at NASA’s Ames Research Center in Silicon Valley who was in charge of MSL’s thermal protection system at the time.
In the wake of those failures, Hwang’s team faced a serious time crunch. It was 2007, and launch was scheduled in less than two years. There were two options, as she saw it: redesign the mission to try to reduce heating conditions, or come up with a new heat-shield material. The first option would limit where the rover could land and the scientific instruments it could carry. The second option meant that they would have to design, develop, test, and build a new heat shield in less than 18 months. That option was risky, but it would allow the mission to do all the science it was meant to do.
They chose the second option.
As human ambitions grow in space, our ingenuity will have to match them. To explore the dense atmospheres of planets like Venus or Saturn, we need ultra-robust heat shields that can handle intense pressures. To send Martian samples back to Earth, we need indestructible heat shields that will prevent any alien life forms from contaminating our planet, or vice versa. Landing humans on other planets will require super-sizing aeroshells, the entry capsules protected by heat shields, to diameters of almost 20 meters (66 feet) across, or more. Nothing close to that scale has ever been flown to Mars before.
The challenges of developing these technologies will be immense, but so will the rewards if they safely deliver robots and humans to new frontiers. Without cutting-edge advances in aeroshells and heat shields, such missions will be pointless—they’ll just burn up in the atmosphere.
If you go into space, there are two reasons to slow down: to return to Earth or to stop at another celestial body. One way to slow down is to use the same method you used to speed up: rockets. But this means carrying more rocket fuel, which adds weight. As a practical matter, it makes sense to use the atmosphere, if there is one. But surviving the resulting heat requires clever materials and cleverly shaped spacecraft.
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.
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.)
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.
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.
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