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, whichhad 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.
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