After seven routine months of spaceflight, NASA is bracing for six minutes of terror in January, when the twin rovers it launched earlier this year punch through the atmosphere of Mars to land on the Red Planet. Each of the unmanned, $400 million rovers must be slowed from 12,000 mph to a complete stop within minutes after first plunging into the planet’s tenuous atmosphere. A strong gust of wind, or a single sharp rock, could destroy either or both rovers.