Two hundred years after Meriwether Lewis and William Clark began preparing for their journey of exploration, NASA is readying two scouts to probe a landscape as alien to much of the world today as was the Northwest in 1803. Engineers at the space agency’s Jet Propulsion Laboratory are scrambling to finish assembling and testing the identical twin robotic rovers for launch on separate rockets to Mars in May and June. The $800 million double mission marks NASA’s second and most ambitious bid yet to roam the surface of Mars.