Just as we mark our lives according to the passage of time, so too do space missions. Most Mars missions have been solar-powered, meaning the spacecraft must do its work during daylight hours. Curiosity has a nuclear generator, but it will still be a solar craft in many ways —- its cameras and other instruments need sunlight to see, and atmospheric phenomena, like the huge temperature shift between day and night on Mars, follows the movement of the sun. So engineers need a reliable method to keep track of time on the planet. Michael Allison, an emeritus professor at the Goddard Institute for Space Studies, has made a hobby of figuring it out. "I actually know Mars time, in a way, better than Earth time," he jokes.