Dagny Carlsson is just 3 years old, but she knows her planets. She can rattle off Mars, Venus and Jupiter like other kids prove their knowledge of colors or counting. “It’s hard to know what a 3-year-old picks up, but she seems interested when we look through the telescope,” said Dagny’s dad, Carl. So when NASA’s two robotic explorers attempt to land on Mars next month, Dagny will be one of 4 million humans whose names, recorded on mini-DVDs, will ride with them to the Red Planet’s surface.