NASA picks finalists to explore comet, Saturn's moon

NASA on Wednesday announced two finalists for billion-dollar robotic missions -- one to explore a comet and another Saturn's moon Titan -- with a launch by the winner planned in the 2020s. "These are tantalizing investigations that seek to answer some of the biggest questions in our solar system today," said Thomas Zurbuchen, associate administrator for NASA's Science Mission Directorate. Titan is Saturn's largest moon, and is seen as one of the most likely hosts of some form of life in the solar system because of its rivers and lakes filled with methane.