There are four terrific candidate missions for the 2007 Mars Scout Mission. NASA can only pick one to go. Over the next two weeks, meet all four of the Principal Investigators on Planetary Radio. Up this week: University of Arizona researcher Peter Smith, PI for Phoenix, and Arizona State University researcher Laurie Leshin, PI for SCIM, or Sample Collection for Investigation of Mars. Emily is observing the Oort Cloud, while Bruce Betts has an Astrobot update and a new, tricky trivia contest.