It’s been three decades since the idea of a mobile robotic Mars rover was seriously proposed, and now, finally, we have one. As a longtime advocate of human Mars exploration, I can only watch this mission with delight. And nothing pleases me more than the news that this week President Bush is expected to announce plans for the human exploration of Mars. Sending humans there is essential because, as wonderful as they are, neither Spirit nor Opportunity nor any other machine will ever tell us what we need to learn about Mars.