The Mars Rovers
Mars arouses deep human curiosity for lots of different reasons
• There is the whole historical notion of little green men, or big scary aliens, coming to Earth from Mars.
• There is the fact that Mars is the only other planet in the solar system that is even vaguely Earth-like, and therefore the only other planet where life might have evolved (another possibility is the ice moon Europa orbiting Jupiter, but that is not a planet).
• Mars is the only planet in the solar system that human beings have any hope of landing on, walking on and exploring in a traditional sense any time soon.
• Mars is the only planet that might be terraformed and turned into an Earth-like planet.
For these reasons and more, humans have sent over 30 different spacecraft to explore Mars. NASA's two Mars Exploration Rovers Spirit and Opportunity are the planet's most recent visitors.