For thousands of years, people have gazed into the night sky and wondered about the planet Mars. It was an object of particular fascination for ancient astrologers who looked to the heavens for guidance about the past and the future. When they first noticed the fiery red planet, they thought it was a star. But as they continued to study the sky, they discovered that stars appear in fixed patterns (now known as constellations) and traveled across the sky with predictable regularity. Mars, however, and the other "wandering stars" behaved differently. They drifted about the sky from constellation to constellation, as if they were following an invisible path. Because of how these celestial bodies differed from stars, the ancient Greeks named them planetes, a word meaning "wanderers."

Although Mars shared certain characteristics with the other planets, sky watchers observed that...