Don’t panic when you peer from the porthole of your spaceship as it plummets to the surface of Mercury: You haven’t taken a wrong turn and touched down on the moon. Cratered, contoured with hills, and covered with dark dust, Mercury’s landscape certainly looks a lot like the moon’s. This itty-bitty planet—the smallest in the solar system—is only slightly larger than our moon, too. But one look up from Mercury’s surface at high noon will tell you you’re far from home. Seen from here, the sun appears three times larger in the sky than it does when viewed from Earth. You’re standing on the closest planet to the sun. Forget your sunglasses? Okay, now you can panic.
Mercury is a planet of extremes. By day (which actually lasts roughly 30 Earth days), it’s one of the solar system’s most scorching spots—more than four times hotter than boiling water. By night, temperatures plummet hundreds of degrees below freezing. (The planet’s weak gravity can’t keep a grip on a heat-trapping atmosphere.) But despite its harsh environment, this hot-and-cold rock offers perks for the spacefarer. A weak magnetic field protects visitors from the sun’s deadly solar radiation (Earth has a similar but much stronger field), and the crater-pocked surface may contain valuable minerals. The speediest of all the planets, Mercury orbits the sun every 88 days—which means any Earthlings born here could celebrate four birthdays for your every one.