Comets

Check out these mysterious flaming snowballs in the sky.

Leave your spaceship on the launch pad for this mission. If there's any heavenly body that’s best seen from Earth’s surface, it’s a comet. These leftovers from the formation of our solar system aren’t much to look at up close. Each is an irregular ball of icy slush, frozen gases, and dark minerals just a few miles or kilometers wide. Comets originate far out in the solar system—some from the so-called Kuiper Belt of icy bodies beyond the orbit of Neptune, and others from a more distant region known as the Oort Cloud. Why journey for years in a cramped spaceship to see a dirty snowball—especially when some comets come to visit us? And when they do, it’s one of the greatest shows on Earth.

Like planets, some comets orbit the sun on a predictable schedule. Halley's Comet, the most famous of these weird wanderers, drops by Earth every 75 years or so (it’s not due for its next visit until July 2061). As a comet nears the sun, ice and dust boil from its slushy center—called a nucleus—to form an atmosphere known as a coma. Sunlight “blows” gas and dust from this coma to create a spectacular tail. Some tails reach 100 million miles (160 million kilometers) long and can be seen from Earth, the comfiest seat in the solar system for comet spotting.