LOCATION: The distant Kuiper Belt and Oort Cloud regions of the solar system

DISTANCE FROM THE SUN: 55 million miles (89 million kilometers) up to 100,000 astronomical units (or 100,000 times the distance between the Earth and the Sun)

GRAVITY: Almost nonexistent


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.





• NASA launched several unmanned missions to study comets. A paparazzi of sensor-laden craft buzzed Halley’s during its last approach in 1986. Another probe actually collided with a comet in 2005.

• Halley’s peanut-shaped nucleus is barely 9 miles (15 kilometers) long by 5 miles (8 kilometers) wide. Its gravitational pull is so slight that you could leap off its surface into space—another reason not to visit!

• For much of human history, comets were not welcome visitors. They were thought to symbolize times of disaster and doom.