A coma is the fuzzy-looking glow that can be seen around the head of a comet.
Photograph by Michael Jäger
Many comets pass close enough to Earth to be visible to the eye, but most small or distant comets can only be seen with a telescope.
Photograph by Damian Peach
A comet's tail can be very long and beautiful. It sometimes stretches for millions of miles.
Illustration by David A. Aguilar
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.