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Comets that take more than 200 years to make one revolution around the Sun are notoriously difficult to study. Because they spend most of their time far from our area of the solar system, many "long-period comets" will never approach the Sun in a person's lifetime. In fact, those that travel inward from the distant Oort Cloud can have periods of thousands or even millions of years.
NASA's WISE spacecraft, scanning the entire sky at infrared wavelengths, delivered new insights about these distant wanderers. Scientists found that there are about seven times more long-period comets measuring at least 0.6 miles (1 kilometer) across than had been predicted previously. They also found that long-period comets are on average up to twice as large as "Jupiter family comets," whose orbits are shaped by Jupiter's gravity and have periods of less than 20 years. Long-period comets that WISE observed probably got kicked out of the Oort Cloud millions of years ago.
The Oort Cloud is lies far beyond Pluto and the most distant edges of the Kuiper Belt. While the planets of our solar system orbit in a flat plane, the Oort Cloud is believed to be a giant spherical shell surrounding the Sun, planets, and Kuiper Belt Objects. It is like a big, thick bubble around our solar system. The Oort Cloud's icy bodies can be as large as mountains — and sometimes larger.
Go farther. Explore the Oort Cloud in Depth ›
The Oort Cloud is a predicted, but undiscovered region of space.
Far, Far Away
The Oort Cloud surrounds our Sun, a star. The Oort Cloud is a spherical shell, occupying space at a distance between about 1,000 and 100,000 astronomical units (AU).
Long Way Round
Long-period comets (which take more than 200 years to orbit the Sun) come from the Oort Cloud.
The Oort Cloud may contain more than a trillion icy bodies.
Closer and Bigger
Comets that originate from the Oort Cloud gain atmospheres (the coma) when they near the sun. This atmosphere collapses when the comet's orbit carries it farthest from the sun.
Too Far to See
There are no known moons of Oort Cloud objects.
The are no known rings around objects in this region of space.
A Long Trip
There have been no missions sent to the Oort Cloud.
Cold and Dark
The Oort Cloud is not capable of supporting life as we know it.
The Oort Cloud is named for the astronomer who predicted its existence during the 1950s: Jan Oort.
Voyager 1 travels at a million miles a day. At that speed, it will take the spacecraft about 300 years to reach the inner layer of the Oort Cloud and probably another 30,000 years to get to the far side. The Oort cloud is likely that thick.