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The Oort Cloud is believed to be a thick bubble of icy debris that surrounds our solar system. This distant cloud may extend a third of the way from our Sun to the next star—between 5,000 and 100,000 astronomical units. Earth is about one astronomical unit from the Sun (roughly 93 million miles or 150 million kilometers).
Consider this: At its current speed of about a million miles a day, NASA's Voyager 1 spacecraft won't reach the Oort Cloud for about 300 years. And it will take about 30,000 years to reach the other side.
In 1950, Dutch astronomer Jan Oort first proposed the idea of this sphere of icy bodies to explain the origins of comets with that take thousands of years to orbit the Sun. These are called long-period comets and most have been seen only once in recorded history. More frequent visitors to the inner solar system are called short-period comets. The Oort Cloud was named for Oort.
There may be hundreds of billions, even trillions, of icy bodies in the Oort Cloud. Every now and then, something disturbs one of these icy worlds and it begins a long fall toward our Sun. Two recent examples are comets C/2012 S1 (ISON) and C/2013 A1 Siding Spring. ISON was destroyed when it passed too close the the Sun. Siding Spring, which made a very close pass by Mars, will not return to the inner solar system for about 740,000 years.
How the Oort Cloud Got its Name
This region of space is named for Dutch astronomer Jan Oort who predicted its existence in 1950.
The Oort Cloud is too far to be seen with current telescopes, so it hasn't been directly seen or discovered. However, it is scientists' best guess about where long-period comets come from. Astronomers have studied several comets believed to have come from this distant region of our solar system.