Oort Cloud

The Oort Cloud is a hypothetical shell of icy bodies that completely surrounds the solar system. It is expected to begin about 2,000 astronomical units (AU) out and extend to 100,000 AU or farther (one AU is the average Earth-Sun distance, about 93 million miles/149 million km). Estimates say it could contain 100 billion bodies to one trillion or more. Most of its members are no more than a few miles in diameter. Passing stars may occasionally knock some of these blobs of ice and rock out of their orbits, either expelling them into interstellar space or sending them hurtling toward the inner solar system. Such bodies may account for long-period comets, which take more than 200 years to orbit the Sun. The Oort Cloud is named for Dutch astronomer Jan Oort, who first proposed its existence in 1950.

Comets

For astronomers, though, a comet’s beauty is not just in its appearance, but in its content: A comet is an

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