This is a drawing of Pluto and its moon Charon.
Click on image for full size
Image courtesy of NASA/JPL. This image is in the public domain.

AU

AU stands for Astronomical Units. It is a useful way to measure the distances in interplanetary space. It is the distance between the Earth and the Sun, which is about 93 million miles.

For reference, Mars is 1.5 AU from the sun, Jupiter is 5 AU from the sun, Pluto is about 39 AU from the sun, the heliopause is about 100 AU away, and the Oort Cloud is about 100,000 AU.

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