Astronomical unit
From Memory Alpha, the free Star Trek reference.
An astronomical unit (abbreviated AU) is a unit of measure on the interplanetary scale, roughly equal to the (average) distance between Earth and Sol. One astronomical unit is equal to 150 million kilometers, or approximately 93 million miles.
The intense energy cloud created by V'Ger was approximately 2 AU in diameter. (Star Trek: The Motion Picture)
The nucleogenic cloud being measured 7 AU in diameter. (VOY: "The Cloud")
When searching for Harry Kim in 2371, the USS Voyager performed a subspace scan with a scan radius of 10 AU. (VOY: "Emanations")
When Captain Sisko's runabout was approached by two Jem'Hadar attack ships in the Badlands in 2373, they initially were 1 AU away. (DS9: "Blaze of Glory")
[edit] Background
In the original version of the film, V'Ger's cloud was actually 82 AUs across. At their farthest point from each other (called 'aphelion'), Earth and Pluto are just over 49 AUs apart. This would have made V'Ger's cloud almost twice that length, completely engulfing the solar system. As a result, the cloud size was toned down to a more reasonable number in the 2001 director's edition but even then, 2 AUs, the diameter of the Earth's orbit, is still an enormous size.
