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 149,597,870,691 ±30 meters, or approximately 150 million kilometers. Because interplanetary distances so quickly run up into the billions of miles or kilometers, (depending on which measurement system one uses), the AU is used to make these distance much easier to quantify in terms of easily discussing large distances throughout the solar system. Upon leaving the solar system and discussing interstellar distances, the term AU fails and the light year or parsec is then brought in.
The intense energy cloud created by V'Ger was approximately 2 AU in diameter. (Star Trek: The Motion Picture)
- 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 is still an enormous size.
References
Background
The uncertainty of the SI length of an AU is due to the imprecision in our current knowledge of the exact gravitational constant. See AU at Wikipedia for more details.