Which is the moon the monolith aliens don't want us near?
In the movie, and the book sequels 2010, 2061, and 3001, it was Europa, around Jupiter. The monolith took a gamble on the walking coral-reef things under the ice on Europa, and turned Jupiter into a sun to give them a chance. And the star-children/space ghosts/whatever of Bowman, HAL, and the Monolith ordered humanity to stay away from Europa.
In the original book version of 2001, the Discovery mission was hastily changed to just fly-by Jupiter, then go to Saturn, which added to the stress and length of the mission and made Bowman and Poole a bit paranoid. This also made HAL's lie (to conceal the monolith found on the moon, and the signal it sent to the "big one" from Bowman and Poole) which drove him insane a bit of a bigger deal. Then the sequel books were just revised to match the movie and everything about Saturn was dropped, since the story otherwise to that point was essentialy the same.
The Saturn moon the big monolith sat on in the book was Iapetus, which was known from Earth teloscopes for some time to be very odd, because we knew for some time that one side is as black as coal, and the other is as white as snow. Clarke decided that having the big monolith that sucked up Bowman and sent him across the galaxy to have caused that coloration by being set down in Iapetus. And the blasted out white part was a big artificial signal to say "Hey! Something's HERE" in case humans got that far without finding TMA1 on the moon first maybe. In the movie and the later books, that monolith just orbited freely around Jupiter, or sat on Europa to protect/guide the life emerging onto the now warm and melted surface from Jupiter being "stellated".
Now with a good look at Iapetus from the Cassini probe, the moon is even
weirder than we initialy thought. Aside from being ice white on one side, and coal-black on the other, it's got a near perfect mountain ridge around it's equator making the whole moon look sort of like a walnut, or like the seams on a soccer ball. It has all sorts of other odd features, probably from some kind of internal collapse, or coming back together from being blasted into chunks.
http://photojournal.jpl.nasa.gov/jpegMod/PIA08376_modest.jpgSo now because of it's oddbal shape, and the ridges, the "face on Mars/big NASA conspiracy tin-foil guys" are now claiming Iapetus is a big ancien Death-Star like space station.
