Although the Hubble telescope is best known for the images it returns of distant objects in space, its very existence proves something entirely different. Since we all know that the stars only come out at night, the Hubble telescope, like all telescopes can only see objects in outer space during the night. During the day there's nothing much to look at so NASA rents time on the telescope to Google which uses it to create the Google maps.
The original design of the Hubble telescope didn't allow for its positioning relative to its orbit around the earth. This design meant that the telescope always pointed the same way. It points away from the earth for twelve hours and then points at the earth for twelve hours. As you can see from the official NASA schematic, the objective lens of the telescope always points in the same direction regardless of its position in its orbit. The telescope points out into space only half the time. The rest of the time it points to earth (for some more detail, see this) .
At first the scientists at NASA thought this arrangement was an error until Sergey Brin explains that it is actually a good thing and offers to buy the daytime images from the Hubble. NASA, being under funded as usual, agrees and sells the images to Brin which he uses to enhance Google. After all, you can't see outer space during the day anyway, right?