That's because the new type of engine doesn't need rocket fuel to make it go, so it doesn't have to carry loads of the really heavy fuel with it.
Once it gets to the moon, around December 2004, Smart-1 will use x-ray scanners to find out what sort of things it is made of.
Depending on what they find, it may prove a theory that the moon was a bit of the Earth that was blown into space when a huge space object hit the planet billions of years ago.
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