The SMART way to travel
The type of ion engine chosen for SMART-1 makes clever use of an effect discovered by the American physicist E.H. Hall in 1879, whereby a current flowing across a magnetic field creates an electric field directed sideways to the current. This is used to accelerate ions (charged atoms) of xenon. A gassy element with atoms about 131 times heavier than hydrogen atoms, xenon is chemically inert.
Drawing electric power of 1350 watts from SMART-1's solar panels, the ion engine generates a thrust of 0.07 newton. That is equivalent to the weight of a postcard.
By accelerating SMART-1 at 0.2 millimetre per second per second, the incredibly gentle thrust could in theory fling the spacecraft right out of the Solar System, if sustained for long enough. In practice, SMART-1 used its ion engine intermittently over 16 months, fighting against the Earth's attraction, to put itself into orbit around the Moon.
For the first 2 or 3 months, the leisurely journey could have brought problems due to SMART-1's exposure to possible harm from energetic atomic particles in the radiation belts that surround the Earth. The electronics and instruments were hardened to resist such damage.