The ion engine used on the spacecraft is said to be 5-10 times more
efficient than an ordinary chemical booster. That is to say, it can provide
5-10 times more thrust than a regular engine, with the same amount of fuel.
The downside of the ion engine is that its thrust is _very_ gentle, so you
can't change your velocity very fast. While an ordinary booster can expend
all its fuel in a matter of seconds/minutes (which can be regarded as
instantaneous, compared to orbital periods in question here), an ion booster
can take months to spend its fuel and accomplish the desired velocity
change. So, in contrast to sending a spacecraft to the target via a standard
Hohmann transfer orbit (when you fire all your booster fuel at launch from
Earth parking orbit, aiming the apogee at the Moon's orbit), you have to
keep the ion engine running a long time and you basically end up with a very
long spiraling orbit to the Moon, which starts at Earth parking orbit and
ends at the Moon's orbit. That's pretty much the whole idea, though in
reality things get more complicated by the Moon's gravitational
perturbations...

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The butler did it.