Lunar missions take a lot of planning – there are a lot of eventualities to take into consideration. One such complication is the fact that the mass (and therefore the gravitational field) of the Moon isn’t distributed evenly. Rather than being a uniform density all across, our nearest neighbour is loaded with mass concentrations (aka ‘mascons’) – big lumps of matter that are significantly heavier than their surroundings, whose distortions of the gravitational field mean that low orbiting satellites can easily be pulled to their doom.