We need to build a gun that's a hundred times bigger than any gun ever built. Then aim it at a moving target over 200,000 miles away. And that's just the start of our problems.

We'll need to recycle the air in the capsule: chlorate of potassium will release oxygen, and caustic potash will absorb carbon dioxide. And we'll need crude rockets to slow us down when we arrive adjust our direction. And diving suits with compressed air so we can leave the capsule. And enough supplies until the people back on Earth can shoot more supplies at us. Then when there we'll need to somehow construct another gun for our return journey. Thankfully it can be much smaller, due to the moon's lower gravity. So many problems to solve! So many unknowns!

The first problem is to find somebody capable of building such a gun. And the next problem is to persuade them to do it! Or have I forgotten anything?