User blog comment:Mrob27/Geology excavation project/@comment-4681795-20111214212507/@comment-4583259-20111215034520

By digging a huge hole.

Really, that's all it takes! Here's another huge hole I dug, and it collected astronauts too:



If you want to catch astronauts on purpose, a long narrow trench that's at least 5 or 6 blocks deep is the quickest way to make something big that will catch a lot:



For more details, see my blog articles Astronaut Wrangling and The moat experiment.