Space
The Moon: why going back is harder than it sounds
It was done sixty years ago with slide rules, and repeating it sustainably is a different and considerably harder problem.
Topic
exploration
Every Muskeology article tagged exploration — 2 pieces across 1 section.
Space
Why getting to Mars is harder than it looks
The launch is the easy part. Entry, landing, radiation, propellant and getting home are each unsolved at the required scale.