Contrary to sci-fi depictions in shows like Iron Man and Star Wars, getting from point A to point B in space is a tough engineering problem. NPR Science Correspondent Geoff Brumfiel explains how space propulsion actually works, and why some new technologies might be needed to get humans to Mars and beyond.Follow Geoff Brumfiel and Short Wave co-host Emily Kwong on Twitter. Email the show at shortwave@npr.org.