/u/danielravennest on I am Jeff Hoffman, MIT Professor of Aerospace Engineering and former NASA Astronaut. Ask Me Anything!19 Feb 2015, 21:11 UTC
Having worked on Mars mission studies at Boeing, I can take a stab at this one:
We should build a variable-g facility in Earth orbit, and study the effects of gravity between zero and one gee on humans and plants. We have lots of data for one gee (obviously), and a good amount on zero gee, but nothing in between. We don't know if the crew can function with 8 months in zero gee followed by some amount of time at 3/8 gee (Mars). Until we do know, we don't know if the Mars ship needs a centrifuge.
We should probably work on high-efficiency propulsion. Solar-thermal needs half the fuel, and electric thrusters (ion or plasma) need 1/10 the fuel as current chemical rocket engines. That cuts how much you need to launch from Earth.
We should probably also work on extracting fuel and other supplies (oxygen, water, bulk shielding) from asteroids, Phobos, and the Martian surface. That also cuts how much you need to launch from Earth.
We don't yet know how to land anything bigger than the Curiosity Rover (900 kg) or more accurately than 20 km. Both need to be improved by at least a factor of ...