Keith Cowing points out the political chicanery and fiscal absurdity of a 2021 attempt with SLS/Orion.
Also, note the technical issue. Orion was designed to come back from the moon, not from Mars. It can barely manage escape velocity on earth entry. Note page 17 of the Plymouth Rock paper:
Reentry velocities are 11.05 to 11.25 km/s for asteroid missions, vs 11.0 km/s for lunar return
TPS enhancement may be required depending on the ultimate capability of Orion lunar TPS
They’ll be coming in a lot hotter than that for Mars. And what will they use for habitat? There’s no way Orion itself is large enough for a mission of that duration.
Note: I do think that the mission is physically and fiscally possible, in that time frame. Just not with SLS/Orion.
[Update a while later]
If Congress was serious about a Venus/Mars flyby in 2021, it would divert all funding from SLS/Orion to hardware actually needed to do it.
— Rand Simberg (@Simberg_Space) February 26, 2014