A 401 -> 4m shroud. 0 SRBs 1 engine Centaur
Which is a bit ironic given the stage was called Centaur because it had two heads (engines).
A 401 is pretty much the baby Atlas V.
Which means there is less to go wrong, which is very attractive on a long ride to Mars. And it's cheaper. All of which are pretty attractive features.
Biting the hand that feeds IT
