With the ATK improvements, Sauvageau says, only those reference missions requiring the full 130-metric-ton capability—a lunar landing and a long-term mission to an asteroid—will require an upper stage. Others, including a lunar orbit flight with a crew and an “advanced” asteroid mission reaching “close proximity” to the target for two weeks, can be accomplished with an SLS core stage carrying four shuttle main engines or their throw-away successors.

“Since the more difficult missions are not anticipated until into the late 2020s or early 2030s, the cost of the upper stage can be deferred for a number of years, thus allowing NASA to develop the other exploration elements necessary to have defined missions,” Sauvageau says.