H2 had been the aft-most piece of the Stack—the train of modules that ran up Izzy’s central axis—until several weeks ago when the Caboose had been launched up from Kourou and mated to its aft end. The main purpose of the Caboose was to house a large rocket engine, burning hydrogen and oxygen, which would do most of the work of boosting Izzy’s orbit. It wasn’t possible to extend Izzy any farther back because, beyond that point, anything tacked onto it would no longer reside safely within Amalthea’s protective envelope. And indeed there had been long discussions of contingency plans for the case where the Caboose took a hit and its engine was destroyed.