Unfortunately, all good rockets must inevitably step aside for newer, more capable vehicles, and for Delta II, the end has been in sight for several years. The rocket's lift capacity is just 6.1 metric tons to low-Earth orbit, yet NASA still paid $96.6 million to launch ICESat-2 including payload processing services. A SpaceX Falcon 9 costs roughly the amount, but can lift spacecraft four times heavier.

The last Delta II to launch from Cape Canaveral was GRAIL in 2011; in July, the dual towers that supported Delta II launches there were demolished. That left just Space Launch Complex 2 at Vandenberg for Delta II flights to polar orbits.

ICESat-2 will only lift off after an operator physically clicks a mouse button as part of the launch sequence. I have known this quirky Delta II fact for a long time, but couldn't remember the reason, so I asked United Launch Alliance (ULA), which operates the rocket, for an explanation. The answer came from Bill Cullen, the director of launch operations for the company's upcoming Vulcan rocket. Cullen has worked on the Delta II program since the beginning. Here is his answer, lightly edited: