Enlarge / ULA stacks its Centaur V upper stage onto the first flight version of the Vulcan rocket at Cape Canaveral Space Force Station. (credit: ULA)

On the evening of March 29, at Marshall Space Flight Center in Alabama, United Launch Alliance started pressurizing the upper stage of its new Vulcan rocket. But then, suddenly, something went wrong with this Centaur upper stage.

Shortly after the incident, to his credit, the chief executive of United Launch Alliance, Tory Bruno, was quick to acknowledge on Twitter that something had happened: "Keeping you posted: During Qual testing of Centaur V structural article at MSFC, the hardware experienced an anomaly."

Unpacking this tweet a little bit, Bruno is saying that during qualification testing—the process of testing rocket engines and stages on the ground to determine their behavior during flight-like conditions—the Centaur stage had a problem. More than a week later, however, there are more questions than answers about the accident.