While fueling the Centaur upper stage with liquid hydrogen for Friday’s launch attempt, the ground crew found a small leak in the Upper Stage Umbilical. The hydrogen is chilled to 20 °K above absolute zero, and the moment it touches open air, frost forms. What the ground crew saw was a ball of ice growing on the umbilical, which United Launch Alliance called “outside their history”. There’s a slim chance it could have led to damage, so they held the launch to diagnose and fix the leak. ULA has a perfect safety record on their Atlas V/Centaur launches, and this kind of caution is why.
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