A crucial mission for both NASA and ESA. NASA, because the project to go to the Moon and Mars is already billions above funding and years beyond timetables, and for ESA because this is the first time ever NASA trusts a non-American supplier to provide a vital, critical component for an else American mission, the service module. If it fails, it means most likely an end to the American political will to fund the project any longer, and to ever trust the Europeans again with providing such a vital compenent.
The political and financial stakes are always very high with space technology, but sometimes they are really-really very high.
