From a New York Times story by Kenneth Chang headlined “At NASA, Dr. Z Was OK With Some Missions Failing”:

Dr. Z has left the building.

After six years, Thomas Zurbuchen concluded his tenure as the head of NASA’s science missions at the end of 2022.

During his time there, he earned his single-letter nickname while presiding over some of the agency’s biggest successes in the exploration of the solar system and the universe: the long-delayed launch of the James Webb Space Telescope; the landing on Mars of the Perseverance rover, which was accompanied by the Ingenuity helicopter; and the slamming of the DART spacecraft into a small asteroid, demonstrating a technique that could be used if a space rock were discovered on a collision course with Earth.