By Joey Roulette WASHINGTON (Reuters) – Thomas Zurbuchen, NASA’s science chief who oversaw some of the U.S. space agency’s ambitious endeavors including the successful deployment of the James Webb Space Telescope and Perseverance Mars rover, is set to step down at the end of the year. The Swiss-American astrophysicist has served as head of NASA’s science mission directorate since 2016, shepherding the agency’s roughly 100 such missions. He announced his planned departure in a memo sent to NASA employees on Tuesday. “This is a difficult decision for me, but I believe it is time for a new beginn…