NASA Administrator Bill Nelson is losing one of his top helping hands as Associate Administrator Bob Cabana is set to retire at the end of the year.

The former Kennedy Space Center director and four-time space shuttle veteran, who is the top civil servant in NASA, will end his NASA career on Dec. 31 after 38 years. He has been part of NASA’s triumvirate of leadership for the past three years along with Nelson and Deputy Administrator Pam Melroy, both of whom were confirmed to their positions by the Senate.

“Bob is one of the finest leaders I’ve ever known. I’ve been fortunate to know him for decades, and I couldn’t be prouder to have had such a great colleague and friend throughout the years,” said Nelson in a NASA press release. “Bob is an example of the American grit, passion, and excellence that are woven into the fabric of our nation.“

Taking his place will be Jim Free, who was chosen to take on NASA’s newly created Exploration Systems Development Mission Directorate overseeing Artemis in 2021. Taking Free’s role will be Free’s deputy associate administrator in the directorate Catherine Koerner, a former program manager for Orion.

The associate administrator role Free is taking over from Cabana is a chief operating officer role for NASA’s 18,000 employees with direct reports from the 10 NASA center directors and the mission directorate associate administrators.

Cabana spent more than a decade leading KSC seeing the final missions of the Space Shuttle Program and its transition to support commercial crew launches.

“Leading the exceptional people at NASA who explore the universe for the benefit of humanity has been a great honor,” said Cabana in the release. “From flying in space to guiding teams across the agency in achieving NASA’s mission, I am grateful for an incredible career at NASA and in the space industry, and thankful for all the enriching friendships made throughout this journey.”

Cabana, who graduated from the U.S. Naval Academy in 1971, became an astronaut candidate in 1985 and made his first shuttle mission on STS-41 as pilot on Space Shuttle Discovery in 1990.

He flew again as the pilot on Discovery on STS-53 in 1992, and then as mission commander on Space Shuttle Columbia on STS-65 in 1994 and STS-88 on Space Shuttle Endeavour in 1998. He retired as a colonel from the U.S. Marine Corps in September 2000.

He joined NASA management in 2000 based at Johnson Space Center and then as director of Stennis Space Center in 2007 before shifting to KSC in 2008.