Robert Louis "Bob" Behnken is a United States Air Force officer, retired NASA astronaut and former Chief of the Astronaut Office. Behnken holds a Ph.D in Mechanical Engineering and holds the rank of Colonel in the U.S. Air Force. Col. Behnken has logged over 1,000 flight hours in 25 different aircraft. He flew aboard Space Shuttle missions STS-123 and STS-130 as a Mission Specialist, accumulating over 378 hours in space, including 19 hours of spacewalk time. Behnken was also assigned as Mission Specialist 1 to the STS-400 rescue mission. He is married to fellow astronaut K. Megan McArthur.