WASHINGTON - Scott J. "Doc" Horowitz, associate administrator for NASA's Exploration Systems Mission Directorate, has announced plans to leave the agency in October.

Horowitz, a retired U.S. Air Force colonel who served as commander or pilot on four space shuttle missions, now leads NASA's efforts to develop the new family of spacecraft that will return astronauts to the moon by 2020.

Horowitz is stepping down for personal reasons. "I need to devote more attention to my family responsibilities," the father of three young children told colleagues this week. "I am very proud of our team and where we are headed - back to the moon," he added.

Horowitz' career plans are not certain. His successor will be named later.

In accepting Horowitz' resignation, NASA Administrator Michael Griffin praised the veteran astronaut's tireless contribution to the Vision for Space Exploration.

"Doc Horowitz has been the key person for NASA's exploration effort during the critical period immediately following definition of the architecture for shuttle replacement and lunar return," Griffin said.

"The Ares I crew launch vehicle concept is Doc's brainchild, a fact that crews launching safely a generation from now will remember with gratitude," the administrator continued. "Doc brought to NASA the perfect combination of integrity, drive, intelligence, engineering intuition, advanced education and flight crew experience. I am grateful for his contributions and his friendship, which will live on for both of us."

Prior to being named associate administrator for exploration in September 2005, Horowitz was director of exploration and space transportation at ATK, Brigham City, Utah. Previously, Horowitz worked as the acting deputy associate administrator for safety and mission assurance at NASA Headquarters in Washington.

Horowitz was an Air Force test pilot, F-15 fighter pilot and master flight instructor. He has worked as a scientist at Lockheed-Georgia Co., Marietta, Ga. He holds doctorate and master's degrees in aerospace engineering from the Georgia Institute of Technology in Atlanta and a bachelor's degree in engineering from California State University at Northridge.

For more information about NASA's exploration mission visit:

http://www.nasa.gov/exploration