"Alan has rendered invaluable service to NASA as the principal investigator for the Pluto/New Horizons mission, as a member of the NASA Advisory Council and as the associate administrator of the Science Mission Directorate," said NASA Administrator Michael Griffin. "While I deeply regret his decision to leave NASA, I understand his reasons for doing so and wish him all the best in his future endeavors."

Stern's reasons for his resignation weren't reported.

Griffin said Edward Weiler, director of NASA's Goddard Space Flight Center, will serve as interim associate administrator. Weiler was appointed to Goddard in August 2004 after serving as the associate administrator for the agency's Space Science Enterprise from 1998 to 2004.

A native of Chicago, Weiler earned his doctorate in astrophysics at Northwestern University in 1976.