NASA's science chief resigned on Wednesday after just a year in the post, with the US space agency giving no reason for his sudden departure.
The US space agency said planetary scientist Alan Stern is quitting as associate administrator for the Science Mission Directorate.
The announcement came a day after NASA rescinded a letter sent last week to the Jet Propulsion Laboratory in California directing budget cuts in one of NASA's leading science missions, the Mars exploration program.
In light of the projected cuts, mission team members said one of the two robot rovers exploring the Martian surface might have to shut down and the activities of the other one limited.
In a Tuesday statement, NASA said agency Administrator Michael Griffin "has stated that no Mars rover operations will be suspended or shut down" and last week's budget-cutting letter "was not fully coordinated" with his office.
Asked if the Mars exploration issue led to Stern's decision, NASA spokesman Robert Jacobs said, "There's no indication one way or the other. The bottom line is that it was his (Stern's) decision to resign."
In an e-mail to NASA colleagues, Stern did not give a reason for resigning and said he would remain "for a few weeks."
Stern will be replaced on an interim basis by Edward Weiler, director of NASA's Goddard Space Flight Center in Greenbelt, Maryland, the agency said.
"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," Griffin said in a statement.