The man who set Canada's record for spacewalks, hovering in the inky sky for 17 hours and 47 minutes, will retire from active astronaut status effective Saturday.
The Canadian Space Agency made the announcement late Friday but did not explain the departure of Williams, 53, except to say that he will move to Ontario with his family and explore other career options.
Neither Williams nor space agency officials were available for comment.
Williams departure comes as NASA enters the final phase of the shuttle program. NASA plans about a dozen more flights, with the final mission due in 2010.
Jim Prentice, federal minister responsible for the space agency, says William's accomplishments in the past decade helped position Canada as a space-faring nation.
Williams inspired "young people across our country to take their place as members of Canada's next space generation," Prentice said in a statement.
Maybe one of them will realize a dream Williams once described: To see a Canadian on the moon by 2019, the 50th anniversary of the first lunar landing by Apollo 11 in July 1969.
"My dream is we can go forward and look at human return to the moon as a potential international collaborative venture," he said in an interview shortly after returning from his last trip into space.
"Wouldn't it be exciting to see Canada part of that."
Since being selected as an astronaut in 1992, Williams has flown in two space shuttle missions.
His first mission was in 1998 aboard Columbia. He spent 16 days on experiments focusing on the effect of weightlessness on the brain. He acted as scientist and subject in 26 experiments.
Williams completed his second space flight as a mission specialist in August 2007 to assist in construction of the International Space Station.
He spent a total of 28 days, 15 hours in space during his missions.
"You really can't see your feet at all but all around you, you've got this panoramic view of the Earth beneath you," he said in an earlier interview. "It's absolutely spectacular."
Williams, who also worked with NASA in Houston, Texas, as director of the space and life sciences directorate, is married to Kathy, an Air Canada pilot, and they have two children.
A specialist in emergency medicine, he did research into patient survival of heart attacks outside hospitals and identification of trauma patients at high risk.
Born in Saskatoon, Sask., Williams holds a doctorate of medicine and a Master of Surgery from McGill University in Montreal.
He is an avid sportsman, enjoying scuba diving, sailing, hiking, and skiing.
