WASHINGTON (Reuters) - NASA chief Sean O'Keefe faced withering questions from senators on Wednesday over his handling of the shuttle Columbia disaster, a week after investigators blamed the U.S. space agency's culture for the accident that killed seven astronauts. "When do you expect this culture to be fixed?" Sen. John McCain, an Arizona Republican, asked O'Keefe at a hearing of the Senate's Commerce, Science and Transportation Committee. "This is going to be a long, long haul," O'Keefe replied, adding that the "beginnings of this change" would be evident in six months to a year. O'Keefe told the senators he planned...