President Barack Obama has taken off for South Africa to attend a national memorial service for Nelson Mandela.

Obama and his wife, Michelle, boarded Air Force One under cold, rainy skies this morning for the long flight.

Former President George W. Bush was already on board, along with former first ladies Laura Bush and Hillary Rodham Clinton.

Bill Clinton is traveling separately from Rio de Janeiro, where he's been attending a Clinton Global Initiative event. And former President Jimmy Carter is planning to jpoin the group in Johannesburg.

The only living U.S. president who won't attend is George H.W. Bush. His spokesman says the 89-year-old is no longer able to travel long distances.