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

President Barack Obama (Kristoffer Tripplaar-Pool/Getty Images)

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 join 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.

The American leaders will join dozens of other dignitaries and tens of thousands of mourners at the memorial service at a stadium in Johannesburg.

Mandela will be buried next week, after a state funeral in his home town.

Biden signs Mandela condolence book at embassy

Vice President Joe Biden signs a book of condolences for former South African President Nelson Mandela at the South African Embassy (Win McNamee/Getty Images)

Vice President Joe Biden is honoring the memory of former South African President and anti-apartheid leader Nelson Mandela by signing a condolence book at the South African Embassy.

Biden traveled to the embassy on a cold, drizzly Monday morning.

Biden's inscription said, quote: "Through his unflagging, unflinching commitment to human dignity and his willingness to forgive, he inspired us and challenged us all to do better."

President Barack Obama left earlier for Johannesburg on Air Force One for Mandela's memorial service.

As a senator more than three decades ago, Biden traveled to South Africa where authorities denied his request to see the imprisoned Mandela. After Mandela's release in 1990, the two men met in Washington.

Biden will speak Wednesday at a Mandela memorial service at the National Cathedral in Washington.

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