US President Barack Obama and first lady Michelle Obama depart Joint Base Andrews in Washington en route to Johannesburg Dec 9, 2013. Obama will attend the memorial service for South Africa's Nelson Mandela on Tuesday.[Photo/Agencies]

ABOARD AIR FORCE ONE - US President Barack Obama embarked on a three-day trip on Monday ?with former President George W. Bush, leading a US contingent to attend a memorial service in South Africa for the late Nelson Mandela.

On board Air Force One for a flight estimated at 16 hours were Obama and his wife, Michelle, Bush and his wife, Laura, and former Secretary of State Hillary Clinton.

Flying separately to Johannesburg were former Presidents Bill Clinton and Jimmy Carter. The only surviving former president not traveling was Bush's father, George H.W. Bush, who is 89 and had a health scare year ago.

The rare gathering of American presidents is a sign of the respect Mandela - who died on Thursday at age 95 - has held among both political parties in the United States.

They will join more than 70 leaders from around the world for a ceremony at Johannesburg's Soccer City stadium.

The journey will likely be the most time that Obama and Bush will have spent in close proximity to each other.

The Bush camp was irked at frequent criticisms of Bush during Obama's 2008 campaign, but over the years the two men have developed a cordial relationship. Obama offered gracious comments about Bush at the opening of the former president's library and museum this year.

The Obama White House has appreciated Bush's disciplined effort to avoid getting involved in the usual partisan rhetoric between Democrats and Republicans.

The 43rd president has a deep interest in Africa and a significant part of his legacy is the PEPFAR program that helps AIDS victims in Africa.

Presidential travel usually requires weeks of preparation, but US officials have scrambled since Thursday to put together logistics for the trip.

Obama has said he was inspired to become involved in politics because of Mandela's struggle against racism.

After attending the Tuesday service, Obama was to return to Washington on Wednesday night.