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US President Barack Obama looks out the window from the cell where Nelson Mandela was once jailed on Robben Island, on June 30, 2013. Paying homage to the 94-year-old former South-African president and anti-apartheid activist who is critically ill in the hospital, Obama stared into the stark cell where Mandela spent two thirds of his 27 years in jail. JIM WATSON/AFP/Getty Images ( 521018277 )

U.S. President Barack Obama in silhouette, center, walks by a bar covered windows during a tour of the prison on Robben Island, South Africa, Sunday, June 30, 2013. Former South African president Nelson Mandela spent 18 years of his 27-year prison term on the island locked up by the former apartheid government. (AP Photo/Carolyn Kaster) ( Obama US South Africa )

U.S. President Barack Obama walks from Section B, prison cell No. 5, on Robben Island, South Africa, Sunday, June 30, 2013. This was former South African president Nelson Mandela's cell where he spent 18 years of his 27-year prison term locked up by the former apartheid government. (AP Photo/Carolyn Kaster) ( Obama US South Africa )

Ahmed Kathrada, left, a former prisoner with Nelson Mandela, gives a tour of the prison on Robben Island in South Africa to the Obama family on Sunday, June 30, 2013. Former South African president Nelson Mandela spent 18 years of his 27-year prison term on the island locked up by the former apartheid government. (AP Photo/Carolyn Kaster) ( Obama US South Africa )

From left, first lady Michelle Obama, Sasha Obama, Ahmed Kathrada, former prisoner with Nelson Mandela guiding the tour, U.S. President Back Obama, Marian Robinson and Leslie Robinson, look out over the courtyard of the prison on Robben Island, South Africa, Sunday, June 30, 2013. Former South African president Nelson Mandela spent 18 years of his 27-year prison term on the island locked up by the former apartheid government. (AP Photo/Carolyn Kaster) ( South Africa US Obama )

U.S. President Barack Obama and first lady Michelle Obama tour Robben Island, South Africa, Sunday, June 30, 2013. Robben Island is a historic Apartheid-era prison that held black political prisoners including former South African president and anti-apartheid hero Nelson Mandela. (AP Photo/Evan Vucci) ( Obama Africa )

US President Barack Obama and First Lady Michelle Obama sign a guestbook in a prison yard as they tour Robben Island outside Cape Town, South Africa, on June 30, 2013. Obama was "deeply humbled" by a visit to the cell where Nelson Mandela spent 18 years as a prisoner, in a solemn homage to the critically ill hero he was unable to see in Pretoria. SAUL LOEB/AFP/Getty Images ( 521021053 )

A note written by U.S. President Barack Obama and signed by both him and first lady Michelle Obama after they toured Robben Island, South Africa, Sunday, June 30, 2013. Robben Island is an historic Apartheid-era prison that held black political prisoners including former South African president and anti-apartheid hero Nelson Mandela. (AP Photo/Evan Vucci) ( Obama Africa )

U.S. President Barack Obama walks with his family as they visit the rock quarry labor camp where Nelson Mandela was forced to work as a prisoner on Robben Island near Cape Town, South Africa on June 30, 2013. The Obamas visited the bleak former prison island off the coast of South Africa on Sunday to pay tribute to ailing anti-apartheid hero Mandela and set the stage for a speech urging Africans to strive for prosperity and democracy. Pictured with the first family is their guide Ahmed Kathrada (L). REUTERS/Jason Reed ( OBAMA-SAFRICA/ )

U.S. President Barack Obama is pictured with his family, as he visits the rock quarry labor camp where Nelson Mandela was forced to work as a prisoner on Robben Island near Cape Town, South Africa on June 30, 2013. The Obamas visited the bleak former prison island off the coast of South Africa on Sunday to pay tribute to ailing anti-apartheid hero Mandela. Pictured with the first family is their guide Ahmed Kathrada (2nd R). REUTERS/Jason Reed ( OBAMA-SAFRICA/ )

US President Barack Obama (3R), First Lady Michelle Obama (C) and their family, tour a limestone quarry where prisoners would work at Robben Island outside Cape Town, South Africa, on June 30, 2013. Obama was "deeply humbled" by a visit to the cell where Nelson Mandela spent years as a prisoner, in a solemn homage Sunday to the critically ill hero he was unable to see in Pretoria. SAUL LOEB/AFP/Getty Images ( 521022880 )

US President Barack Obama (R), and First Lady Michelle Obama (2L) listen to former prisoner Ahmed Kathrada with Michelle Obama's mother Marian Robinson (L) as they tour the limestone quarry where prisoners worked on Robben Island off the coast of Cape Town, South Africa on June 30, 2013. President Barack Obama was "deeply humbled" by a visit to the cell where Nelson Mandela spent years as a prisoner, in a solemn homage Sunday to the critically ill hero he was unable to see in Pretoria. SAUL LOEB/AFP/Getty Images ( 521018977 )

U.S. President Barack Obama, second from left, and the first family tour a rock quarry where Robben Island prisoners once worked near Cape Town, South Africa, Sunday, June 30, 2013. Robben Island is an historic Apartheid-era prison that held black political prisoners including former South African president and anti-apartheid hero Nelson Mandela. From left, tour guide Ahmed Kathrada, Obama, daughter Sasha, first lady Michelle Obama, and daughter Malia. (AP Photo/Evan Vucci) ( Obama Africa )

U.S. President Barack Obama arrives in Cape Town, South Africa on June 30, 2013. REUTERS/Jason Reed ( AFRICA-OBAMA/ )

Hundreds watch as Marine One helicopter, with US President Barack Obama aboard, lands at a field prior to Obama's tour of the Desmond Tutu HIV Foundation Youth Centre in Cape Town, South Africa, on June 30, 2013. SAUL LOEB/AFP/Getty Images ( 521022848 )

Counter assault team members stand on the roof of the Desmond Tutu HIV Foundation Youth Centre as U.S. President Barack Obama takes part in a health event there with youth in Cape Town, South Africa on June 30, 2013. REUTERS/Jason Reed ( AFRICA-OBAMA/ )

Protesters hold placards on a bridge near the University of Cape Town ahead of a visit by U.S. President Barack Obama in Cape Town, South Africa on June 30, 2013. REUTERS/Mark Wessels ( AFRICA-OBAMA )

Protesters hold placards on a bridge near the University of Cape Town ahead of a visit by U.S. President Barack Obama in Cape Town, South Africa on June 30, 2013. REUTERS/Mark Wessels ( AFRICA-OBAMA/ )

US President Barack Obama chats with Desmond Tutu following a tour of the Desmond Tutu HIV Foundation Youth Centre in Cape Town, South Africa, on June 30, 2013. Desmond Tutu on Sunday hailed Nelson Mandela as an "incredible, incredible person" who was still uniting South Africans from the hospital bed where he is battling for his life. SAUL LOEB/AFP/Getty Images ( 521019815 )

US President Barack Obama tours a classroom alongside Nobel peace laureate Archbishop Desmond Tutu (L) at the Desmond Tutu HIV Foundation Youth Centre in Cape Town, South Africa, on June 30, 2013. SAUL LOEB/AFP/Getty Images ( 521022891 )

U.S. President Barack Obama listens to young rapper Aviwe Ntongana, who calls himself 'Katmeister', as he tours the Desmond Tutu HIV Foundation Youth Centre and takes part in a health event in Cape Town, South Africa on June 30, 2013. REUTERS/Jason Reed ( AFRICA-OBAMA/ )

U.S. President Barack Obama looks at a computer with a youth as he tours the Desmond Tutu HIV Foundation Youth Centre and takes part in a health event in Cape Town, South Africa on June 30, 2013. REUTERS/Jason Reed ( AFRICA-OBAMA/ )

US President Barack Obama speaks about US and South African relations and younger generations at the University of Cape Town in Cape Town, South Africa, on June 30, 2013. Obama was "deeply humbled" by a visit to the cell where Nelson Mandela spent years as a prisoner. SAUL LOEB/AFP/Getty Images ( 521021452 )

A member of the audience takes a picture of U.S. President Barack Obama as he delivers remarks at the University of Cape Town in South Africa on June 30, 2013. REUTERS/Jason Reed ( AFRICA-OBAMA/ )

The audience reacts as U.S. President Barack Obama delivers remarks at the University of Cape Town in South Africa on June 30, 2013. REUTERS/Jason Reed ( AFRICA-OBAMA/ )

U.S. President Barack Obama delivers a speech at the University of Cape Town on June 30, 2013 in Cape Town, South Africa. This is Obama's first official visit to South Africa where bilateral talks with President Jacob Zuma have been held, as well visiting Robben Island where Nelson Mandela was imprisoned for 27 years for his fight against apartheid. (Photo by Michelly Rall/Getty Images) ( 171980297 )

People listen to US President Barack Obama's speech on US - African relations and the younger generation at the University of Cape Town in South Africa on June 30, 2013. Obama made Nelson Mandela the keystone of his address at the University of Cape Town, citing his unifying legacy as a blueprint for a new generation in emerging Africa. SAUL LOEB/AFP/Getty Images ( 521022865 )