Armstrong and Aldrin were the first men to go to the moon in 1969, exactly 50 years and a day ago. A third astronaut, Michael Collins, remained in the control module and picked up his colleagues later. Mr. Armstrong died in 2012 at the age of 82 and Mr. Aldrin has issued a harrowing statement.
He said he was disappointed that they would not be able to celebrate the 50th anniversary of the Moon by landing together in 2019.
In an official statement, he wrote: "I am deeply saddened by the death of my good friend and space exploration companion, Neil Armstrong today."
He added: "I really hoped that on July 20, 2019, Neil, Mike and I would stand together to commemorate the 50th anniversary of our landing on the moon, as we also anticipated the continued expansion of humanity in space, which our little mission helped make it possible.
"Unfortunately, this does not have to be. Neil will surely be there with us in spirit."
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He added on Twitter: "I know I'm joined by millions of other people grieving over Neil's death – a true American hero and the best pilot I've ever met."
Mr Armstrong, Aldrin and Collins only had six frantic months to get to know each other before their mission and apparently "felt the weight of the world" on them, Mr. Collins later admitted.
Mr. Collins is the least known of the three and on previous anniversaries he was glad to be forgotten.
However, with "the huge void" left by Mr. Armstrong's death, Collins said he felt compelled to talk though "my first inclination to celebrate the 50th anniversary is to go and hide under a rock somewhere ".
His two daughters helped the avalanche of requests for 88-year-olds.
The extraordinary Apollo 11 mission, 50 years ago, was the culmination of decades of work by hundreds of thousands of people in the fields of science, technology and engineering.
The cost is around $ 25 billion – which is £ 20 billion or £ 323 billion in today's money.
They achieved the incredible feat by using the technology developed long before modern computers.
In fact, it has been calculated that an i-Phone has about 100,000 times the processing power of the computers used in the 1969 moon landings.
When they finally reached the moon, Armstrong's famous words appropriately marked the decisive moment of the century.
Obviously he said: "A small step for man, a giant leap for humanity".