The ISS crew members - American Suni Williams and two Russians, Commander Fyodor Yurchikhin and Flight Engineer Oleg Kotov - answered questions from British-born NASA astronaut Michael Foale, who stood next to the Queen. The video link was one-way, so crew members were unable to see the Queen.

"It's a wonderful place to live so thank you very much for joining us today." said Suni Williams. "This is a stepping stone to going back to the moon and on to Mars.", she added.

"Very fascinating," the queen said after hearing the three crew members describe life in space.

Later in the day, Queen Elizabeth II remembered the sacrifices of World War II by placing a wreath at a US memorial in Washington.

"For those of us who have witnessed the peace and stability and prosperity enjoyed in the United Kingdom and the rest of Europe over these postwar years, we have every reason to remember that this has been founded on the bedrock of the Atlantic Alliance," she said.

Last week, Elizabeth visited Jamestown, Virginia, which is celebrating 400 years since its founding as the first British colony in the New World. The queen, an avid horse fan, also attended the Kentucky Derby.

In a speech at the White House Monday, Elizabeth called her visit "a window on the future" of US-British relations, and said she was looking forward to seeing "something of how the cutting edge of science and technology can take us to the next phases of discovery and exploration in human endeavour."