US President Barack Obama joked with astronauts in a call to the International Space Station

When he came on the line, the American leader said: "This is President Obama, who am I talking to?"

After the crew introduced themselves, he quipped: "Well, that's funny because I was just dialling out for pizza; I didn't expect to end up in space."

Mr Obama called the ISS to pay tribute to the crew of space shuttle Atlantis on its final visit to the station.

When Atlantis returns to Earth