They spoke during a phone call between the Vatican and six crew members on the space station.
The Pope asked the crew - three Americans, two Russians and an Italian - what gives them the most joy in their work.
Cmdr Bresnik, a US Marine who flew combat missions during the Iraq War, said what strikes him is that in space there are "no borders, there is no conflict, it's just peaceful".
He added: "People cannot come up here and see the indescribable beauty of our Earth and not be touched in their souls.
"You see the thinness of the atmosphere, it makes you realise how fragile our existence here is."
Francis asked the crew how they understand Dante's verse that love is the force that moves the universe.
Russian cosmonaut Alexander Misurkin said he had been reading St Exupery's The Little Prince in space and was taken by the child's understanding of love.
He told Francis: "Love is the force that gives you strength to give your life for someone else."
Francis praised his response, saying: "It's clear you have understood the message that St Exupery so poetically explained, and that you Russians have in your blood, in your humanistic and religious tradition."
It was the second papal phone call to space: Benedict XVI rang the space station in 2011 and peppered its residents with questions about the future of the planet and the environmental risks it faced.
Italian astronaut Paolo Nespoli was aboard the orbiting lab for that call and was also around to chat with Francis, who considered a career in chemistry before becoming a priest.
