Despite the inherent dangers of space exploration and the serious nature of NASA's scientific work, the agency has long had a playful side. Here the astronauts of NASA's Expedition 45, which journeys to the International Space Station in September, pose for a poster in "Star Wars" costumes -- complete with light sabers.
Astronaut Leland Melvin included his dogs, Jake and Scout, in his official NASA photo. Posted on Twitter in early February, it was retweeted thousands of times.
Astronauts aboard the International Space Station wear Hawaiian shirts for an April 2001 group portrait inside a space module.
James S. Voss, Expedition Two flight engineer, reads the book "The Last of the Mohicans" while riding a stationary ergometer cycle aboard the International Space Station in 2001.
Astronaut Steven A. Hawley runs on a treadmill on the middeck of the Space Shuttle Columbia in 1999.
Astronauts pose in a "starburst" formation to celebrate a successful shuttle radar topographic mission aboard NASA's space shuttle Endeavour in 2000.
Astronauts Brent W. Jett, Jr., left, and William M. Shepherd observe an old Navy tradition of ringing a bell to announce the arrival of someone to a ship -- in this case, the International Space Station in 2000.
In 2001 astronauts James S. Voss, left, and James M. Kelly share a friendly moment onboard the International Space Station's U.S. laboratory Destiny in spite of the longstanding rivalry between their respective alma maters, Auburn University and the University of Alabama.
In June 2014, NASA astronauts Reid Wiseman and Steve Swanson and ESA astronaut Alexander Gerst take a break to watch the World Cup between science experiments aboard the International Space Station.