Astronauts and CNN iReporters alike describe space shuttle Discovery's last flight as "bittersweet."

When Discovery touched down at Dulles International Airport near Washington on Tuesday, it marked the end of the shuttle program - and the end of an era.

To commemorate the shuttle's final flight, CNN teamed up with NASA and the Smithsonian to collect images from people who spotted the shuttle on its journey, atop a specially modified 747, from Kennedy Space Center in Florida. It will be transferred into the Smithsonian's collection at a public ceremony on April 19 at the museum's Udvar-Hazy Center in Chantilly, Virginia.

Check out the most stunning images of Discovery's journey in the gallery above, and visit our Open Story to see more submissions on a map and submit your own.