Mission controllers have given Atlantis a "go" for an on-time undocking from the International Space Station at 2:28 a.m., the last by a shuttle.
"This is the that we are transitioning between a period in which space shuttles visited the International Space Station to a time in which they no longer will," Kwatsi Alibaruho, the lead shuttle flight director, told his team before the poll, according to NASA TV commentator Rob Navias.
Navias described an "almost reverent silence" in the shuttle and station mission control rooms, "in awe of the moment."
The station has maneuvered into its undocking orientation, with Atlantis now positioned in front of it in the direction of travel, and solar arrays have been feathered to avoid potential contamination from thruster plumes.
Atlantis' docking system is powered up.
The two spacecraft are flying more than 240 miles above the planet.
The crew awoke around 10 p.m. Monday to a tune perhaps appropriate for pilot Doug Hurley, who will be at the controls as Atlantis backs away from the station and performs a final flyaround of the station: "Don't Panic," by Coldplay.
"We get to do one last lap of Atlantis around the ISS and then start our trip home," Hurley radioed to Mission Control.
"Sounds great, Atlantis," replied Shannon Lucid, an astronaut communicating with the crew from Houston. "Have a good flyaround."