The space shuttle Endeavour and its crew are scheduled to return to Earth early Wednesday morning, completing NASA's second-to-last shuttle flight. There is only one more flight on the schedule, by the shuttle Atlantis -- the end of a 30-year program.
Endeavour undocked from the International Space Station late Sunday. The entry flight control team will evaluate weather conditions at the landing site at Merritt Island, Fla., before giving the approval to land, NASA said.
The 25th and final flight for Endeavour also marks the last flight for the six men flying Endeavour. The astronauts, all veterans of previous flights, are wrapping up a 16-day mission, where they added the last major components to the U.S. section of the International Space Station.
Endeavour's crew includes Cmdr. Mark Kelly; pilot Greg Johnson; spacewalkers Mike Fincke, Drew Feustel and Greg Chamitoff; and European Space Agency astronaut Roberto Vittori, who's handling robotics.
"To see the horizon out there, with all that hardware beneath you and to understand what we've done as a nation and as a world to build that international space station; it's spectacular," astronaut Feustel said.
Spacewalker Fincke, who has spent months living on the space station in years past, now holds the U.S. record for time in space, at 380 days.
"I hope my record is soon broken," Fincke said.
Endeavour delivered a $2 billion cosmic ray detector, called Alpha Magnetic Spectrometer-2 (AMS), which will remain mounted on the space station for the next decade. The cosmic ray detector is searching for antimatter and dark matter, which scientists hope will shed light on the origins of the universe.
"Understanding what the universe is -- the universe we live in, how we all got here, how it was formed -- these are fundamental questions, and this is what AMS is going to be trying to understand for us," Chamitoff said.
The mission has so far been a complete success for NASA.
Space Shuttle: Emotional Trip for Mark Kelly
It has been a special voyage for the entire crew, and especially so for Kelly, whose wife, U.S. Rep. Gabrielle Giffords, is still recovering from a gunshot wound to the head after the assassination attempt she survived in Tucson, Ariz., almost five months ago.
Kelly almost gave up his final opportunity to fly in space, but was finally persuaded his wife was in good hands and would want him to go. She underwent a successful cranioplasty surgery in Houston two weeks ago, after his launch, to replace bone that was lost after the shooting.
"I think it was the right decision. She was ready for the surgery," Kelly said.
NASA arranged for a videoconference between Giffords and Kelly, so they would be able to see each other during Endeavour's mission. There is also an Internet phone on the space station, and astronauts in orbit have access to email.
Speaking with ABC News in a crew interview from space, Kelly also shared some new good news about Giffords' progress.
"She'll be doing rehab for some continuing period of time," he said. "But at some point pretty soon she'll become an outpatient. So we're really looking forward to that. And it's all really encouraging."
After Endeavor lands at the Kennedy Space Center in Florida Wednesday, there will be only one more shuttle flight: Atlantis is currently scheduled for launch July 8. After Endeavour lands, it will be "safed" by engineers -- engines and explosive bolts will be removed, and traces of toxic fuel will be cleared from its plumbing. It will ultimately go on display in a Los Angeles museum.
What's next for America's astronaut program? That's not clear. The Obama administration has talked about having private companies launch astronauts to the station in yet-to-be-completed spacecraft, and NASA has been asked to make plans for trips to asteroids and, eventually, Mars. But all that is years away, and space people already talk wistfully of the space shuttle era as its end approaches.