Astronauts Rex Walheim, Sandra Magnus, Chris Ferguson and Doug Hurley, pictured left to right, train for NASA's final shuttle mission. Photo Credit/NASA photo

Four astronauts are training for NASA’s final shuttle mission, a 12-day voyage to the International Space Station that has the smallest shuttle crew in 28 years ready to the stock orbiting science lab with a year’s worth of vital supplies.

Atlantis is scheduled to lift off from NASA’s Kennedy Space Center on June 28 at 3:40 p.m., EDT.

Commander Chris Ferguson leads the smallest crew since the sixth shuttle mission, a five-day satellite deployment flown in 1983.

Discussion of the mission surfaced early last year. In January, NASA committed to the flight that was initially conceived of as a possible rescue mission for the crew of Endeavour. Endeavour’s six astronauts are prepared to launch on April 19 to the space station with the Alpha Magnetic Spectrometer, a $2 billion astronomical observatory.

The supplies that Atlantis will carry in the Rafaello Multipurpose Logistics Module will help to ensure the space station can remain staffed with astronauts carrying out scientific research even if new U. S. commercial cargo companies encounter delays in their initial missions.

In addition, Ferguson, pilot Doug Hurley, Sandra Magnus and Rex Walheim will retrieve an external space station coolant pump that sustained an unexplained electrical failure in late July. Engineers intend to disassemble the motor once it is returned to Earth do determine the cause of the breakdown.

“I think there is still some speculation as to whether we will actually fly this flight,” Ferguson told a gathering of news reporters at NASA’s Johnson Space Center on Wednesday.  “But with every passing day, I’m more and more convinced that NASA has the funding put aside and we have the tacit approval of Congress.”

Congress has yet to agree on a budget for the 2011 fiscal year. But lawmakers included the mission in the 2010 NASA Authorization Act that was passed by Congress and signed by President Obama.

Without a shuttle on standby to serve as their rescue vehicle, Ferguson’s crew is also training more extensively than usual in space station operations. Should Atlantis reach the station with heat shield damage that would prevent the orbiter from returning safely to Earth, the small shuttle crew would rotate back to Earth aboard a succession of Russian  Soyuz flights.

Walheim would return to Earth after three months, Ferguson after six months, Magnus after nine months and Hurley after a year.

The odds of serious difficulties with Atlantis, which last flew in May 2010, are small.

The flight will be the 33rd for Atlantis, which was first launched in October 1985.

Discovery completed her 39th and final flight earlier this month.  Endeavour will equip the station with the Alpha Magnetic Spectrometer, an external astronomical observatory, on her 25th and final flight, a 14-day voyage.

Though the inevitability of the shuttle program’s looming retirement, after 135 flights, has long sunk in among most of NASA’s astronaut corps, the final mission is still difficult to contemplate.

“It’s definitely a reality check,” said Magnus. “It’s going to be very sad. I will probably be crying.”