For 27 years, the space shuttle has been the center of the U.S. space program and the only means to send crews up to space. But, in a year and a half, the shuttle -- now on its 125th mission -- will be retired by presidential mandate. With only nine or 10 flights left to go, the program is on the brink of a major transition.
So, what's next for NASA when the space shuttle quits flying? In effect, it will be back to the future. NASA will launch two rockets -- not one as they did in the Apollo days, when man first landed on the moon.
The Ares V rocket will take a lunar landing vehicle into Earth orbit, and another will carry a crew of four astronauts into a similar orbit to rendevous with the cargo rocket.
"The Ares V will be the largest launch vehicle that anyone has ever built," said Jeff Hanley, manager of NASA's Constellation Program, charged with paving the way for America's re-entry into space exploration. "[Ares V] will be assembled down at the Kennedy Space Center in the same building that the Saturn V was assembled in. But it will be able to lift substantially more than what Saturn was able to do.
"One of the things we wanted to do was separate the crew from the major cargo of the flight, which would be the lunar lander, all of the experiments and gear that the crew would take with them to the lunar surface," Hanley said.
The astronauts in their capsule will link with the spacecraft carrying their lander, then will reignite the engine from the larger rocket and head for a three-day trip to the moon. All four of the astronauts will get into the lander to go to the moon's surface.
Hanley told ABC News' Charles Gibson that it will be very demanding to achieve the goals of going back to the moon, and eventually, getting to Mars.
"We will go to the moon -- anywhere on the moon. We will stay twice as long as Apollo did with twice as many people, and be able to bring that crew home any time," he said.
The plan is for astronauts to live there for weeks at a time and perfect the systems that will later take man even deeper into space.
"The plan that we've laid out will actually lead humans to Mars some time in the next two or three decades," Hanley said.
There will be a five-year gap between the end of the space shuttle and the flight of the Ares V rockets in 2015. During that time, the United States will have no access to space except for seats on the Russian Soyuz -- seats that cost $32 million.
Hanley said he worries about the gap in the nation's space exploration, but it is out of his control.
"Surely it will have some impact on the public's perception of how out-front and leading NASA is," he said. "This gap is because of the hard decisions that had to be made about the amount we're willing to spend, we as a country, on this endeavor.
"And so, the close-down of the shuttle, the disposition of all of its assets and its facilities, and those that we're using then for Constellation, converting them to our use, and getting that system designed and tested and ready to fly -- there's going to be a natural gap there at some point," he said. "It could have been shorter."
Now, Discovery and its crew are flying the STS 119 mission, and it is going well, with just a few minor problems. Weather permitting, the crew will land in Florida Saturday.
Veteran astronaut John Phillips, flying on the current mission, said the legacy of the space shuttle is in its uniqueness.
"We have had a vehicle that would fly for 30 years. It is still the only reusable spacecraft. No one else has one," Phillips said. "It will fly for 30 years and do things no other spacecraft can do, and no other spacecraft in the foreseeable future can do -- carry these big things up and grab it with a robot arm and dock to the space station and bolt something on the side and send spacewalkers out and launch satellites and do scientific laboratory experiments, all of these things together. ... It is a remarkable vehicle."
The first mission to the moon may not happen until 2020, but the work to get there is intensive. It's no small task to design rockets, space capsules, lunar landers and lunar rovers.
But NASA has a head start.
They've been there, and they've done that.